ARTICLE IN PRESS Progress in Quantum Electronics 27 (2003) 211–266 Review Continuous-wave silica-based erbium-doped ﬁbre lasers Antoine Bellemare D!epartement de Physique, de Ge!nie Physique et d’Optique, Universit!e Laval, Qu!ebec (Qc), Canada G1K 7P4 Abstract This review paper on erbium-doped ﬁbre laser (EDFL) covers a broad range of designs and applications related to the ﬁeld of optical ﬁbre telecommunication. After a brief historical overview of EDFL technology in Section 1, Section 2 will present the theoretical background necessary to appreciate the experimental results and applications discussed later. A detailed review of EDFL developments will be given in Section 3, which is divided in three parts. The ﬁrst part will focus on tuneable EDFLs, while the second part is concerned with multifrequency EDFLs. The third sub-section will be devoted to superﬂuorescent ﬁbre sources. Throughout Section 3, illustrative examples of various EDFL designs and applications will be presented. Section 4 will conclude this review by recalling the key issues related to EDFL development and will offer some insights concerning future research trends. r 2003 Elsevier Ltd. All rights reserved. PACS: 42.55.Wd; 42.60.D; 42.81; 42.79.Sz Keywords: Fibre lasers; Laser ampliﬁers; Fibre optics; Optical communication systems Contents 1. Historical development of erbium-doped ﬁbre lasers . . . . . . . . . 212 2. Theoretical background . . . . . . . . . . . . . . . . . 2.1. The physics of ampliﬁcation in erbium-doped ﬁbre 2.2. Erbium-doped ﬁbre parameter measurement . . . . 2.2.1. Signal emission and absorption coefﬁcients 214 215 218 218 E-mail address: [email protected] (A. Bellemare). 0079-6727/03/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0079-6727(02)00025-3 . . . . . . . . . . . . . . . . . . . . . . . . ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 212 2.2.2. 2.3. 2.4. The fundamental and metastable state pump absorption coefﬁcients . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Metastable state lifetime . . . . . . . . . . . . . . . . Algorithm of a space/frequency resolved simulation program . Typical erbium-doped ﬁbre ampliﬁer performance curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 220 220 221 3. Review of erbium-doped ﬁbre laser development . . . . . . 3.1. Tuneable lasers . . . . . . . . . . . . . . . . . . . . . 3.1.1. Background . . . . . . . . . . . . . . . . . . 3.1.2. Linear cavity EDFL . . . . . . . . . . . . . . 3.1.3. Ring cavity EDFL . . . . . . . . . . . . . . . 3.1.4. Coupled cavity EDFL . . . . . . . . . . . . . 3.2. Multifrequency lasers . . . . . . . . . . . . . . . . . . 3.2.1. Background . . . . . . . . . . . . . . . . . . 3.2.2. Room temperature operation of EDFL . . . . 3.2.3. Liquid nitrogen cooled multifrequency EDFL . 3.3. Superﬂuorescent ﬁbre source . . . . . . . . . . . . . . 3.3.1. Background . . . . . . . . . . . . . . . . . . 3.3.2. Typical SFS performance curves . . . . . . . . 3.3.3. Principal work published on SFS . . . . . . . . . . . . . . . . . . . . . 228 228 228 229 231 236 238 238 239 240 243 243 246 252 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 1. Historical development of erbium-doped ﬁbre lasers The discovery of the light ampliﬁcation process in rare-earth doped ﬁbres dates back more than 40 years. In fact, research on optical ﬁbre-based lasers has begun shortly after the famous proposal of the optical maser by Schawlow and Townes  in 1958 and the ﬁrst demonstration of the laser effect by Maiman  in 1960. In January 1961, Snitzer  proposed to use an optical ﬁbre as the gain medium and resonant cavity, and later that year he demonstrated it experimentally  using a neodymium-doped barium crown glass ﬁbre. In 1963, Prokhorov  modelled ampliﬁcation in an optical ﬁbre. That same year, Wolff et al.  demonstrated the ﬁrst plastic ﬁbre laser. In 1964, Koester et al.  showed 47 dB of internal ampliﬁcation of a light pulse at 1.06 mm in a neodymium-doped multimode ﬁbre pumped by a ﬂash lamp. It is interesting, therefore, that a signiﬁcant body of work pre-dates the important paper of Kao and Hockham , who in 1966 ﬁrst discussed the telecommunication potential of optical ﬁbres . Notwithstanding this impressive pioneering work, the glasses made at that time had large intrinsic losses (about 4 dB/m), and the appearance of practical rare-earth-based ﬁbre ampliﬁers and lasers was delayed to the mid 1980s. Nevertheless, Stone et al. [10,11] realised the ﬁrst silica-based ﬁbre lasers in the early 1970s. Co-linearly pumped with the signal by semiconductor diode lasers and emitting continuously at room temperature, these ﬁbre lasers showed good promise for ﬁbre-optic telecommunications applications . A breakthrough in the modiﬁed chemical ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 213 vapour deposition (MCVD) optical ﬁbre fabrication process allowed the incorporation of rare-earth ions in the core of a preform , and subsequently the fabrication of low background loss singlemode ampliﬁer ﬁbre. This breakthrough led to the fabrication, in 1985, of the ﬁrst singlemode ﬁbre lasers by a group at Southampton University . These neodymium-doped ﬁbre lasers, in a linear or ring cavity, emitted a few milliwatts of output power around 1.08–1.09 mm, and would pave the way to other rare-earth-doped ﬁbre lasers and ampliﬁers. In particular, the erbium ion, which has a radiative transition around 1.55 mm corresponding to the lowest loss transmission window in silica ﬁbre, has attracted most of the interest for ﬁbre-optic telecommunications. Despite it forming a threelevel laser system, the erbium ion in silica can be made into a very potent gain medium once put in ﬁbre form. The guided-wave approach has many advantages over a bulk gain medium : * * * High pump intensity: Since glass ﬁbre cores can be made only a few microns in diameter, the small mode ﬁeld diameter of the waveguided pump light yields a much higher pump intensity in a ﬁbre laser than in a bulk device, and therefore a reduced lasing threshold. This feature is particularly important for three-level systems. Signal and pump light waveguiding: The excellent mode overlap between the signal and pump light and the guaranteed parallelism between the two light beams allows efﬁcient laser operation. Independence of pump spot size and gain medium length: In a bulk gain medium, since the pump beam is divergent, there follows a relation between the optimum gain medium length (Lopt ) and the pump spot size (o0 ) for the case of a TEM00 pump laser of wavelength l: Lopt E3po20 =l: * * However, for a ﬁbre laser, the two parameters are independent. This added degree of freedom allows the doping density to be kept sufﬁciently low to avoid unwanted ion–ion interactions like cooperative up-conversion. Having the possibility to use an arbitrarily long gain medium allows for weak pump absorption lines to be used to excite the lasing transition. This can be of practical importance if that weak pump band were to coincide with the wavelength of commercially available diode lasers. It also permits the use of weak radiative transitions, as in the case of the L-band erbium-doped ﬁbre ampliﬁers. Compact gain medium: In ﬁbre form, the gain medium can be arbitrarily long, yet compact. High quality silica ﬁbre can be coiled to small bend radii (about 10 mm) and can be packaged in many different ways. Heat dissipation: The small diameter of the ﬁbre allows for good heat dissipation, thus greatly reducing the occurrence of heat-related problems like thermal lensing, thermal gradient-induced stresses and reduced ﬂuorescence at high temperature. ARTICLE IN PRESS 214 * * A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 Beam quality: A singlemode optical ﬁbre will provide a diffraction limited beam up to very high output powers. Robustness: An all-ﬁbre laser cavity is much more robust to mechanical perturbations than a free-space laser; once every component is spliced, there is no need for further optical alignment. The ﬁrst erbium-doped ﬁbre laser has been realised in 1986 at Southampton University by Mears et al. . It was also the ﬁrst time that a three-level laser system was operated in continuous-wave (CW) at room temperature, clearly demonstrating the advantages of waveguided lasers over bulk lasers. The following year, the same group published the ﬁrst results concerning the erbium-doped ﬁbre ampliﬁer (EDFA) . Although it is not the subject of this review to discuss EDFAs in detail, their development has driven a lot of the research on erbium-doped ﬁbre lasers (EDFL), since they are so closely related. Furthermore, the invention of the EDFA has greatly inﬂuenced the development of optical ﬁbre telecommunication by rendering practical the concept of wavelength-division multiplexing (WDM). Therefore, it is relevant to present some background information on EDFAs. Mears and his colleagues showed that a 3 m ﬁbre backward-pumped by an Argon laser (655–675 nm, 60 mW) delivered a peak gain of 28 dB at 1536 nm (gain>10 dB over a 30 nm band) and a saturation power of 7 dB m. That novel optical ampliﬁer became a solid alternative to previously developed optical ampliﬁers like semiconductor optical ampliﬁers (SOA)  and Raman ﬁbre ampliﬁers . The advantages of optical ampliﬁers over optoelectronic regenerators are: the possibility for simultaneous ampliﬁcation of multiwavelength signals, the transparency to the modulation format and the bit rate, power efﬁciency, reliability and cost. However, optoelectronic regenerators do have the advantage of removing signal dispersion (chromatic or polarisation) and nonlinear effects and having no additive noise (though they may introduce bit errors in the data). The EDFA distinguishes itself from other optical ampliﬁers by its compatibility with telecommunication optical ﬁbre, its low crosstalk, low excess noise, its polarisation independence, its high output power and efﬁciency, and its relative compactness. In order to emerge at the commercial level, compact semiconductor pump lasers had to be developed for EDFA applications. Many pump wavelength bands are now accessible through semiconductor diode lasers emitting at 0.67 mm , 0.8 mm [20,21], 0.98 mm , 1.48 mm  and 1.53 mm . A comparison of the efﬁciency of each band will be made in Section 2, where theoretical background information on optical ampliﬁcation in EDFLs will be presented. 2. Theoretical background This section will present the basics of erbium-doped ﬁbre ampliﬁcation modelling. The physical process of ampliﬁcation in erbium-doped ﬁbre will be detailed and the simulation program algorithm will be highlighted. ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 215 2.1. The physics of amplification in erbium-doped fibre Erbium (chemical symbol Er) is a transition metal of the rare-earth series. In a glass matrix, erbium forms trivalents ions with a [Xe]4f115s25p6 electronic structure. All visible and infrared erbium transitions come from electrons in the 4f layer . Since this layer is partially isolated by the 5s and 5p layers, the erbium absorption and emission spectra are relatively unaffected by the type of glass matrix used. However, the phonon energy of the host glass has a signiﬁcant inﬂuence on the lifetimes of excited energy levels . As for many rare-earths, the metastable level of erbium is well separated from the lower laser level, rendering non-radiative transitions difﬁcult and favouring ﬂuorescence . The principal energy levels of the erbium ion are illustrated in Fig. 1, where we have indicated the possible pump wavelengths, the nomenclature of energy levels in spectroscopic notation along with their lifetimes. Many different pump band levels exist and all are reachable with a known laser system. These lasers pump erbium ions from the fundamental level (4I15/2) up to an excited pump level, e.g. 4I9/2 level for a pump in the 0.8 mm wavelength band. From there, ions will quickly and non-radiatively decay to the metastable level 4I13/2. The very short lifetime of pump levels relative to the metastable level lifetime allows the Fig. 1. Energy levels of the erbium ion with lifetimes taken from Ref. . ARTICLE IN PRESS 216 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 accumulation of ions in the metastable level and the formation of a population inversion between the lower (fundamental) and higher (metastable) laser levels. However, since the erbium ion forms a three-level laser system, i.e. the laser transition is between the metastable and fundamental levels, there is a need to counteract the signal absorption from the fundamental level before transparency is obtained, something that is not the case in four-level systems like neodymium, where gain is observed at very low pump powers. The three-level nature also causes erbium to be a gain medium with an optimal length, as we will see later. To properly model the erbium ion, we need to consider a fourth energy level to account for excited state absorption (ESA). Excited state absorption can occur either at the pump or at the signal wavelengths. Fig. 1 illustrates the transitions for pump excited state absorption at 514 and 800 nm. ESA reduces the overall ampliﬁcation efﬁciency by exciting ions from the metastable level to an upper energy state, e.g. 2 H11/2 for a pump at 0.8 mm, from which it may relax non-radiatively to the 4S3/2, and subsequently decay straight to the fundamental level by spontaneously emitting a green photon at 530–550 nm . Overall this process wastes a pump photon each time it happens and, therefore, reduces efﬁciency. The simpliﬁed energy diagram considered in the model is illustrated in Fig. 2. For each energy state i; we associate a density of ions Ni : Level 1 is the fundamental state, level 2 is the metastable state, level 3 is the pump state and level 4 is associated with the pump ESA state. Arrows represent the possible transition. Depending on the associated labelling, the transition can either be a pump transition (R: pump rate), a signal transition (W : absorption or stimulated emission rate), or a spontaneous transition (A: radiative or non-radiative spontaneous rate). The ion density for each state is modelled by the following rate equations: dN1 ¼ R13 N1 þ R31 N3 W12 N1 þ W21 N2 þ AR 21 N2 ; dt dN2 NR ¼ W12 N1 W21 N2 AR 21 N2 þ A32 N3 þ R42 N4 R24 N2 ; dt Fig. 2. Erbium ion energy level model. R: pump transition, W: signal transitions and A: spontaneous transitions (R: radiative, NR: non-radiative). Non-radiative spontaneous transitions are dues to phonon relaxations and are identiﬁed by slanted wavy lines. ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 217 dN3 NR ¼ ANR 32 N3 þ R13 N1 R31 N3 þ A43 N4 ; dt dN4 ¼ ANR 43 N4 þ R24 N2 R42 N4 ; dt r0 ¼ N1 þ N2 þ N3 þ N4 ; where r0 is the erbium concentration, Rij ¼ Pp Gp sij =SEr hnp the pump rate, Wij ¼ Ps Gs sij =SEr hns the signal absorption and emission rate, np ; ns the pump and signal frequencies, tij ¼ 1=Aij the excited state lifetime, and Pp ; Ps are the pump and signal power respectively, Gp ; Gs the overlap between the LP01 mode and doping proﬁle respectively, sij the cross-section of state i to j transition, and SEr ¼ pa2Er is the erbium-doped ﬁbre area. To solve for Ni in the steady-state regime, we set all time derivatives equal to zero: dN1 dN2 dN3 dN4 ¼ ¼ ¼ ¼ 0: dt dt dt dt Since the non-radiative transitions towards the metastable level are sufﬁciently fast compared to the metastable level lifetime, it is possible to neglect the population densities of levels 3 and 4 (N3 ¼ N4 ¼ 0). After a few simple algebraic manipulations , we obtain the basic modelling equations that can be used in a simulation. The variable nðzÞ ¼ N2 ðzÞ=r0 represent the relative population inversion as a function of position along the erbium-doped ﬁbre (EDF). In steady-state we obtain dnðzÞ ap ¼ ½1 nðzÞfp ðzÞ þ dt r0 Z gðnÞ þ nðzÞ ¼ 0; fxðnÞ½1 nðzÞ 1g ½f ðn; zÞ þ f s ðn; zÞ dn r0 s t where the ﬁrst term is a function of pumping, the second term corresponds to the signal acting on the erbium ion population, and the last term represents spontaneous emission. The signal photon ﬂux per unit frequency travelling along (+) or against () the pump photon ﬂux fp ðzÞ; is represented by f7 s ðn; zÞ: The pump absorption coefﬁcient is ap and gðnÞ ¼ Gs ðnÞs21 ðnÞr0 is the emission coefﬁcient. The overlap integral between the signal and the transverse distribution of erbium is Gs ðnÞ: Finally, we have posed xðnÞ ¼ 1 þ s12 ðnÞ=s21 ðnÞ; where s12 and s21 are respectively the signal absorption and emission cross-section. Furthermore, the pump and signal (forward and backward) evolution for each frequency interval is represented by the following equations: dfp ðzÞ ¼ fap ½1 nðzÞ þ aESA nðzÞgfp ðzÞ; dz df7 s ðn; zÞ ¼ 7gðnÞfxðnÞ½nðzÞ 1 þ 1gf7 s ðn; zÞ72dngðnÞnðzÞ: dz The spontaneous emission noise is modelled by the introduction of two photons (one per polarisation state) per frequency interval dn  while ESA is considered by the inclusion of the excited state pump absorption coefﬁcient aESA : ARTICLE IN PRESS 218 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 2.2. Erbium-doped fibre parameter measurement The proper measurement of the erbium-doped ﬁbre parameters is crucial to the accurate modelling of the EDFA. These parameters are the signal emission and absorption coefﬁcients, the pump absorption coefﬁcient for the fundamental and metastable state, and the metastable state lifetime. In the following, measurement techniques will be presented for these parameters. 2.2.1. Signal emission and absorption coefficients The signal absorption coefﬁcient can be measured over a wide wavelength range by the cutback method , with a tuneable laser and a calibrated optical powermeter, or with a broadband source and an optical spectrum analyser (OSA). The launched power needs to be low enough or else saturated absorption may occur in the EDF. However, the ﬁbre needs to be short enough so that there is still some signal power detected at the output. The cutback method consists of measuring the output spectrum at the output of the EDF, Pðn; z ¼ LÞ; then cutting back a portion DL of the EDF, and repeating the output spectrum measurement. The signal absorption coefﬁcient is computed by the formula ln½Pðn; z ¼ L DLÞ=Pðn; z ¼ LÞ as ðnÞ ¼ : DL For the measurement of the emission coefﬁcient, two different methods exist. First, the emission coefﬁcient can be obtained from the ﬂuorescence spectrum FðnÞ of an over-pumped (highly inverted) short piece of ﬁbre (a few cm). The emission coefﬁcient is then computed using the following relation: FðnÞ gðnÞ ¼ ZMAX as ðnMAX Þ ; FMAX where nMAX is the frequency where the absorption coefﬁcient is highest, ZMAX ¼ gðnMAX Þ=as ðnMAX Þ ¼ 0:95 from Ref.  and FðnÞ=FMAX is the normalised ﬂuorescence. The ﬂuorescence spectrum can be easily measured from one end of the ﬁbre with an OSA; however this spectrum suffers from some distortion caused by the ampliﬁed spontaneous emission (ASE) occurring along the ﬁbre length. It is possible to alleviate the spectrum narrowing caused by the ASE by measuring the ﬂuorescence spectrum from the side of the ﬁbre. However, this measurement is much more difﬁcult to achieve because only a very small amount of light is available. It must be mentioned that accurate measurement of the emission coefﬁcient is difﬁcult to obtain because it depends on nMAX ; and FðnÞ varies quickly around nMAX : It has been shown  that it is possible to obtain directly gðnÞ from as ðnÞ using the following expression: h½nMAX n gðnÞ ¼ ZMAX as ðnÞexp : kB T This method is precise because there is no uncertainty about the ﬂuorescence spectrum. Furthermore, it is much simpler to compute the emission coefﬁcient than to do the experiment. A typical parameter measurement result is present in Fig. 3. ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 Fig. 3. Absorbtion (- - -) and emission ( 219 ) coefﬁcient for the erbium-doped ﬁbre INO-920213-1. Fig. 4. Energy distribution of erbium ions in the 4I13/2 and 4I15/2 manifolds. The arrows represent the most probable transitions. It is worth noting the wavelength shift between the emission and absorption coefﬁcients in Fig. 3. This fact is explained by the simpliﬁed energy diagram of Fig. 4, where it can be seen that the Boltzmann energy distribution of ions in the 4I13/ 4 2 and I15/2 manifolds creates a long wavelength shift of the emission cross-section relative to the absorption cross-section. The most probable absorption transition is from the highly populated (low energy) sub-levels of 4I15/2 to the less populated (high energy) sub-levels of 4I13/2. Conversely, the most probable emission transition is from the highly populated (low energy) sub-levels of 4I13/2 to the less populated (high energy) sub-levels of 4I15/2. 2.2.2. The fundamental and metastable state pump absorption coefficients The fundamental state pump absorption coefﬁcient can also be measured by the cutback method, in the same way the signal absorption coefﬁcient is measured. The ARTICLE IN PRESS 220 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 metastable state pump absorption coefﬁcient measurement requires that the EDF is fully inverted by a pump laser. A second light source is then used to measure the ESA absorption coefﬁcient . ESA is not a necessary measurement if the EDF is to be pumped at 980 or 1480 nm since these pump bands do not suffer from ESA . 2.2.3. Metastable state lifetime The measurement of the metastable state lifetime is made by modulating a pump laser injected into the EDF and observing the exponential decay of ﬂuorescence. A typical value for ﬂuorescence lifetime is 10 ms when there are no erbium ion clustering effects . At high erbium concentrations (>0.1% wt) ﬂuorescence lifetime is reduced and detrimental effects like cooperative up-conversion can occur to reduce the ampliﬁer’s efﬁciency . 2.3. Algorithm of a space/frequency resolved simulation program The objective of the simulation program is to compute the output spectrum of the EDFA over the wavelength range from l1 to l2 with a spectral resolution of dl: To do so, the program must solve 2½ðl2 l1 Þ=dl þ Ns þ Np coupled differential equations, that is, one differential equation for each spontaneous emission wavelength bin, one for each signal wavelength, and one for each pump, all in the forward and backward direction, since signal/pump reﬂections at EDF ends caused by a reﬂector or by unwanted reﬂections are taken into account. Since there is no way of knowing a priori the forward (+) and backward () ampliﬁed spontaneous emission (ASE) distribution along the ﬁbre, the simulation program must make an assumption about the backward ASE at the signal input end (z ¼ 0) if the signal is travelling along with the pump, in order to perform the integration of the equations over the length of the ﬁbre with steps dz: Essentially there are two ways to solve such a two boundary value problem: the shooting method and the relaxation method. The boundary values are ASEþ ðz ¼ 0Þ ¼ 0 but ASE ðz ¼ 0Þ is unknown, while ASE ðz ¼ LÞ ¼ 0 and ASEþ ðz ¼ LÞ is unknown. The shooting method is a trial and error approach in which the backward ASE spectrum at z ¼ 0 is ﬁrst estimated before the system of equations is integrated from z ¼ 0 to L: Since the backward ASE must be null at the signal output end (z ¼ L) it is possible to correct the ﬁrst guess for ASE ðz ¼ 0Þ and iterate until the boundary condition ASE ðz ¼ LÞ ¼ 0 is met. This method is not recommended since its convergence is slow, the method is complex, and results may be arbitrarily inﬂuenced by the backward ASE spectrum estimation method. The relaxation method makes iterative adjustments to the solution. A ﬁrst integration of the equations is made along the signal propagation direction (from z ¼ 0 to L) without considering backward ASE. Then, considering backward ASE, integration in the reverse direction (from z ¼ L to 0) is performed using the previously computed forward ASE distribution, and a ﬁrst estimate of the backward ASE distribution is obtained. By iterating this way with the boundary conditions of the previous iteration it is possible to obtain a fast and precise convergence to a ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 221 solution by the nature of the rate equation involved. The fact that we ignored backward ASE in the ﬁrst integration overestimates signal gain and forward ASE, and underestimates backward ASE in the second iteration. However, in each iteration backward ASE is less and less underestimated and the solution is reached asymptotically, contrary to the shooting method that can lead to oscillations around the actual solution. The iteration process is stopped by adding an accuracy condition between iterations, e.g. a computed signal gain value change of less than 0.001 dB. Finally, only a few adjustments need to be made to a working EDFA simulation program in order to model EDFL operation. Instead of the previously stated boundary conditions on the ASE distribution, the new boundary conditions would become ASEþ ðz ¼ 0Þ ¼ R1 ASE ðz ¼ 0Þ and ASE ðz ¼ LÞ ¼ R2 ASEþ ðz ¼ LÞ for the case of a linear cavity laser, where R1;2 is the reﬂectivity of the back and front reﬂector. An excellent commercial example of such a program is OASiX 3.1 developed by Lucent Technologies. With this program, it is possible to simulate ampliﬁers and linear lasers. EDFAs can be simulated with up to 80 input signals, in the 1500– 1650 nm range, in various ampliﬁer conﬁgurations comprising up to 6 stages. ASE noise bins are spaced 2 nm apart in the 1520–1620 nm range. Each ampliﬁer stage can be pumped bi-directionally either at 960–999 or 1450–1499 nm. The model accounts for parasitic ﬁbre end reﬂections, components loss, Rayleigh backscattering, temperature and spectral hole burning (SHB). Pump and wavelength dependent signal reﬂectors along with inter-stage ﬁlters, isolators and pump bypasses can be included to study complex ampliﬁer structures. The program outputs gain, noise ﬁgure, ASE noise power, residual pump power, backscattered signal and averaged inversion level. In the following section we will illustrate a few EDFA concepts by presenting characteristic simulation results. 2.4. Typical erbium-doped fibre amplifier performance curves An accurate modelling program enables an EDFA design to be optimised in order to meet the required performance much faster than a trial-and-error experimental method. Still, good engineering practice requires that the design be validated experimentally before EDFA prototypes are made. For example, we can ﬁnd the optimal EDF length required to generate the highest possible gain for a given signal and under certain pump constraints. Fig. 5 illustrates the simulation results of the ampliﬁcation of a 20 dB m (10 mW) signal at 1550 nm in a Lucent Technologies HG-980 ﬁbre with a pump power of 50 mW. The HG-980 ﬁbre has a peak signal absorption as ¼ 8 14 dB/m, a numerical aperture NA=0.29, a mode-ﬁeld diameter MFD=3.6–5.2 mm, and a cut-off wavelength lc ¼ 8002950 nm. A few interesting concepts can be introduced by the results of Fig. 5. First, it is evident that an optimum EDF length exists for each pump conﬁguration. This is due to the three-level nature of the erbium ion, as previously mentioned. It is also clear that 1480 nm pumping delivers more gain than 980 nm. This is because, for the same amount of power, 1480 nm pumping provides about 50% more pump photons than ARTICLE IN PRESS 222 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 35 30 Gain [dB] 25 20 15 10 5 0 0 5 10 15 20 EDF length [m] 25 30 Fig. 5. Gain as a function of EDF length for a single-stage EDFA under different pumping conﬁgurations: forward at 1480 nm (&), forward at 980 nm (W), backward at 1480 nm (J) and backward at 980 nm (B). Ps ¼ 20 dB m, ls ¼ 1550 nm, Pp ¼ 50 mW. Table 1 Maximum gain, optimal EDF length, linear gain coefﬁcient and noise ﬁgure as a function of pump conﬁguration Pump conﬁguration Gmax (dB) Lopt (m) g (dB/m) F (dB) Forward at 1480 nm Forward at 980 m Backward at 1480 nm Backward at 980 nm 32.2 29.3 33.1 30.2 16.5 8.0 17.5 9.5 3.1 5.8 3.1 5.8 5.1 3.5 7.6 6.8 980 nm pumping. However, since 1480 nm pumping excites ions directly in the 4I13/2 level, only incomplete inversion is obtained. This leads to a lower gain coefﬁcient g¼ maxfdG=dLg; longer optimal EDF length Lopt and higher noise ﬁgure (F ). The noise ﬁgure is a measure of the signal-to-noise ratio deterioration between the input and the output of the EDFA. Noise ﬁgure will be formally deﬁned below. Table 1 resumes these parameters. Considering that an EDFA is usually required to be both high gain and low noise, the best conﬁguration would be forward pumping at 980 nm for a single-stage ampliﬁer. However, it is possible to obtain higher gain while maintaining an excellent noise ﬁgure in a two-stage ampliﬁer  composed of a low noise preampliﬁer stage followed by a booster ampliﬁer stage capable of outputting a high power signal but having a mediocre noise ﬁgure. Then the two-stage ampliﬁer output power performance is dictated by the booster stage and the noise ﬁgure is determined by the preampliﬁer since the noise ﬁgure of the cascade is FTOT ¼ Fpreamp þ ðFbooster 1Þ= Gpreamp . Fig. 6 presents typical results for a single-stage EDFA with various pump wavelengths. It is clear from Fig. 6 that the pump wavelengths of 514.5 and 810 nm, ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 223 Fig. 6. Gain as a function of EDF length for a single-stage forward pumped EDFA for different pump wavelengths: lp ¼ 514:5 nm (B), 532 nm (W), 670 nm ( * ), 810 nm (J) et 980 nm ( ). L ¼ 2 m, Pp ¼ 50 mW, Ps ¼ 10 mW, and ls ¼ 1550 nm. Fibre INO-920213-1: as ¼ 31 dB/m, NA=0.19, MFD=7.3 mm, and lc ¼ 850 nm. known to suffer from ESA, show signiﬁcantly less gain than the pump wavelengths of 532 and 980 nm which are free from ESA. Figs. 5 and 6 allow the EDFA designer to select the proper pump source for its application. It must be mentioned that gain is not the only criteria by which the choice of a pump source is to be made, as source practicality (size, power consumption, reliability, etc.) must also be considered. Semiconductor diode lasers can offer output powers of hundreds of milliwatts coupled in the ﬁbre at wavelengths of 670, 800, 980 and 1480 nm. Also, frequency doubled mini-Nd:YAGs emitting at 532 nm can be very appealing for power ampliﬁer applications . However, for most applications, it is preferred that the pump be singlemode in the EDF, thus limiting the choice to 980 and 1480 nm pumping. From now on, 980 nm pumping is assumed unless speciﬁcally noted. This EDFA study can now be pursed by showing EDFA gain as a function of pump power (Fig. 7). From these curves it is possible to extract the following parameters: threshold pump power (Pp;th ¼ fPp jG ¼ 0g) and gain efﬁciency ZG ¼ maxfG=Pp g; resumed in Table 2. It can be observed that the signal wavelength of 1530 nm has the highest gain and best gain efﬁciency. However, the lowest threshold is for 1565 nm, since the higher the signal wavelength is, the more the erbium ion operates like a four-level system, that is without signal ground state absorption (see Fig. 3). To obtain the output saturation power, the ampliﬁer power efﬁciency and the quantum efﬁciency, one must compute the gain versus input signal power curve as shown in Fig. 8. Table 3 shows the values of saturated output power, power efﬁciency (Z ¼ ½Psat Ps =Pp ) and quantum efﬁciency for a signal input power of 0 dB m at different wavelengths. Quantum efﬁciency (Zq ¼ Zls =lp ) is deﬁned as the conversion efﬁciency of pump photons into signal photons. The quantum efﬁciency is not 100% ARTICLE IN PRESS 224 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 35 30 Gain [dB] 25 20 15 10 5 0 0 20 40 60 Pump power [mW] 80 100 Fig. 7. Gain as a function of forward pump power for a single-stage EDFA for different signal wavelength: 1520 nm (J), 1530 nm (B), 1550 nm (W) and 1565 nm (&). Ps ¼ 20 dB m, L ¼ 8 m (HG980) and lp ¼ 980 nm. Table 2 Threshold pump power, gain at 50 mW pump power and gain efﬁciency as a function of signal wavelength ls (nm) Pp;th (mW) G @ Pp ¼ 50 mW (dB) ZG (dB/mW) 1520 1530 1550 1565 5.7 4.8 3.8 3.1 21.2 31.5 29.3 25.4 1.2 2.2 2.2 2.1 40 35 Gain [dB] 30 25 20 15 10 5 0 -30 -20 -10 0 10 Input signal power [dBm] Fig. 8. Gain as a function of input signal power for a single-stage EDFA for different signal wavelength: 1520 nm (W), 1530 nm (&), 1550 nm (J) and 1565 nm (B). Pþ p ¼ 50 mW, ls ¼ 1550 nm, L ¼ 8 m (HG980) and lp ¼ 980 nm. because a portion of the pump photons is converted to ASE noise, because another part is not absorbed by the ampliﬁer and thus is wasted, and also because the erbium ion is a three-level system with a threshold level. ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 225 Table 3 Saturated output power, power efﬁciency and quantum efﬁciency as a function of signal wavelength ls (nm) Psat (dB m) Z (mW/mW) Zq (%) 1520 1530 1550 1565 12.5 12.7 13.3 13.4 0.34 0.36 0.41 0.41 52.4 55.5 64.3 66.0 40 35 Gain [dB] 30 25 20 15 10 5 0 1510 1520 1530 1540 1550 1560 1570 Wavelength [nm] Fig. 9. Single-wavelength gain as a function of wavelength for a single-stage EDFA for different signal powers: 30 dB m (&), 20 dB m (J), 10 dB m (B) and 0 dB m (W). Pþ p ¼ 50 mW, L ¼ 8 m (HG-980) and lp ¼ 980 nm. It is also very instructive to compute the gain spectrum of an EDFA under various channel loading conditions in order to understand the concept of ampliﬁer saturation. In the single-wavelength case we can observe that the gain curve becomes quite ﬂat for high input signal power. In Fig. 9, for an input power of 10 dB m, the gain is ﬂat to better than 0.7 dB over the 1524–1564 nm wavelength range. This ﬂat gain characteristic is especially important in broadly tuneable ﬁbre ring lasers [35,36]. In those lasers, cavity losses are reduced to a minimum to force the gain stage into deep saturation operation. It is well known that the round-trip gain is equal to the cavity losses for any continuous-wave laser above threshold. So if cavity losses are kept low then circulating intracavity power will quickly increase to very high values in order to saturate the ampliﬁer stage gain value down to the steady-state value. According to Fig. 9, the gain will be very ﬂat. So if the laser is tuned using an intracavity ﬁlter, a highly desirable ﬂat tuning curve will be obtained. In the multiwavelength case, the same ampliﬁer will perform differently. Fig. 10 illustrates the multiwavelength gain for a single-stage EDFA with 35 input wavelengths spaced by 1 nm between 1529 and 1563 nm. We observe a peak-topeak gain non-uniformity of nearly 5 dB for the 10 dB m input power gain curve. ARTICLE IN PRESS 226 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 40 35 Gain [dB] 30 25 20 15 10 5 0 1520 1530 1540 1550 Wavelength [nm] 1560 1570 Fig. 10. Multi-wavelength gain as a function of wavelength for a single-stage EDFA for different signal powers: 20 dB m (&), 10 dB m (W) and 0 dB m (J). Pþ p ¼ 50 mW, L ¼ 8 m (HG-980) and lp ¼ 980 nm. Fig. 11. Effect of a saturating signal on the gain curve depending on the type of atomic transition spectral broadening. (a) Homogeneous broadening (b) inhomogeneous broadening. This is due to the fact that, at room temperature, the gain mostly saturates homogeneously in an EDFA. It is worthwhile to note that two types of atomic transition spectral broadening exist: homogenous and inhomogeneous. A gain medium is said to be purely homogeneous if all atomic transitions undergo identical spectral broadening. Conversely, a gain medium is said to be purely inhomogeneous if each atom’s transition frequency is shifted to a different and distinct extent so that the total broadening of the transitions is a combination of all these individual shifts . These two cases are depicted in Fig. 11, where the thin and thick lines represent the unsaturated and saturated gain curves respectively. In a homogeneous gain medium, ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 227 gain is saturated uniformly over all frequencies. In an inhomogeneous medium, the gain is not saturated evenly for all frequencies. The saturation occurs mostly for the channels around the saturating signal. Thus, it is possible to have a strong gain at certain frequencies even under strong saturation. Many phenomena determine the linewidth of an atomic transition. For free ions, the intrinsic linewidth broadening is related to the lifetime of the levels (i; j) involved in the transition by Doa ði; jÞ ¼ 1=ti þ 1=tj ; where tk ¼ tR;k þ tNR;k is the level lifetime, tR;k is the radiative lifetime and tNR;k is the non-radiative lifetime. When ions are incorporated into a glass matrix, the high frequency lattice vibrations generate phonon broadening. For solid-state lasers the phonon broadening is the dominant effect. Intrinsic and phonon broadening are homogeneous effects . Other broadening processes are inhomogeneous. The Stark effect, which is induced by the crystalline electric ﬁeld surrounding the erbium ion, lifts the atomic state degeneracy. If the fundamental level 4I15/2 and the metastable level 4I13/2 are respectively split into g1 and g2 sub-levels, then the transition between these levels is the superposition of g1 g2 transitions between the sub-levels. For a purely electric perturbation, g ¼ J þ 1=2; so that g1 ¼ 8 and g2 ¼ 8: The transition line width is thus broadened. Strictly speaking, Stark broadening is not a homogeneous process because each transition between the Stark manifolds has its own characteristics. However, all sub-levels are strongly coupled by fast thermalisation effects caused by their small energy gap, and by the important spectral overlap between Stark components. This is especially true at room temperature. This strong coupling explains why, at room-temperature, saturation in erbium-doped ﬁbres is mostly homogeneous. At very low temperature, thermalisation is much less efﬁcient and saturation becomes inhomogeneous. It is then possible to observe spectral hole burning (SHB) effects in EDFAs . Inhomogeneous broadening is also possible at room temperature. In fact, in a glass matrix, the electric ﬁeld and crystalline vibrational modes vary according to the local glass structure . Thus at room temperature, the main factor causing partial inhomogeneous broadening in erbiumdoped ﬁbres is the random site-to-site ﬂuctuation of the Stark and phonon broadening. Finally, it is possible to evaluate the noise ﬁgure from the ampliﬁer output spectrum (see Fig. 12). Noise ﬁgure, under the condition that signal-ASE beat noise is dominant, is computed according the formulae S=Nin 1 PASE F þ1 ; ¼ S=Nout G Dnhn where G is the EDFA gain, PASE is the ASE in the noise bandwidth Dn; n is the signal optical frequency and h ¼ 6:626 1034 J s is Planck’s constant. For the present case of Fig. 12 we ﬁnd F ¼ 3:5 dB which is very close to the quantum limit of 3 dB for high gain ampliﬁers. It must be mentioned that the strict quantum limit of a fully ARTICLE IN PRESS 228 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 10 Output power [dBm/nm] 5 0 -5 -10 -15 -20 -25 -30 1520 1530 1540 1550 1560 Wavelength [nm] 1570 1580 Fig. 12. Output power spectral density (resolution=1 nm) of a single-stage EDFA with Ps ¼ 20 dB m, ls ¼ 1550 nm, Pþ p ¼ 50 mW, L ¼ 8 m (HG-980) and lp ¼ 980 nm. inverted ampliﬁer is expressed as Fq ¼ 2 1=G . For example, an ampliﬁer with gain G ¼ 2 (3 dB) could have a noise ﬁgure as low as F ¼ 1:5 (1.76 dB). 3. Review of erbium-doped ﬁbre laser development This section will review the major achievements in erbium-doped ﬁbre laser technology throughout its historical development. The review is divided in three subsections focusing on three different types of erbium-doped ﬁbre sources, namely: tuneable single-frequency lasers, multifrequency lasers and superﬂuorescent ﬁbre sources. Each erbium-doped ﬁbre source types has its own challenges and applications that are worthwhile to discuss. 3.1. Tuneable lasers 3.1.1. Background As previously mentioned in Section 1, the pioneering work of Snitzer and Koester on ﬁbre lasers at the American Optical company dates back to the early days of laser research. Since then, ﬁbre lasers have progressed substantially and now ﬁnd application in a multitude of ﬁelds. Tuneable ﬁbre lasers are found in ﬁbre-based sensors systems  and can be used in spectroscopy [25,41]. Fibre laser transmitters have been studied in direct detection digital [42–50] and analog  ﬁbre-optic communication systems. Narrow line width lasers have been used in receivers for coherent communication system experiments . Tuneable EDFL can also be applied to the characterisation of WDM ﬁbre-optic components [35,53]. High power double-clad ﬁbre lasers  are used in medical and material processing applications. Q-switched ﬁbre lasers are anticipated to ﬁnd applications in non-linear optics, distributed sensing, optical time domain reﬂectometry, range ﬁnding and LIDAR systems . Visible ﬁbre lasers  can serve in displays or data storage applications. ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 229 The historical development of erbium-doped ﬁbre lasers (EDFL), especially those related to optical ﬁbre telecommunication, can be traced as follows. Mears et al. , while at Southampton University, demonstrated the ﬁrst EDFL in 1986. In that paper, they also published the ﬁrst results on EDFL wavelength tuning. It was Reekie et al.  that reported the ﬁrst diode laser-pumped EDFL. While studying the effect of erbium-doped ﬁbre length on the EDFL they also showed that EDFLs could oscillate in the L-band. Initially mounted in linear cavities, EDFL have since then been assembled in various conﬁguration. Urquhart has made a complete review of ﬁbre laser resonators in Ref. . In 1988, the ﬁrst coupled-cavity EDFL has been made by Barnsley et al. . Using a Fox–Smith resonator, they demonstrated the ﬁrst single-longitudinal mode ﬁbre laser. A year later, Scrivener and his co-worker  presented the ﬁrst ﬁbre ring EDFL. By adding a non-reciprocal element in the cavity, it is possible force a ring EDFL to operate as a travelling-wave laser instead of as a standing-wave laser. The required non-reciprocity can be obtained either by an optical isolator or by a cavity asymmetry [60,61]. Morley et al.  demonstrated, in 1990, that travelling-wave operation eliminates spatial hole burning effects due to interfering standing waves and allows single-longitudinal mode operation of ring EDFLs. It is also possible to eliminate standing waves in a linear cavity by forcing counter-propagating waves to be orthogonally polarised , circularly polarised  or to have different optical frequencies . Finally in 1994, it is Ball et al.  who conceived the ﬁrst tuneable single-longitudinal mode EDFL tuneable without mode hops. Next, major developments in single-frequency and tuneable EDFL technology will be reviewed for each type of resonators, namely: linear cavity, ring cavity, and coupled cavity. The resonator types will be presented and published results will be analysed and summarised in tables. 3.1.2. Linear cavity EDFL The linear, or Fabry–Perot, cavity is the most common laser cavity, and the ﬁrst EDFL cavity that was explored. Its main advantages are its simplicity and the possibility to make very short cavities. It is thus well suited for robust singlelongitudinal mode operation. They are also suitable for master oscillator power ampliﬁer (MOPA)  applications since it is usually easy to recover unabsorbed pump power at the output coupler. An example of a linear cavity is presented in Fig. 13. In a forward pumped linear cavity EDFL, the pump light is injected through a wavelength-dependent reﬂector (WDR) which is, ideally, perfectly transparent at the pump wavelength and perfectly reﬂective at the signal wavelength. The output coupler completes the linear cavity. It is preferable that the output coupler be highly reﬂective at the pump wavelength to recycle unused pump power thus providing optimised pumping and no residual pump at the output. The output coupler must also have a reﬂectivity at the signal wavelength that optimises the output power . The output coupler reﬂectivity in the signal band can either be broadband, leading to a lasing wavelength determined by the erbium-doped ﬁbre gain curve, or wavelength-selective, leading to a lasing wavelength selected, and possibly tuned, by the output coupler. ARTICLE IN PRESS 230 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 Fig. 13. General schematic diagram of a linear cavity EDFL. M1: pump WDR mirror, M2: output coupler, EDF: erbium-doped ﬁbre, ISO: optical isolator. 14 Output power [mW] 12 10 8 6 4 2 0 0 20 40 60 80 Output coupler reflectivity [%] 100 Fig. 14. Output power against narrowband output coupler reﬂectivity (R2 ) for HG-980 EDF lengths of 1 m (&) and 3 m (J). P P ¼ 50 mW, ls ¼ 1550 nm, lp ¼ 980 nm, and R1 ¼ 100% (broadband). Fig. 14 shows a computational example of output coupler reﬂectivity optimisation. We observe that optimal output coupler reﬂectivity is a function of EDF length. It is also a function of pump power, pump wavelength, pump direction and lasing wavelength. And since output power is a function of EDF length for a given output coupler reﬂectivity, it is clear that we need to ﬁnd the proper pair (L; R) to ﬁnd the optimal output power. In Fig. 14, it is interesting to see that the optimal length for laser operation Lopt;laser ¼ 3 m is only about 30% the optimal length for standard ampliﬁer operation Lopt;EDFA ¼ 9:5 m. The ﬁbre laser by Ball et al. , shown in Fig. 15, is an excellent example of a linear cavity. This laser uses ﬁbre Bragg gratings (FBG) at each ends of the doped ﬁbre. Fibre Bragg gratings are wavelength-dependent reﬂectors resulting from UVwritten periodic modulation of the optical ﬁbre effective index of refraction [67–73]. An ingenious stretching/compression system of the ﬁbre allows this laser to be continuously tuned over 32 nm without mode-hops. Furthermore, this MOPA laser has a relaxation oscillation noise reduction circuit that modulates the pump drive current out of phase with detected temporal variations in the laser intensity . Table 4 summarises the principal results published with linear cavity EDFLs. Linear cavities are ideal for compact single-longitudinal mode lasers and for high power applications. ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 231 Fig. 15. Block diagram depicting a continuously tuned EDFL in MOPA conﬁguration. Taken from Ref. . 3.1.3. Ring cavity EDFL The ring cavity design is the most common type of EDFL conﬁguration found in the literature. It is a simple cavity to build, in its simplest form the output of an EDFA is fed to its input using a coupler and a ring cavity laser is obtained, as illustrated in Fig. 16. Besides its simplicity, a ring cavity that includes an optical isolator has the advantage of travelling-wave operation, unlike the standing-wave operation of most linear cavity lasers. Travelling-wave operation is advantageous because it prevents spatial hole burning. When a standing-wave pattern is established in a laser cavity, it will form a gain grating in the erbium-doped ﬁbre. For the wave that forms this gain grating, round-trip gain is reduced because gain is locally saturated on the standingwave pattern. Frequency hopping to other longitudinal cavity modes is possible since neighbouring modes may have a higher (unsaturated) gain. Usually, when no cavity ﬁlters are used, linear cavity lasers are less stable in power and frequency then ring cavity lasers. Ring cavity EDFLs use the gain provided by the erbium-doped ﬁbre more efﬁciently and have a cavity free spectral range (FSR) that is twice as large for the same cavity length, compared to linear cavity lasers . In a ﬁbre ring laser (Fig. 17), pump light, ideally from a high power compact ﬁbre pigtailed laser diode, is injected into the erbium-doped ﬁbre through a wavelengthdivision multiplexer (WDM). The WDM can be a wavelength-dependent fused ﬁbre coupler or it can be a thin-ﬁlm interference ﬁlter. An optical isolator forces unidirectional laser operation. Finally, an all-ﬁbre polarisation controller (PC), either Lefevre’s loops  or Yao’s controller , may be used to optimise the polarisation state of the cavity wave. A bandpass optical ﬁlter can be easily added to the laser cavity if tuneable operation is required. The laser optical signal-to-ASE noise ratio (OSNR) will be improved by placing the ﬁlter just before the output coupler. Conversely, if the ﬁlter is placed after the output coupler, the output power is improved at the expense of the OSNR. The coupling ratio of the output coupler can be optimised either for optimal output power, usually a high output coupling ratio, or laser line width, requiring a long photon cavity lifetime and thus a low coupling ratio. The principal disadvantage of ring EDFLs is the length of the cavity, usually in the 1–100 m range. Thus, the cavity FSR is in the 1–100 MHz range. If we consider an erbium gain bandwidth of the order of 10 THz, then it is easy to understand that a very selective ﬁlter must be used if single longitudinal mode operation, out of 105–107 ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 232 Table 4 Principal results published on linear cavity EDFL Ref. Tuning (nm) Linewidth Power (mW) Pump source Remarks                Possible 0 50 0 0 66 23 Possible 0 43 18 21 6 0 8 4.9 GHz ? o2 nm ? 1 nm 100 MHz 0.3 nm 47 kHz 1 MHz o0.1 nm o0.1 nm ? o0.1 nm 5 kHz o0.1 nm 5.5 ? 0.5 ? 1.35 270 13.5 5 7.5 ? 0.46 ? 0.5 0.2 2.5 Etched ﬁbre grating Fibre loop mirrors EDF length l-setting First Er:Yb ﬁbre laser Mirror-less Diffraction grating Acousto-optic ﬁlter First DBR laser Micro etalon ﬁlter Polarisation stable Acousto-optic ﬁlter Output coupling tuning DBR active ﬁlter AO phase modulator Optical control of wavelength            21 4.5 32 Possible 16.7 Possible Possible Possible 1.4 40 Possible 70 kHz 550 kHz ? 300 kHz o0.1 nm 15 kHz 13 kHz o0.2 nm o0.11 nm 1 MHz 100 kHz 1 0.02 3 3.2 ? 5.4 1.2 ? ? 0.08 0.24 650 nm, 125 mW 806 nm 514 nm, 320 mW 800–845 nm 514 nm, 280 mW 980 nm, 540 mW 514 nm, 830 mW 980 nm, 50 mW 980 nm, 38 mW 980 nm 1480 nm, 28 mW 980 nm 980 nm, 20 mW 980 nm, 80 mW 1060 nm, 60 mW & 980 nm, 25 mW 532 nm, 200 mW 980 nm, 60 mW 1480 nm, 80 mW 980 nm, 120 mW 980 nm, 200 mW 1480 nm, 50 mW 980 nm, 120 mW 1480 nm 980 nm 1480 nm, 55 mW 980 nm, 90 mW   Possible Possible ? 25 kHz 3 9.8 924 nm, 110 mW 980 nm, 110 mW   Possible Possible 260 kHz ? 17 0.06 524 nm, 190 mW 980 nm, 25 mW      Possible Possible 25 Possible 4.7 ? ? o26 MHz 500 kHz 18 kHz 7.6 0.03 13 58 0.58 980 nm, 648 nm, 980 nm, 980 nm, 980 nm, 35 mW 6 mW 70 mW 500 mW 100 mW        Possible 32 2.4 3 5 40 27 ? ? o0.05 nm 35 kHz ? (narrow) 4.6 kHz ? (narrow) ? 0.6 B0.15 B0.14 0.62 0.08 1 980 nm 980 nm, 980 nm, 980 nm, 980 nm, 980 nm, 976 nm, 30 mW 300 mW 15 mW 60 mW 90 mW 70 mW Frequency locked to 13C2H2 Micro-laser (100 mm) FBG compression tuning First DFB Step-tuning with sampled FBG Permanent DFB Single-scan permanent DFB FBG Michelson mirror Chirp FBG Polarisation stable Single polarisation twisted DFB Intra-cavity pumping Single sided output DFB Efﬁcient 524 nm pumping Permanent single polarisation DFB B/Ge photosensitive ring Low-cost 650 nm pumping FBG micro-laser FBG micro-laser Accurate step-tuning with 100 GHz sampled FBG Self-injection locking Polarimetric tuning Fast l-switching Electronically tuned DFB L-band DFB All-ﬁbre acousto-optic ﬁlter Mechanically tuned DFB ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 233 Fig. 16. Basic ring cavity EDFL. Fig. 17. General schematic diagram of tuneable unidirectional ring EDFL. possible modes, is to occur. Furthermore, the long laser cavity is still sensitive to acoustical noise, and mode-hopping may occur even if single longitudinal mode operation is obtained with a narrowband ﬁlter. Active cavity stabilisation schemes [116–118] have been proposed to counteract laser mode-hopping. Table 5 recapitulates the principal results obtained with ring cavity EDFL. Ring cavities are remarkable in the sense that they can integrate a wide variety of ﬁbre-optic components (ﬁbre Bragg gratings, modulators, switch, etc.) in cascade, even if these components have high back reﬂections, through the use of optical isolators. Thus, ring cavities are well adapted for broadly tuneable EDFL applications. A particularly valid example of a broadly tuneable EDFL in a ring cavity has been presented by Bellemare et al. . In that work, record tuneability (1506–1618 nm) for an EDFL was demonstrated through the use of a low loss ring cavity with an EDF working in the deep saturation regime. The wide tuneable range is achieved as follows: in a laser the gain (G) is locked to the ring cavity losses (b); if b is small then G will be small. The large amount of feedback power (Ps ) entering the EDF due to the low cavity loss deeply saturates the gain of the EDF. It is well known that the gain of a deeply saturated EDFA is ﬂat over a wide wavelength range . Fig. 18 illustrates the computed gain against signal wavelength for different signal powers in the deep saturation regime. Fig. 19 plots the computed EDF gain against output power for different signal wavelengths and illustrates the various operational regimes of the laser’s EDF gain stage. In general, an EDFA is used as a preampliﬁer or as an in-line ampliﬁer in the small signal regime or as a booster ampliﬁer in the saturation regime. However, to ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 234 Table 5 Principal results published on ring cavity EDFL Ref. Tuning (nm) Linewidth Power (mW) Pump source Remarks           45 0 45 2.8 Possible 40 30 61 45 44 o0.1 nm 60 kHz 100 kHz 1.4 kHz 10 kHz 10 kHz o1 MHz o0.1 nm 5.5 kHz 10 kHz 0.14 0.9 1.4 1.3 10 1.8 ? 4.2 3 0.32 532 nm, 80 mW 980 nm, 45 mW 980 nm, 50 mW 1480 nm, 78 mW 980 nm, 40 mW 980 nm, 60 mW 980 nm 1480 nm, 32 mW 1480 nm, 35 mW 980 nm, 10 mW         43 45 33 0 7 7 39 Possible 295 kHz 23 kHz o0.1 nm o2.5 MHz 200 MHz o0.3 nm o0.1 nm o16 kHz 1.9 4.5 0.6 15 0.02 1.9 2.7 6.2 1480 nm, 26 mW 980 nm, 44 mW 1480 nm, 30 mW 1480 nm, 70 mW 980 nm, 27 mW 980 nm, 21 mW 980 nm, 125 mW 1064 nm, 175 mW       18 4.5 38 Possible 5 30 o0.1 nm 1.2 kHz o0.05 nm 2 kHz 52 kHz o0.2 nm 23 ? 60 0.25 5 1.3 980 nm, 80 mW 1480 nm 980 nm, 250 mW ? 1480 nm, 80 mW 980 nm, 15 mW     Possible 39 39 10 10 kHz 5 kHz o0.1 nm 0.06 nm 1 0.1 10 0.2 980 nm? 980 nm, 10 mW ? 1060 nm  0 possible ? 170 kHz 1.6 1480 nm, 70 mW  47 6 GHz 3 980 nm, 100 mW  12.1 8 kHz 0.7 980 nm, 100 mW   50 112 o126 kHz 1.2 GHz 7 6 972 nm, 85 mW 980 nm, 180 mW     80 21 40 10 300 MHz 0.018 nm o4 kHz o0.05 nm 2 2 3 0.8 1480 nm, 84 mW 1480 nm, 24 mW 980 nm, 150 mW 980 nm, 200 mW   41 50 o0.1 nm o0.1 nm 6.3 0.5 980 nm, 60 mW 980 nm, 125 mW Tuneable etalon ﬁlter Unidirectional operation Liquid-crystal ﬁlter All PM ﬁbre cavity Etched ﬁbre grating Acousto-optic tuneable ﬁlter Tandem ﬁbre F–P ﬁlters Fibre Fabry–Perot ﬁlter Tandem ﬁbre F–P ﬁlters GRIN lens Fabry–Perot ﬁlter Bulk diffraction grating Tuneable etalon ﬁlter Polarimetric tuning Unidirectional w/o isolator Polarisation ﬁlter S-ring cavity Reﬂective Mach–Zehnder Un-pumped EDF tracking ﬁlter FBG with circulator FBG Fabry–Perot etalon S-ring cavity DFB-type FBG ﬁlter Overlay ﬁlter Non-reciprocal cavity with polarisation splitter Transmission FBG Twisted EDF Liquid crystal ﬁlter TE-TM converter and crossed polarisers Chirped FGB etalon and overlay ﬁlter 100 GHz solid Fabry–Perot micro etalon Precise 100 GHz step-tuning with sampled FBG 50 GHz step-tuning Low loss cavity for ultrawide tuning range Wide tuning range FBG array Highly stretchable FBG 50 GHz step-tuning with Sagnac birefringence ﬁlter Temperature-tuned FBG FBG-enhanced efﬁciency ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 235 Fig. 18. EDFA gain as a function of signal wavelength: L ¼ 15 m (HP-980), P p ¼ 180 mW, lp ¼ 980 nm, Ps ¼ 110 : 5 (m), 52.4 (E), 28.6 (K), and 16.6 mW (’). Taken from Ref. . Fig. 19. EDF ampliﬁcation stage (L ¼ 15 m (HP-980), forward pump power of Pþ p ¼ 180 mW and lp ¼ 980 nm) gain against output power for ls ¼ 1510 (ddd), 1540 (– –), 1570 (—) and 1600 nm (– – –). Small signal, saturation and deep saturation regimes are illustrated for ls ¼ 1540 and 1570 nm. Taken from Ref. . ARTICLE IN PRESS 236 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 Fig. 20. Laser tuning curve with and without automatic power control (APC) for L ¼ 27 m, P p ¼ 180 mW, lp ¼ 980 nm. Experiment (—) and simulation (K). The APC is set for 0 dB m of output power. Taken from Ref. . realise a broadband tuneable EDFL, the EDF gain stage must be operated in the deep saturation regime where, for a small-valued gain clamped to the ring cavity loss by lasing, the output power spectral uniformity is greatly improved. Fig. 20 shows the laser-tuning curve where good spectral uniformity can be observed. 3.1.4. Coupled cavity EDFL A typical coupled cavity EDFL is the Fox–Smith resonator laser  presented in Fig. 21. The Fox-Smith resonator is a directional coupler resonator with three mirrors creating two coupled cavities. Each of the cavities operating individually will produce a set of resonances. The ﬁrst cavity is made of dielectric mirror r1 ; which has a high reﬂection coefﬁcient at the signal wavelength and is highly transparent at the pump wavelength, and the dielectric mirror r4 : The second cavity is composed of mirror r1 (common mirror) and the diffraction grating. The transmission of the complete resonator is high when both of the constituent cavities are on resonance and low when only one cavity is on resonance. If the cavity lengths are chosen to be in the ratio of two integers which are close, but not equal in value, the frequency spacing of the high transmission peaks will be made to be signiﬁcantly further apart than the spacing of either of the transmission peaks of the cavities acting individually. This is an example of the Vernier effect. When a gain medium, such as erbium dopant, is incorporated in the ﬁbre, lasing will take place preferentially at the frequencies which simultaneously satisfy the resonance conditions of both cavities. The lasing modes are then sufﬁciently apart that one of them may be selected with the use of the diffraction grating . ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 237 Fig. 21. Schematic diagram of a ﬁbre Fox-Smith resonator laser. Taken from Ref. . Table 6 Principal results published on coupled cavity EDFL Ref. Tuning (nm) Linewidth Power (mW) Pump source Remarks   Possible 60 o8.5 MHz o1.6 MHz 0.08 ? 514 nm, 400 mW 529 nm      Possible Possible 9.6 45 Possible 40 kHz 1.5 MHz 26 kHz 5 kHz 37 kHz 0.4 o1 0.05 21 1.6 980 nm, 200 mW 980 nm, 25 mW 980 nm, 45 mW 980 nm, 90 mW 1480 nm, 70 mW    8 Possible 38.4 68 kHz 240 kHz Singlemode 5.2 5 1 1480 nm, 80 mW 1480 nm, 80 mW ?  Possible 200 kHz 166  11.2 200 kHz 62 980 nm, 160 mW 1480 nm, 158 mW 980 nm, 160 mW Fox–Smith resonator Two short EDF coupled cavities Multiple FBG DFB laser injection Micro-laser All-ﬁbre sub-cavity FBG and broadband mirror All-ﬁbre sub-cavity All-ﬁbre sub-cavity Fabry–Perot semiconductor optical ampliﬁer ﬁlter DFB in a DBR cavity DFB in a DBR cavity, enhanced temperature tuning Also, ﬁbre-optic sub-cavities [157–160] with cavity lengths controlled by piezoelectric elements allow laser frequency stabilisation and mode-hop free operation of ring lasers. Table 6 gives an overview of the body of work on coupled cavity EDFL. ARTICLE IN PRESS 238 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 3.2. Multifrequency lasers 3.2.1. Background Multifrequency lasers are of great interest as head-end transmitters in wavelength routed local area networks (LAN) . Multifrequency lasers also have a great potential in the ﬁbre-optic test and measurement of WDM components. The requirements for such optical sources are: a high number of channels over large wavelength span, moderate output powers (of the order of 100 mW per channel) with good OSNR and spectral ﬂatness, single longitudinal mode operation of each laser line, tuneability and accurate positioning on the ITU frequency grid . Reaching all these requirements simultaneously is a difﬁcult task, and many different approaches using semiconductor or erbium-doped ﬁbre technology have been proposed and experimented in order to obtain multifrequency laser oscillation. Gain-guided Fabry–Perot semiconductor lasers naturally offer a multiline spectrum. However, the output spectrum is highly non-uniform. Furthermore, the channel spacing is determined by the length of the cavities when the wafer is sliced into chips. Thus, it is very difﬁcult to obtain the proper channel spacing since a precision of the order of 0.1 mm is required, even considering temperature/current tuning of the laser. A more direct approach to obtaining a uniform output spectrum is to combine in the same ﬁbre the light from a laser array. Young et al.  have realised an array of sixteen DBR lasers, with each laser being followed by an electro-absorption modulator. A 16 1 multiplexer combines all the signals into a semiconductor optical booster ampliﬁer stage before the output. However, due to its complexity, this structure is fabricated with very low yields and is costly. Furthermore, channel spacing regularity becomes very difﬁcult to obtain when the channel count becomes high. Laser oscillation at speciﬁc frequencies can be forced by an external ﬁlter in order to obtain channel spacing with good regularity. In 1991, Farries and his colleagues  demonstrated a hybrid cavity laser composed of a diffraction grating in a Czerny–Turner conﬁguration acting as a transmission ﬁlter between a Fabry–Perot laser array and a ﬁbre loop mirror. The laser array had an anti-reﬂection (AR) coated facet and a high-reﬂectivity (HR) facet. They obtained ﬁve 2.5 nm-spaced wavelengths, tuneable over 80 nm. Poguntke et al.  showed in 1993 that the multistripe array grating integrated cavity (MAGIC) laser could be used as a multifrequency laser with nine lines spaced by about 2 nm. This laser integrates an active array with a curved grating acting as a reﬂective multi/demultiplexer by diffracting and focusing light into the individual stripes of the laser array. A year later, Zirngibl et al.  integrated an arrayed waveguide grating (AWG), acting as a transmissive multi/demultiplexer, with semiconductor optical ampliﬁers (SOA) to generate twelve wavelengths spaced by 3.2 nm. In 1996, Zirngibl et al.  reﬁned their laser to emit 18 lines spaced 103 GHz apart. Fibre lasers also offer great possibilities as multifrequency sources. Their ease of fabrication has yielded many ingenious designs. The main challenge in producing a multiline output with an EDFL is the fact that the erbium ion saturates mostly ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 239 homogeneously at room temperature, preventing stable multifrequency operation. This phenomenon was described in Section 2. As we will see below, one straightforward way around this problem is to use a single gain medium per wavelength. It is also possible to generate a multifrequency spectrum from a single gain medium using some clever schemes, as will be explained. Finally, the brute force approach to solving this problem is to cool the erbium-doped ﬁbre to liquid nitrogen temperature, thus rendering the gain medium inhomogeneous and allowing multifrequency operation to take place. 3.2.2. Room temperature operation of EDFL 188.8.131.52. Multiple gain medium. In a manner similar to semiconductor laser arrays, it is possible to create multifrequency EDFLs that use a single gain medium per wavelength. In 1994, Takahashi et al.  demonstrated a multifrequency ring EDFL oscillating simultaneously over four wavelengths spaced 1.6 nm apart by using an 8 8 AWG and four EDFAs. Later, Miyazaki and his co-worker  showed a ring EDFL that lases on 15 lines separated by 1.6 nm. Again, the laser consists of 15 EDFAs placed between two 16 16 AWGs. One interesting scheme using a single pump laser was proposed by Kim et al.  and is depicted in Fig. 22. The light from a 1480 nm pump laser is evenly distributed to N ﬁbre segments by a 1 N broadband coupler. Each segment is composed of a piece of EDF followed by an optical isolator, a tuneable optical ﬁlter and variable attenuator. By adjusting each attenuator it is possible to establish multifrequency oscillation is this ring cavity. Independent wavelength tuning of each laser line is the main feature of this structure. 184.108.40.206. Single gain medium. The very ﬁrst attempts [178,179] at room temperature operation of single gain stage multifrequency EDFLs showed, notwithstanding their inefﬁciency, the great potential of these sources. Later, Hubner . et al.  proved that a multifrequency EDFL could be obtained through writing a series of DFB ﬁbre Bragg gratings in a single erbium-doped ﬁbre. Their laser produced ﬁve lines over a 4.2 nm range. The use of speciality doped ﬁbre has also led to very elegant designs. A twincore EDF was used by Graydon et al.  as an inhomogeneous gain medium in a multifrequency ring EDFL. In that ﬁbre, wavelength-dependent periodic coupling between the two cores partially decouples the available gain for each wavelength, since they interact with a different subset of erbium ions. Poustie et al.  used a multimode ﬁbre to create a frequency periodic ﬁlter based on spatial mode beating and showed multi-wavelength operation over four lines spaced by 2.1 nm. In 1992, Abraham et al.  conceived a multifrequency hybrid laser composed of a 980 nm pump laser diode with antireﬂection coating coupled to an EDF with a ﬁbre mirror. That laser produced an output spectrum with six lines spaced by 0.44 nm. In 1997, Zhao et al.  demonstrated that the control of optical feedback in a modiﬁed S-type cavity allowed stable multifrequency operation. Finally, a very interesting scheme to realise room temperature operation of a multifrequency EDFL was demonstrated by Sasamori et al. . They used an acousto-optic modulator to prevent the laser from reaching steady-state operation ARTICLE IN PRESS 240 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 Fig. 22. Schematic diagram of a multifrequency EDFL using a single pump. Taken from Ref. . (see Fig. 23). Initially, the authors believed that the repeated frequency shifting of the circulating ASE by the acousto-optic modulator prevented laser oscillation and yielded an incoherent source. Recently, it was shown that this source is in fact a laser and its potential as a frequency reference was demonstrated [186–188]. 3.2.3. Liquid nitrogen cooled multifrequency EDFL The most obvious way to force multifrequency operation in a single gain medium EDFL is to cool the EDF by immersion in a bath of liquid nitrogen (77 K). At these temperatures the erbium ions become inhomogeneous, and multifrequency operation is much easier. It must be noted that this complex and unreliable approach is not recommended for ﬁeld applications. Nonetheless, many potent experimental results have been published using this method, and it is worthwhile to review them. In 1996, Chow et al.  published results concerning a multifrequency ring EDFL using two different types of frequency periodic ﬁlters. They obtained eleven laser peaks spaced by 0.65 nm using a Fabry–Perot ﬁlter based on chirped ﬁbre Bragg gratings , and ﬁve laser peaks spaced by 1.8 nm with a sampled ﬁbre Bragg grating (see Fig. 24). ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 241 Fig. 23. Schematic diagram of the multifrequency EDFL based on an acousto-optic modulator, (a) ring cavity, (b) multiwavelength optical bandpass ﬁlter, (c) high-power EDFA. Taken from Ref. . Fig. 24. Schematic diagram of a nitrogen-cooled multifrequency EDFL. Taken from Ref. . That same year, Yamashita et al.  proposed a single-polarisation linear cavity multifrequency EDFL. This laser does not use polarisation-maintaining ﬁbre and operates in a travelling-wave mode, thus preventing spatial hole burning, since cavity feedback is provided by Faraday mirrors. A Fabry–Perot etalon is used as the frequency periodic ﬁlter. A polariser and a Faraday rotator are placed on each side of the etalon to prevent parasitic reﬂections. With this setup, the authors obtained simultaneous oscillation over 17 wavelength spaced by 0.8 nm. Simultaneous lasing of up to 24 wavelengths has been demonstrated by Park et al.  using controlled polarisation evolution in a ring cavity and liquid nitrogen cooling to enhance ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 242 Table 7 Principal results published on multi-frequency EDFL Ref. Range (nm) (# of lines) Power/ch. (dB m) (ﬂatness (dB)) Pump Cooled/ uncooled Remarks  21 (5) 22 (3.3) Uncooled GRIN-lens F–P ﬁlter   29 (6) 2.2 (6) 11 (5.7) ? (1) Uncooled Uncooled  4.5 (3) ? (1.7) Grating WDM ﬁlter Semiconductor intracavity F–P etalon serial FBG linear cavity  6.4 (4) 12.7 (4.0) 980 mm, o5 mW 980 nm 980 nm, 31 mW 980 nm, 30 mW 1480 nm  5.3 (4) 5.6 (5.2)  5.3 (8) 12 (5.9)  6.5 (11) 14.1 (10.4)  13 (17) 21.7 (16.4)  17.6 (17) 16 (4.2)  14.8 (29) 23.3 (13.3)   22.4 (15) 4.2 (5) +5 (0.7) 14.8 (7.9)  18.0 (3) 13.6 (0.9)  12.6 (4) +13 (0.2)  7.0 (4) 7 (o1)  6.0 (16) 12 (4)  12.4 (9) 2.7 (9.5)  9.2 (11) 15.2 (1.6)  10.4 (14) 3.7 (14.6)  0.7 (3) 9.5 (o2)  11.2 (8) 23.5 (5.6)  11.8 (16) ? (5.4)  27.2 (6) 11.7 (3.3)  28.0 (34) 18.8 (10) 1480 nm, 4 45 mW 980 nm, 70 mW 980 nm, 300 mW 1480 nm, 30 mW 980 nm, 75 mW 980 nm, 38 mW 15 EDFAs 1480 nm, 60 mW 980 nm, 95 mW 1480 nm, 307 mW 980 nm 980 nm, 400 mW 976 nm, 50 mW 980 nm, 60 mW 980 nm, 150 mW 980 nm, 70 mW 980 nm, 240 mW 980 nm, 80 mW 980 nm, 90 mW 980 nm, 94 mW Uncooled Uncooled Uncooled MM ﬁbre spatial mode beating ﬁlter Single AWG WDM ﬁlter Er-doped twincore ﬁbre Cooled Chirped grating F–P Cooled Cooled F–P etalon in linear cavity Lyot ﬁlter Cooled mm-long EDFL Uncooled Uncooled AWG WDM pair DFB array Uncooled Cooled Self-generated optical feedback Frequency shifted feedback serial FBG linear cavity with MOPA DFB array with pump redundancy dual-pass MachZehnder comb ﬁlter MQW waveguide comb ﬁlter Frequency shifted feedback FBG Sagnac loop Cooled overlap-written FBG Cooled Hi-bi ﬁbre loop Uncooled MM ﬁbre spatial mode beating ﬁlter Frequency shifted feedback Uncooled Uncooled Uncooled Uncooled Cooled Uncooled Uncooled Uncooled ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 243 Table 7 (continued) Ref. Range (nm) (# of lines) Power/ch. (dB m) (ﬂatness (dB)) Pump Cooled/ uncooled Remarks  18.1 (4) 5 (0.5) Uncooled FBG tree ﬁlter  11.7 (9) 16.0 (6) 980 m, 90 mW 1480 nm Cooled  6.4 (6) 20.8 (11) Uncooled Spacing-tuneable comb ﬁlter based on PMF L-band operation  11.2 (15) 1.8 (13) Cooled Single-mode ring EDFL  30 (4) 2.8 (o1) Uncooled L-band operation  2.1 (3) 7.7 (1.5) Uncooled Elliptical core EDF  12.2 (3) 3.3 (1.5) Uncooled Serial FBG linear cavity 1480 nm, 100 mW 980 nm, 120 mW 980 nm, 90 mW 980 nm, 72 mW 1480 nm, 100 mW spectral hole burning, polarisation hole burning, and polarisation selectivity. A polariser and a polarisation controller were placed before a piece of polarisation maintaining ﬁbre to form a Lyot ﬁlter with a free spectral range of 1.1 nm. Finally, Yamashita et al.  realised a multiwavelength Er:Yb Fabry–Perot micro-laser with 29 0.4 nm-spaced lines. Table 7 presents the principal results published on multi-frequency EDFLs. 3.3. Superfluorescent fibre source 3.3.1. Background In the last decade, erbium-doped superﬂuorescent ﬁbre sources (SFS) have attracted a lot of consideration in various areas of ﬁbre-optic technology. SFS have found application as broadband incoherent sources in optical low coherence reﬂectrometry (OLCR) [211–213], ﬁbre-optic components test and measurement , and in optical sensors . In telecommunication, SFS have been used as transmitters in spectrum-sliced WDM systems [216–218] and as pump sources in Raman ﬁbre ampliﬁers . They are also the preferred source for navigational grade ﬁbre-optic gyroscopes (FOG) . In all these applications the source requirements are the following: high output power, broadband spectrum or wavelength tuning, spectral uniformity and stability. Simultaneous optimisation of all these parameters in a single SFS is quite challenging, and design trade-offs are often made to reach the required application functionality with a practical source implementation. Over the years, many different SFS designs have been proposed and Fig. 25 illustrates the most notable. Single-pass SFS, shown in Fig. 25a, is the simplest conﬁguration: the waveguided spontaneous emission generated along the erbiumdoped ﬁbre in both directions travels only once through the EDF where it is ampliﬁed, thus becoming ampliﬁed spontaneous emission (ASE) . The ASE ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 244 Laser Diode Pump (a) SPB EDF WDM Laser Diode Pump (b) DPB SPF EDF WDM Laser Diode Pump EDF WDM (c) Laser Diode Pump EDF WDM DPF Laser Diode Pump EDF WDM Output Fig. 25. Generic SFS conﬁgurations: (a) single-pass (b) double-pass (c) two-stage. travelling along with the pump power is called forward ASE and the ASE propagating against the pump power is called backward ASE. While the single-pass design is simple and robust to parasitic laser oscillation, it is not as power efﬁcient as the double-pass conﬁguration of Fig. 25b. In the double-pass conﬁguration, a reﬂector is used to recover the ASE from one end of the EDF and redirect it to the main output . It must be noted here that the use of an optical isolator at the output is always recommended if the source has to have a stable output. This is even more important in the double-pass conﬁguration due to the risk of laser oscillation caused by a parasitic reﬂector outside of the source . Finally, an effective design for high output power generation is the double-stage conﬁguration  presented in Fig. 25c. By splitting the ampliﬁcation in two isolated stages, the ASE seed stage and the power ampliﬁer stage, the risk of lasing caused by parasitic reﬂection, including ampliﬁed Rayleigh backscattering, is reduced and high output powers can be reached. Before we present some experimental and simulation results concerning superﬂuorescent ﬁbre sources, it is worthwhile to review some deﬁnitions of important ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 245 performance parameters. First of all, we need to deﬁne the optical bandwidth of the source. In the following we will use the deﬁnition of Morkel et al. : R N 2 pðnÞ dn ; Dn ¼ R 0N 2 0 p ðnÞ dn where pðnÞ is the power spectral density in W/Hz. This optical bandwidth deﬁnition is more representative than the common full width at half maximum (FWHM) line width, Dn3 dB ; deﬁnition used for lasers. In fact, it has been demonstrated  that the signal-to-noise ratio (SNR) of an SFS is given by /IS ; SNR ¼ 2eB þ /ISB=Dn where /IS is the time average photocurrent and B is the electrical bandwidth of the photodetection scheme. In the case of a sufﬁciently large detected signal, excess photon noise due to the beating of the Fourier components within the broadband spectrum is the dominant source of noise compared to shot noise. The signal is then said to be excess-noise limited and has SNR ¼ Dn=B: Furthermore, it has been shown that for FOG the minimum measurable rotation rate is given by  c2 pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ Omin ¼ B=Dn; 2pLDn where c is the velocity of light and L and D are the ﬁbre-loop length and diameter, respectively. Also of interest is the mean optical frequency of the source, deﬁned as the power weighted average of signal frequencies : RN pðnÞn dn n% ¼ R0N : 0 pðnÞ dn In a FOG, the rotation rate O is obtained at the output of a Sagnac interferometer as c2 DF ; 2pLDn% where DF is the phase difference between the two counter-propagating beams. Since the scale factor, c2 =2pLDn% ; is inversely proportional to the source mean optical frequency, it is crucial that the SFS have a very stable mean optical frequency if it is going to be used in a navigational-grade gyroscope where long-term stabilities of about 1 ppm are required. The mean optical frequency of a SFS can be inﬂuenced by ﬂuctuations in EDF temperature, pump power, pump wavelength, pump or output signal states of polarisation, and optical feedback from the FOG . The use of an isolator will prevent optical feedback effects, and the polarisation effects can be substantially reduced by adding ﬁbre Lyot depolarisers to the SFS . The other ﬂuctuation effects are related to temperature and can be expressed by the following relation : dn% qn% qn% qPpump qn% qnpump ¼ þ þ : dT qT qPpump qnpump qT qT O¼ ARTICLE IN PRESS 246 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 The ﬁrst term is the intrinsic temperature dependence of the active medium, which is the result of variations in the Boltzmann distributed occupation of the laser manifolds. The second and third terms are due to the temperature dependence of output power and emission frequency in pump laser diodes. With proper SFS design [223,226,229,230] it is possible to reduce these dependencies to levels where standard temperature control (0.11C) of laser diode and EDF will yield very stable sources. 3.3.2. Typical SFS performance curves In this section, we will introduce important SFS concepts while depicting a few typical SFS performance curves. Fig. 26 presents the simulated SFS output power as a function of EDF length for different source conﬁgurations. From that graph it is important to observe that the single-pass backward design does not have an optimal length, but asymptotically converges towards its optimum output power value for L-N; contrary to other conﬁgurations. Therefore, it is advantageous, in that design, to use a ‘‘long’’ piece of EDF since the un-pumped ﬁbre far end will be absorptive and will reduce detrimental back-reﬂections that can lead to lasing. Additionally, the ﬁbre far end can be terminated with an angle-cleave and a bend loss to further prevent feedback. For L ¼ 25 m of EDF pumped by 50 mW at 980 nm, a backward output power of 17.56 mW is generated at a mean wavelength of 1540.47 nm, and this translates into a 55.2% conversion efﬁciency of pump photons into backward-signal photons. The conversion of over 50% of pump photons to backward-signal photons in a single-pass SFS is only possible in a three-level medium like erbium. For this conﬁguration, a four-level medium, like neodymium, produces equal amounts of forward and backward ASE . Also, it is clear that double-pass designs are more efﬁcient than single-pass ones and, in the case of the double-pass backward (DPB), even more than the combined 25 Output power [mW] 20 15 10 5 0 0 5 10 15 20 25 EDF length [m] Fig. 26. SFS output power as a function of EDF length for different source conﬁgurations: single-pass forward (d), single-pass backward (—), double-pass forward ( ) and double-pass backward (— —). Pp ¼ 50 mW, lp ¼ 980 nm, Fibre HG-980. ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 247 outputs of a single-pass device. This is caused by the ASE second pass in the SFS efﬁciently stimulating further collinear emission from the erbium ions rather than letting the ions slowly spontaneously decay isotropically in the ﬁbre. The fact that backward-pumped designs are more efﬁcient than the forward-pumped designs is because the output power is directed towards the EDF end that is strongly pumped, and thus more capable of further amplifying the already strong optical signals. As the EDF length of a DPB SFS is increased beyond the optimal length, the output power converges towards the single-pass backward (SPB) limit. This is simply caused by the EDF far end being highly absorptive and effectively nulling the broadband mirror reﬂectivity. Finally, for the single-pass forward (SPF) and doublepass forward (DPF) designs, the output power converges to zero as the EDF length is increased beyond the optimal length, because all the signal power is reabsorbed in the output end of the under-pumped EDF. The mean wavelength evolution with EDF length for different SFS designs is plotted in Fig. 27. The form of these plots is explained by consideration of the interplay between the gain and loss spectra of Fig. 3. For short pieces of EDF of about 5 m for single-pass designs and 3 m for double-pass designs, the entire ﬁbre is highly pumped and the SFS output spectrum resembles the gain spectrum of Fig. 3. The mean wavelength is near 1533–1534 nm and the optical bandwidth is about 13–15 nm (see Fig. 28). As the ﬁbre length is increased, signal saturation and pump depletion produce regions where the EDF is poorly inverted. Such conditions favour emission at longer wavelengths where the gain coefﬁcient exceeds the absorption coefﬁcient . Fig. 28 shows the relation between optical bandwidth and EDF length. For example, in the case of the SPF structure, as the EDF length is augmented from the region of minimum optical bandwidth (L ¼ 6 m) we observe a shift of the peak emission wavelength from 1.53 mm to 1.56 mm. When these two peaks are well 1.58 Mean wavelength [µm] 1.57 1.56 1.55 1.54 1.53 0 5 10 15 20 25 EDF length [m] Fig. 27. SFS mean wavelength as a function of EDF length for different source conﬁgurations: single-pass forward (d), single-pass backward (—), double-pass forward ( ) and double-pass backward (— —). Pp ¼ 50 mW, lp ¼ 980 nm, Fibre HG-980. ARTICLE IN PRESS 248 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 60 Optical bandwidth [nm] 50 40 30 20 10 0 0 5 10 15 20 25 EDF length [m] Fig. 28. SFS optical bandwidth as a function of EDF length for different source conﬁgurations: singlepass forward (d), single-pass backward (—), double-pass forward ( ) and double-pass backward (— —). Pp ¼ 50 mW, lp ¼ 980 nm, Fibre HG-980. 0 Output power [dBm/nm] L= 6 m -10 L= 14 m -20 -30 L= 20 m -40 -50 1520 1530 1540 1550 1560 1570 1580 Wavelength [nm] Fig. 29. Broadband emission in the C-band from a SPF SFS. L ¼ 6; 14 and 20 m, lp ¼ 980 nm, Pp ¼ 50 mW, Fibre HG-980. balanced (L ¼ 14 m), we obtain broadband emission in the C-band as illustrated in the output spectrum of Fig. 29. If we go beyond that EDF length (L ¼ 20 m) then the spectrum is again single-peaked but at 1.56 mm. Furthermore, by using 1.48 mm-band pumping, it is possible to generate high power broadband emission in the C and L-bands with the simple DPB structure. With 200 mW of pump power, a power spectral density of at least 10 dB m/nm can be generated from 1529.4 to 1617.0 nm. The optical bandwidth of this source is 57.1 nm with a mean wavelength of 1579.6 nm and 141.6 mW of total output power (Fig. 30). As previously mentioned, navigational-grade ﬁbre-optic gyroscopes require broadband incoherent sources with temperature-insensitive mean wavelength ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 249 10 Output power [dBm/nm] 5 0 -5 -10 -15 -20 -25 -30 1520 1540 1560 1580 Wavelength [nm] 1600 1620 Fig. 30. Broadband emission from an optimised DPB SFS. L ¼ 64 m, lp ¼ 1480 nm, Pp ¼ 200 mW, R ¼ 100%; Fibre HG-980. Table 8 Required length of EDF for temperature independent SFS designs 980 nm 1480 nm SPF SPB DPF DPB 11.7 m (This work) 16.5 m (This work) 15.8 m (lp =990 nm)  —  7.8 m  10.3 m  7.5 m  9.5 m (This work) operation. In the following, we will discuss the stability of SFS against EDF temperature and temperature-induced pump source ﬂuctuations. Firstly, it is possible to design a SFS in order to obtain to eliminate the intrinsic temperature dependence of the gain medium around room-temperature, that is dl=dTjT¼251C ¼ 0: Table 8 summarises such designs for various SFS conﬁgurations using HG-980 ﬁbre and Pp ¼ 75 mW. References to the ﬁrst demonstrations of temperature-independent operation are also given. Fig. 31 displays typical results concerning the intrinsic temperature dependence of various SFS designs. For example, the SPF conﬁguration shows a U-shaped temperature dependence that can be adjusted to yield zero ﬁrst order temperature dependence at room temperature. An almost temperature independent behaviour can be obtained through a DPB design, contrary to the SPB where temperature independent operation could not be demonstrated, and a typical 5.6 ppm/1C dependence is observed when lp ¼ 980 nm. Even though it is shown in Table 8 that it is possible to obtain temperature independent operation for the SPB design, it must be mentioned that the pump wavelengths used in these experiments were 962 and 1475 nm. In our case, if lp ¼ 990 nm and L ¼ 15:8 m, then the SPB will be temperature independent but will suffer more from pump wavelength instabilities, as will be shown in Fig. 33. Furthermore the highly doped EDF that was used in Refs. [223,226] may have different emission and absorption coefﬁcients which may ARTICLE IN PRESS 250 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 1546 (b) Mean wavelength [nm] 1545 1544 1543 1542 1541 (a) 1540 (c) 1539 -20 0 20 40 60 80 100 Temperature [°C] Fig. 31. SFS intrinsic thermal behaviour for different conﬁgurations: (a) uncompensated SPB (L ¼ 20 m, lp ¼ 980 nm), (b) compensated SPF (L ¼ 11:7 m, lp ¼ 980 nm) and (c) compensated DPB (L ¼ 9:5 m, lp ¼ 1480 nm). Fibre HG-980, Pp ¼ 75 mW. Table 9 Required length of EDF for pump power independent SFS designs 980 nm 1480 nm SPF SPB DPF DPB — — 20.0 m  36.5 m  — — 13.0 m  35.0 m (This work) be more appropriate for temperature insensitive operation than HG-980, where pump detuning in the 1.48 mm band could not yield a temperature independent design. Still, we have demonstrated, for the ﬁrst time to our knowledge, the possibility of temperature-independent operation in the SPF and 1.48 mm-pumped DPB conﬁgurations. The second parameter that must be controlled in a high stability SFS is the pump power dependence. Table 9 shows designs for various SFS conﬁgurations that meet the condition dl=dPp jPp ¼75 mW ¼ 0: Again references to the ﬁrst demonstrations of pump power-independent operation are given. Fig. 32 also illustrates all the designs cases of Table 9. Finally, the last parameter that must be controlled is the pump wavelength dependence. Figs. 33 and 34 plot the mean wavelength dependence on pump wavelength for the 0.98 and 1.48 mm bands respectively. While the 0.98 mm band has a V-shaped dependence and the 1.48 mm band is more U-shaped, both have local minima, at 979 and 1483 nm respectively, that can be exploited to reduce the ﬁrstorder dependence of mean wavelength on pump wavelength. Still, the 0.98 mm band is particularly sensitive to pump wavelength instabilities, with slopes of about 70.3 nm/nm. Considering that the pump wavelength has a temperature coefﬁcient of about 400 ppm/1C, this yields a temperature-related mean wavelength dependence of 76 ppm/1C, thus requiring 70.011C control the laser diode pump ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 251 1.57 Mean wavelength [µm] (d) 1.56 (b) (c) 1.55 1.54 1.53 0 (a) 20 40 60 80 100 Pump power [mW] Fig. 32. SFS mean wavelength as a function of pump power for different conﬁgurations: (a) SPB (L ¼ 20 m, lp ¼ 980 nm), (b) SPB (L ¼ 36:5 m, lp ¼ 1480 nm), (c) DPB (L ¼ 13 m, lp ¼ 980 nm) and (d) DPB (L ¼ 35 m, lp ¼ 1480 nm). Fibre HG-980. 1547 Mean wavelemgth [nm] 1546 1545 1544 1543 1542 1541 1540 960 970 980 990 Pump wavelength [nm] 1000 Fig. 33. Mean wavelength stability against pump wavelength in the 0.98 mm band for a SFS in the SPB conﬁguration. Pp ¼ 75 mW, L ¼ 20 m, Fibre HG-980. Me]an wavelemgth [nm] 1560 1559 1558 1557 1556 1555 1450 1460 1470 1480 1490 Pump wavelength [nm] 1500 Fig. 34. Mean wavelength stability against pump wavelength in the 1.48 mm band for a SFS in the SPB conﬁguration. Pp ¼ 75 mW, L ¼ 36:5 m, Fibre HG-980. ARTICLE IN PRESS 252 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 temperature. To insure highly stable long-term operation of a SFS for navigationgrade ﬁbre-optic gyroscopes, a distributed feedback (DFB) structure may be warranted, in the 0.98 mm band, to maintain single-mode operation at the desired wavelength . With slopes one order of magnitude smaller, the 1.48 mm band is less temperature dependent and requires only 70.11C temperature control. 3.3.3. Principal work published on SFS In the following, examples of various SFS conﬁgurations will be presented in order to illustrate the many possible designs and applications of such sources. Fig. 35 presents the high power SFS developed by Gray et al. . This triplestage design uses a single 6 W Nd:YLF laser operating at 1054 nm to backward pump each stage. Wavelength division multiplexer (WDM) bypasses are used in between the isolated stages to send the pump power from one Er/Yb ﬁbre to the other. A total of 1.03 W of output power in a 4 nm bandwidth is obtained from this source. By adding a FBG ﬁlter in between the seed source and the pre-ampliﬁer, a 1.3 W narrowband (0.5 nm) signal at 1535 nm can be obtained with 6.8 W of pump power. Another high power SFS is shown in Fig. 36. This SFS was developed by Masuda et al.  as a Raman pump unit for a 1.65 mm band discrete Raman ﬁbre ampliﬁer (RFA). Again the source uses a triple-stage design, but it also includes angletuneable dielectric-multilayer optical bandpass ﬁlters (BPF) for tuning the narrowband incoherent output of the SFS. Output powers of about 1.06 W are obtained from this SFS over the 1540–1563 nm tuning range. SFS spectral linewidths varied from 1.4 nm (1540 nm) to 0.8 nm (1563 nm) due to the BPF angle-dependent linewidths. With that SFS, the RFA had a peak gain of 33 dB at 1652 nm and it offered, over the 1640–1670 nm range, a gain better than 20 dB and a noise ﬁgure under 6.2 dB. Fig. 35. High power SFS using a triple-stage conﬁguration. Taken from Ref. . ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 253 Fig. 36. Wavelength-tuneable high power SFS for Raman ﬁbre ampliﬁer applications. Taken from Ref. . Fig. 37. Spectrally ﬂattened broadband SFS. Taken from Ref. . In 2000, Espindola et al.  demonstrated a spectrally ﬂattened broadband SFS having a 83 nm bandwidth. Fig. 37 shows the setup that was used in their experiment. It consisted of two stages that shared a single 1480 nm pump, the DPF ﬁrst stage generates L-band ASE that is injected in the SPB second stage where it is ampliﬁed. The second stage also generates the C-band ASE much like a standard SPB source. For optimal spectrum ﬂatness, a long period grating-based wavelength-dependent loss ﬁlter is placed inside the second stage. Overall, the source spectrum has only a 1 dB ripple over 80 nm, and a 20 mW output power for 135 mW of pump. To conclude this section on superﬂuorescent ﬁbre sources, Table 10 provides an overview of the most important published accounts of experimental work on SFS. ARTICLE IN PRESS 254 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 Table 10 Principal results published on SFS (* indicates a FWHM bandwidth measurements) Ref. Bandwidth (nm) Power (mW) Pump Remarks       B5 B2 25* B2 20* 22 0.11 3 5 3.8 16.7 0.3 514 nm, 145 Mw 980 nm, 40 mW 514 nm, 460 mW 1480 nm, 77.3 mW 1475 nm, 33 mW 1480 nm 85 0.3 14 21 20 — 980 nm, 250 mW 1480 nm, 26.6 mW 1480 nm, 50 mW 980 nm 980 nm, 60 mW 980 nm, 15 mW First SFS SPF First DPF report First diode-pumped SFS First DPB report Spectrum broadened Alco-doped EDF SPB SPB First polarised SFS Four-pass conﬁguration LD&EDFA tandem Flattened with blazed FBG Double-stage conﬁg. SPB EDF length adjusted for dl=dP ¼ 0 SOA&EDFA tandem SPB LPG ﬂattened DPB Polarised SPB Double-stage conﬁg. DPB Triple-stage conﬁg. DPB conﬁg. using MM ﬁbre DPB with chirped FBG Double-pass, double pump design       >20 16* 24 10* 21* 35*    1.4 — 18.8 150 123 26          22* 36 36* 27 — 11 27 4* 36* 110 27 6 18.6 12.5 100 26 1030 770   27 83.4*  B1   39 B25–30   90* 20 — 180 980 nm, 100 mW 980 nm, 100 mW     20* 83* 30 33* 530 20 19 25 975 nm, 1550 mW 975 nm, 2000W 1480 nm, 135 mW 980 nm, 56 mW 980 nm, 100 mW 27 1 1060 27 6.7 980 nm, 440 mW 976 nm, 272 mW 980 nm, 100 mW 1480 nm, 1047 nm 965 nm, 125 mW 1480 nm 980 nm, 80 mW 1470 nm, 31.3 mW 980 nm, 1150 mW 980 nm, 82 mW 1054 nm, 6000 mW 978 nm, 1330 mW 980 nm, 107 mW 980 nm, 56 mW 1480 nm, 11 mW Power EDFA and 1480 nm, 50 mW 980 nm, 85 mW 980 nm, 60 mW Triple-stage conﬁg., tuneable SFS (1540– 1563 nm) DPB with LPG ASE reuse in unpumped EDF Er/Tm co-doped ﬁbre Double-stage conﬁg. with o1 ppm l-stability Polarised SFS DPF with LPG DPB with thin-ﬁlm ﬁlter DPF with Sagnac loop and FBG equaliser ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 255 4. Conclusion This paper has shown that erbium-doped ﬁbre devices have an array of applications in ﬁbre-optic telecommunications. Of course, the advent of the EDFA had the most inﬂuence on the design of ﬁbre-optic telecommunication systems, but the EDFL is increasingly used in ﬁbre-optic test and measurement applications either as a tuneable or broadband light source. In summary, after a brief overview of EDFL historical development in Section 1, we reviewed the basic theory of ampliﬁcation in EDF and introduced important EDFA concepts in Section 2. In Section 3, our review of erbium-doped ﬁbre laser development was divided into three different subjects. We ﬁrst discussed the tuneable laser, where it was shown that different laser cavity designs offer different advantages, either for single-frequency operation or broadband tuning. Afterwards we considered multifrequency lasers and highlighted the various design strategies such as liquid nitrogen cooling of the EDF, multiple gain media, and a single gain medium with forced inhomogeneity. Finally, superﬂuorescent ﬁbre sources have been studied in the last sub-section of Section 3. Different broadband source conﬁgurations were presented and analysed, with a focus on power efﬁciency and stable wavelength operation. For some conﬁgurations, temperature insensitive designs were also demonstrated for the ﬁrst time. Even though erbium-doped ﬁbre devices have reached some degree of technological maturity, there is still active research in that ﬁeld. In particular, externally modulated DFB-EDFLs are particularly appealing in high-speed and high-density WDM systems due to their inherent wavelength stability, power scalability, reliability and potential low cost. Also system-ready uncooled EDFAs with footprints smaller than a credit card have been commercially introduced for metropolitan area network (MAN) applications, thus demonstrating the degree of reﬁnement attainable by current technology. In the future, the experience gained with erbium-doped ﬁbre devices will allow the accelerated development of other doped-ﬁbre devices like praseodymium-doped ﬂuoride ﬁbre ampliﬁers for the 1.3 mm window or the newly introduced semiconductor cylinder ﬁbre light ampliﬁers [252,253]. References  A.L. Schawlow, C.H. Townes, Infrared and optical masers, Physical Review 112 (6) (1958) 1940–1949.  T.H. Maiman, Stimulated optical radiation in ruby, Nature 187 (1960) 493–494.  E. Snitzer, Proposed ﬁber cavities for optical masers, Journal of Applied Physics 32 (1) (1961) 36–39.  E. Snitzer, Optical maser action of Nd3+in a barium crown glass, Physical Review Letters 7 (12) (1961) 444–446.  A.M. Prokhorov, Ampliﬁcation properties of a dielectric ﬁlament, Optics and Spectroscopy 14 (1963) 38–40.  N.E. Wolff, R.J. Pressley, Optical maser action in an Eu3+-containing organic matrix, Applied Physics Letters 2 (8) (1963) 152–154. ARTICLE IN PRESS 256 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266  C.J. Koester, E. Snitzer, Ampliﬁcation in a ﬁber laser, Applied Optics 3 (10) (1964) 1182–1186.  K.C. Kao, G.A. Hockham, Dielectric-ﬁbre surface waveguides for optical frequencies, Proceedings of the Institution of Electrical Engineers 113 (7) (1966) 1151–1158.  P. Urquhart, Review of rare earth doped ﬁbre lasers and ampliﬁers, IEE ProceedingsOptoelectronics 135 (6) (1988) 385–407.  J. Stone, C.A. Burrus, Neodymium-doped silica lasers in end-pumped ﬁber geometry, Applied Physics Letters 23 (7) (1973) 388–389.  J. Stone, C.A. Burrus, Neodymium-doped ﬁber lasers: room-temperature cw operation with an injection laser pump, Applied Optics 13 (6) (1974) 1256–1258.  J.C. Fletcher, C. Elachi, G.A. Evans, C. Yeh, Fiber distributed feedback laser, United States Patent No. 3958188, 1976, 9pp.  R.J. Mears, L. Reekie, S.B. Poole, D.N. Payne, Neodymium-doped silica single-mode ﬁbre lasers, Electronics Letters 21 (17) (1985) 738–740.  P.W. France, Optical ﬁbre lasers and ampliﬁers, CRC Press, Boca Raton, 1991, 259pp.  R.J. Mears, L. Reekie, S.B. Poole, D.N. Payne, Low-threshold, tunable cw and Q-switched ﬁbre laser operating at 1.55 mm, Electronics Letters 22 (3) (1986) 159–160.  R.J. Mears, L. Reekie, I.M. Jauncey, D.N. Payne, Low-noise erbium-doped ﬁbre ampliﬁer operating at 1.54 mm, Electronics Letters 23 (19) (1987) 1026–1028.  R.P. Webb, W.J. Devlin, Traveling-wave laser ampliﬁer experiments at 1.5 mm, Electronics Letters 20 (17) (1984) 706–707.  R.H. Stolen, E.P. Ippen, Raman gain is glass optical waveguides, Applied Physics Letters 22 (1973) 276–278.  M. Horiguchi, K. Yoshino, M. Shimizu, M. Yamada, 670 nm semiconductor laser diode pumped erbium-doped ﬁbre ampliﬁers, Electronics Letters 29 (7) (1993) 593–595.  T.J. Whitley, Laser-diode pumped operation of Er3+-doped ﬁbre ampliﬁer, Electronics Letters 24 (25) (1988) 1537–1539.  M. Horiguchi, M. Shimizu, M. Yamada, K. Yoshino, H. Hanafusa, Highly efﬁcient optical ﬁbre ampliﬁer pumped by a 0.8 mm band laser diode, Electronics Letters 26 (21) (1990) 1758–1759.  M. Yamada, M. Shimizu, T. Takeshita, M. Okayasu, M. Horiguchi, S. Uehara, E. Sugita, Er3+doped ﬁber ampliﬁer pumped by 0.98 mm laser diodes, IEEE Photonics Technology Letters 1 (12) (1989) 422–424.  M. Suyama, K. Nakamura, S. Kashiwa, H. Kuwahara, 14.4-dB gain of erbium-doped ﬁber ampliﬁer pumped by 1.49 mm laser diode, OFC’89 Postdeadline Papers, paper PD6, 1989.  H. Nakamura, A. Fujisaka, H. Ogoshi, Gain and noise characteristics of erbium-doped ﬁber ampliﬁer pumped at 1530 nm, Technical Digest of OFC’96, paper WK9, 1996, pp. 158–159.  R. Larose, R!ealisation et applications d’un laser a" ﬁbre a" l’erbium monofr!equence, Ph.D. Thesis, Universit!e Laval, Sainte-Foy, Canada, 1995, 173pp.  P.A. Krug, M.G. Sceats, G.R. Atkins, S.C. Guy, S.B. Poole, Intermediate excited-state absorption in erbium-doped ﬁber strongly pumped at 980 nm, Optics Letters 16 (24) (1991) 1976–1978.  A. Bellemare, Lasers a" ﬁbre dop!ee a" l’erbium accordables et multifr!equences applicables aux t!el!ecommunications par ﬁbre optique, Ph.D.Thesis, Universit!e Laval, Sainte-Foy, Canada, 2000, 250pp.  E. Desurvire, Erbium-doped ﬁber ampliﬁers: principles and applications, Wiley, New York, 1994, 770pp.  M. Movassaghi, Characterization of erbium doped ﬁbers, Master Thesis, University of British Colombia, Vancouver, Canada, 1996, 85pp.  R.I. Laming, S.B. Poole, E.J. Tarbox, Pump excited-state absorption in erbium-doped ﬁbers, Optics Letters 13 (12) (1988) 1084–1086.  R.J. Mears, S.R. Baker, Erbium ﬁbre ampliﬁers and lasers, Optical and Quantum Electronics 24 (1992) 517–538.  H. Masuda, A. Takada, High gain two-stage ampliﬁcation with erbium-doped ﬁbre ampliﬁer, Electronics Letters 26 (10) (1990) 661–662. ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 257  H. Taub, D.L. Schilling, Principles of Communications Systems, 2nd Edition, McGraw-Hill, New York, 1986, 759pp.  M.C. Farries, P.R. Morkel, R.I. Laming, T.A. Birks, D.N. Payne, E.J. Tarbox, Operation of erbium-doped ﬁber ampliﬁers and lasers pumped with frequency-doubled Nd: YAG lasers, Journal of Lightwave Technology 7 (10) (1989) 1473–1477.  A. Bellemare, Ch. Riviere, F. Babin, G. He, G.W. Schinn, Widely tunable erbium-doped ﬁber ring laser for ﬁber-optic test and measurement in the C and L bands, Technical Digest Optical Ampliﬁers and their Applications (OAA’2000), 2000, pp. 123–125.  A. Bellemare, Ch. Riviere, M. Kar!asek, F. Babin, G. He, G.W. Schinn, Wideband tunable erbiumdoped ﬁber ring laser: simulation and experiment, Technical Digest of ECOC’2000, paper P1.1, 2000.  S.L. Chin, Fundamentals of Laser Optoelectronics, World Scientiﬁc, Singapore, 1989, 362pp.  A.E. Siegman, Lasers, University Science Books, Mill Valley, 1986, 1283pp.  E. Desurvire, J.L. Zyskind, J.R. Simpson, Spectral gain hole-burning at 1.53 mm in erbium-doped ﬁber ampliﬁers, IEEE Photonics Technology Letters 2 (4) (1990) 246–248.  S.H. Yun, D.J. Richardson, B.Y. Kim, Interrogation of ﬁber grating sensor arrays with a wavelength-swept ﬁber laser, Optics Letters 23 (11) (1998) 843–845.  S.K. Kim, G. Stewart, W. Johnstone, B. Culshaw, Mode-hop-free single-longitudinal-mode erbiumdoped ﬁbre laser with a ﬁber ring resonator, Applied Optics 38 (24) (1999) 5154–5157.  J.L. Zyskind, J.W. Sulhoff, P.D. Magill, K.C. Reichmann, V. Mizrahi, D.J. DiGiovanni, Transmission at 2.5 Gbit/s over 654 km using an erbium-doped ﬁbre grating laser source, Electronics Letters 29 (12) (1993) 1105–1106.  V. Mizrahi, D.J. DiGiovanni, R.M. Atkins, S.G. Grubb, Y.-K. Park, J.-M.P. Delavaux, Stable single-mode erbium ﬁber-grating laser for digital communication, Journal of Lightwave Technology 11 (12) (1993) 2021–2025.  J.-M.P. Delavaux, Y.K. Park, V. Mizrahi, D.J. DiGiovanni, Long-term bit error rate transmission using an erbium ﬁber grating laser transmitter at 5 and 2.5 Gb/s, Optical Fiber Technology 1 (1994) 72–75.  G. Bonfrate, F. Vaninetti, F. Negrisolo, Single-frequency MOPA Er3+DBR ﬁber laser for WDM digital telecommunication systems, IEEE Photonics Technology Letters 10 (8) (1998) 1109–1111.  C. Gaeta, W. Ng, S. Bourgholtzer, R. Stephens, Compact single frequency erbium ﬁber-laser transmitter module with asymmetric output ports, Proceedings of LEOS’98, paper FJ6, 1998, pp. 365–365.  M. Ibsen, A. Fu, H. Geiger, R.I. Laming, Fibre DFB lasers in a 4 10 Gbit/s WDM link with a single sinc-sampled ﬁbre grating dispersion compensator, Technical Digest of ECOC98, 1998, pp. 107–111.  W.H. Loh, B.N. Samson, L. Dong, G.J. Cowle, K. Hsu, High performance single frequency ﬁber grating-based erbium: ytterbium-codoped ﬁber lasers, Journal of Lightwave Technology 16 (1) (1998) 114–118. *  H.N. Poulsen, P. Varming, A. Buxens, A.T. Clausen, I. Munoz, P. Jeppesen, C.V. Poulsen, J.E. Pedersen, L. Eskildsen, 1607 nm DFB ﬁbre laser for optical communication in the L-band, Technical Digest of ECOC’99, paper MoB2.1, 1999, pp. 70–71.  S.-K. Liaw, K.-P. Ho, C. Lin, High output power erbium-doped ﬁber grating laser, Technical Digest of CLEO’97, paper CThL69, 1997, pp. 398–399.  C.-C. Lee, Y.-K. Chen, S.-K. Liaw, Single-longitudinal-mode ﬁber laser with a passive multiple-ring cavity and its application for video transmission, Optics Letters 23 (5) (1998) 358–360.  K. Iwatsuki, H. Okamura, M. Saruwatari, Optical homodyne detection with an injection-locked Erdoped-ﬁber ring laser, Optics Letters 15 (24) (1990) 1437–1439.  P. Kiiveri, S. Tammela, Spectral gain and noise measurement system for ﬁber ampliﬁers, Optical Engineering 34 (9) (1995) 2592–2595.  V. Dominic, S. MacCormack, R. Waarts, S. Sanders, S. Bicknese, R. Dohle, E. Wolak, P.S. Yeh, E. Zucker, 110 W ﬁber laser, CLEO’99 Postdeadline Papers, paper CPD11, 1999. ARTICLE IN PRESS 258 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266  M.J.F. Digonnet, Rare Earth Doped Fiber Lasers and Ampliﬁers, 2nd Edition, Marcel Dekker, New York, 2001, 777pp.  R. Vall!ee, P. Laperle, A. Chandonnet, Lasing characteristics of a thulium-doped ZBLAN ﬁber laser at 481 nm, Proceedings of ICAPT’98, SPIE 3491 (1998) 224–229.  P. Urquhart, Fibre laser resonators, in: D.R. Hall, P.E. Jackson (Eds.), The Physics and Technology of Laser Resonators, Bristol, Adam Hilger, 1989, pp. 209–219.  P. Barnsley, P. Urquhart, C. Millar, M. Brierley, Fiber Fox-Smith resonators: application to singlelongitudinal-mode operation of ﬁber lasers, Journal of the Optical Society of America A 5 (8) (1988) 1339–1346.  P.L. Scrivener, E.J. Tarbox, P.D. Maton, Narrow linewidth tunable operation of Er3+-doped singlemode ﬁbre laser, Electronics Letters 25 (8) (1989) 549–550.  E.M. Dianov, T.R. Martirosian, O.G. Okhotnikov, V.M. Paramonov, Unidirectional singlefrequency all-ﬁber ring laser, Technical Digest of OFC’93, paper WG6, 1993, pp. 103–104.  Y. Shi, M. Sejka, O. Poulsen, A tunable quasi-unidirectional Er3+doped ﬁber ring laser without isolator, IEEE Photonics Technology Letters 7 (3) (1995) 290–292.  I.Yu. Khrushchev, V. Tanko, I.M. Dianov, Tunable travelling-wave laser with linear cavity conﬁguration, Electronics Letters 31 (11) (1995) 895–896.  D.J. Chang, M.J. Guy, S.V. Chernikov, J.R. Taylor, H.J. Kong, Single-frequency erbium ﬁbre laser using twisted-mode technique, Electronics Letters 32 (19) (1996) 1786–1787.  R. Stolte, R. Ulrich, Er-ﬁbre lasers: suppression of spatial hole burning by internal modulation, Electronics Letters 29 (19) (1993) 1686–1688.  G.A. Ball, W.W. Morey, Compression-tuned single-frequency Bragg grating ﬁber laser, Optics Letters 19 (23) (1994) 1979–1981.  G.A. Ball, C.E. Holton, G. Hull-Allen, W.W. Morey, 60 mW 1.5 mm single-frequency low-noise ﬁber laser MOPA, IEEE Photonics Technology Letters 6 (2) (1994) 192–194.  R. Kashyap, Photosensitive optical ﬁbers: devices and applications, Optical ﬁber Technology 1 (1) (1994) 17–34.  I. Bennion, J.A.R. Williams, L. Zhang, K. Sugden, N.J. Doran, UV-written in-ﬁbre Bragg gratings, Optical and Quantum Electronics 28 (1996) 93–135.  B. Poumellec, F. Kherbouche, The photorefractive Bragg gratings in the ﬁbers for telecommunications, Journal de Physique III 6 (12) (1996) 1595–1624.  J.-L. Archambault, S.G. Grubb, Fiber gratings in lasers and ampliﬁers, Journal of Lightwave Technology 15 (8) (1997) 1378–1390.  C.R. Giles, Lightwave applications of ﬁber Bragg gratings, Journal of Lightwave Technology 15 (8) (1997) 1391–1404.  K.O. Hill, G. Meltz, Fiber Bragg grating technology fundamentals and overview, Journal of Lightwave Technology 15 (8) (1997) 1263–1276.  A. Othonos, Fiber Bragg gratings, Review of Scientiﬁc Instruments 68 (12) (1997) 4309–4341.  G.A. Ball, G. Hull-Allen, C.E. Holton, W.W. Morey, Low noise single frequency linear ﬁbre, Electronics Letters 29 (18) (1993) 1623–1625.  I.M. Jauncey, L. Reekie, R.J. Mears, C.J. Rowe, Narrow-linewidth ﬁber laser operating at 1.55 mm, Optics Letters 12 (3) (1987) 164–166.  I.D. Miller, C.A. Millar, B.J. Ainslie, D.B. Mortimore, J.R. Armitage, Rare-earth doped ﬁbre lasers and ampliﬁers for optical communications, 14th Congress of the International Commission for Optics (ICO-14’87), SPIE 813, paper A9.2, 1987, pp. 323–324.  Y. Kimura, M. Nakazawa, Lasing characteristics of Er3+-doped silica ﬁbers from 1553 up to 1603 nm, Journal of Applied Physics 64 (2) (1988) 516–520.  D.C. Hanna, R.M. Percival, I.R. Perry, R.G. Smart, A.G. Tropper, Efﬁcient operation of an Ybsensitised Er ﬁbre laser pumped in the 0.8 mm region, Electronics Letters 24 (17) (1988) 1068–1069.  Y. Kimura, K. Suzuki, M. Nakazawa, Laser-diode-pumped mirror-free Er3+-doped ﬁber laser, Optics Letters 14 (18) (1989) 999–1001.  R. Wyatt, High-power broadly tunable erbium-doped silica ﬁbre laser, Electronics Letters 25 (22) (1989) 1498–1499. ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 259  P.F. Wysocki, M.J.F. Digonnet, B.Y. Kim, Electronically tunable 1.55 mm erbium-doped ﬁber laser, Optics Letters 15 (5) (1990) 273–275.  G.A. Ball, W.W. Morey, W.H. Glenn, Standing-wave monomode erbium ﬁber laser, IEEE Photonics Technology Letters 3 (7) (1991) 613–615.  G. Grasso, A. Righetti, F. Fontana, Single longitudinal mode operation of an erbium-doped ﬁber laser with microoptics Fabry–Perot interferometer, Technical Digest of ECOC’91, paper TuB3, 1991, pp. 149–152.  I.N. Duling III, R.D. Esman, Single-polarization ﬁber ampliﬁer, Electronics Letters 28 (12) (1992) 1126–1128.  K. Doughty, D.E.L. Vaughan, K. Cameron, D.M. Bird, Novel acoustically tuned ﬁbre laser, Electronics Letters 29 (1) (1993) 31–32.  T. Rosadiuk, J. Conradi, Output coupling induced wavelength shifts in erbium-doped ﬁber lasers, IEEE Photonics Technology Letters 5 (7) (1993) 758–760.  D. Abraham, R. Nagar, M.N. Ruberto, G. Eisenstein, U. Koren, J.L. Zyskind, D.J. DiGiovanni, Frequency tuning and pulse generation in a ﬁber laser with an intracavity semiconductor active ﬁlter, IEEE Photonics Technology Letters 4 (4) (1993) 377–379.  W.H. Loh, P.R. Morkel, D.N. Payne, Wavelength selection and tuning by optical control in a twosegment erbium-doped ﬁber laser, IEEE Photonics Technology Letters 6 (1) (1994) 43–46.  R. Larose, D. Stepanov, C. Latrasse, M. T#etu, F. Ouelette, M.A. Duguay, Simple frequency tuning technique for locking a singlemode erbium-doped ﬁbre laser to the centre of molecular resonances, Electronics Letters 30 (10) (1994) 791–793.  K. Hsu, C.M. Miller, J.T. Kringlebotn, E.M. Taylor, J.E. Townsend, D.N. Payne, Single-mode tunable erbium: ytterbium ﬁber Fabry–Perot microlaser, Optics Letters 19 (12) (1994) 886–888.  J.T. Kringlebotn, J.-L. Archambault, L. Reekie, D.N. Payne, Er3+:Yb3+-codoped ﬁber distributedfeedback laser, Optics Letters 19 (24) (1994) 2101–2103.  M. Ibsen, B.J. Eggleton, M.G. Sceats, F. Ouellette, Broadly tunable DBR ﬁber laser using sampled ﬁber Bragg gratings, Electronics Letters 31 (1) (1995) 37–38.  M. Sejka, P. Varming, J. Hubner, . M. Kristensen, Distributed feedback Er3+-doped ﬁbre laser, Electronics Letters 31 (17) (1995) 1445–1446.  W.H. Loh, R.I. Laming, 1.55 mm phase-shifted distributed feedback ﬁbre laser, Electronics Letters 31 (17) (1995) 1440–1442.  K. Sugden, I. Bennion, K. Byron, H. Rourke, S. Davies, Grating Michelson mirrors for optimised ﬁbre laser performance, Topical Meeting on Photosensitivity and Quadratic Nonlinearity in Glass Waveguides: Fundamentals and Applications, paper PMD3, 1995, pp. 261–264.  A. Othonos, X. Lee, D.P. Tsai, Spectrally broadband Bragg grating mirror for an erbium-doped ﬁber laser, Optical Engineering 35 (4) (1996) 1088–1092.  Y. Takushima, S. Yamashita, K. Kikuchi, K. Hotate, Single-frequency and polarization-stable oscillation of Fabry–Perot ﬁber laser using a nonpolarization-maintaining ﬁber and an intracavity etalon, IEEE Photonics Technology Letters 8 (11) (1996) 1468–1470.  Z.E. Harutjunian, W.H. Loh, R.I. Laming, D.N. Payne, Single polarisation twisted distributed feedback ﬁbre laser, Electronics Letters 32 (4) (1996) 346–348.  W.H. Loh, B.N. Samson, Z.E. Harutjunian, R.I. Laming, Intracavity pumping for increased output power from a distributed feedback erbium ﬁbre laser, Electronics Letters 32 (13) (1996) 1204–1205.  W.H. Loh, L. Dong, J.E. Caplen, Single-sided output Sn/Er/Yb distributed feedback ﬁber laser, Applied Physics Letters 69 (15) (1996) 2151–2153.  W.H. Loh, S.D. Butterworth, W.A. Clarkson, Efﬁcient distributed feedback erbium-doped germanosilicate ﬁbre laser pumped in 520 nm band, Electronics Letters 32 (22) (1996) 2088–2089.  H. Stor^y, B. Sahlgren, R. Stubbe, Single polarisation ﬁbre DFB laser, Electronics Letters 33 (1) (1997) 56–58.  L. Dong, W.H. Loh, J.E. Caplen, J.D. Minelly, K. Hsu, L. Reekie, Efﬁcient single-frequency ﬁber lasers with novel photosensitive Er/Yb optical ﬁbers, Optics Letters 22 (10) (1997) 694–696.  C.R. Giles, V. Mizrahi, Single-frequency 1559-nm erbium-doped ﬁber laser pumped by a 650-nm semiconductor laser, Applied Optics 36 (24) (1997) 5859–5861. ARTICLE IN PRESS 260 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266  K. Hsu, W.H. Loh, L. Dong, C.M. Miller, Efﬁcient and tunable Er/Yb ﬁber grating lasers, Journal of Lightwave Technology 15 (8) (1997) 1438–1441.  A. Bellemare, J.-F. Lemieux, M. T#etu, S. LaRochelle, Erbium-doped ﬁber ring laser steptunable to exact multiples of 100 GHz step-tunable single-frequency erbium-doped ﬁber lasers, Proceedings of Infrared Glass Optical Fibers and their Applications, SPIE 3416 (1998) 220–228.  S. Yamashita, K. Hsu, Single-frequency, single-polarization operation of tunable miniature erbium: ytterbium ﬁber Fabry–Perot lasers by use of self-injection locking, Optics Letters 23 (15) (1998) 1200–1202.  N. Azami, A. Sa.ıssy, M. De Micheli, G. Monnom, D.B. Ostrowsky, Improved polarimetric tuning of an Er3+-doped ﬁber laser, Optics Communications 158 (1998) 84–88.  N.J.C. Libatique, R.K. Jain, Precisely and rapidly wavelength-switchable narrow-linewidth 1.5-mm laser source for wavelength division multiplexing applications, IEEE Photonics Technology Letters 11 (12) (2001) 1584–1586.  H.Y. Yoon, K.M. Chu, S.B. Lee, S.S. Choi, D. Park, Tunable Er3+-doped ﬁber distributedfeedback laser, Proceedings of LEOS’00, 2, paper WA4, 2000, pp. 401–402.  P. Varming, V.C. Lauridsen, J.H. Povlsen, J.B. Jensen, M. Kristensen, Design and fabrication of Bragg grating based DFB ﬁber lasers operating above 1610 nm, Technical Digest of OFC’00, 3, paper ThA6, 2000, pp. 17–19.  S.H. Chang, I.K. Hwang, B.Y. Kim, H.G. Park, Widely tunable single-frequency Er-doped ﬁber laser with long linear cavity, IEEE Photonics Technology Letters 13 (4) (2001) 287–289.  M. Ibsen, S.Y. Set, G.S. Goh, K. Kikuchi, Broad-band continuously tunable all-ﬁber DFB lasers, IEEE Photonics Technology Letters 14 (1) (2002) 21–23.  H.C. Lefevre, Single-mode ﬁbre fractional wave devices and polarisation controllers, Electronics Letters 16 (20) (1980) 778–780.  X.S. Yao, Apparatus and method for connecting polarization sensitive devices, United States Patent No. 5561726, 1996.  N. Park, J.W. Dawson, K.J. Vahala, Frequency locking of an erbium-doped ﬁber ring laser to an external ﬁber Fabry–Perot resonator, Optics Letters 18 (11) (1993) 879–881.  H. Sabert, Active stabilisation of singlemode operation in a ﬁbre laser, Electronics Letters 29 (11) (1993) 1004–1005.  H. Sabert, Suppression of mode jumps in a single-mode ﬁber laser, Optics Letters 19 (2) (1994) 111–113.  C.Y. Chen, M.M. Choy, M.J. Andrejco, M.A. Saiﬁ, C. Lin, A widely tunable erbium-doped ﬁber laser pumped at 532 nm, IEEE Photonics Technology Letters 2 (1) (1990) 18–20.  P.R. Morkel, G.J. Cowle, D.N. Payne, Travelling-wave erbium ﬁbre ring laser with 60 kHz linewidth, Electronics Letters 26 (10) (1990) 632–634.  M.W. Maeda, J.S. Patel, D.A. Smith, C. Lin, M.A. Saiﬁ, A. Von Lehman, An electronically tunable ﬁber laser with a liquid-crystal etalon ﬁlter as the wavelength-tuning element, IEEE Photonics Technology Letters 2 (11) (1990) 787–789.  K. Iwatsuki, H. Okamura, M. Saruwatari, Wavelength-tunable single-frequency and singlepolarisation Er-doped ﬁbre ring-laser with 1.4 kHz linewidth, Electronics Letters 26 (24) (1990) 2033–2035.  G.J. Cowle, D.N. Payne, D. Reid, Single-frequency travelling-wave erbium-doped ﬁbre loop laser, Electronics Letters 27 (3) (1991) 229–230.  D.A. Smith, M.W. Madea, J.J. Johnson, J.S. Patel, M.A. Saiﬁ, A. Von Lehman, Acoustically tuned erbium-doped ﬁber ring laser, Optics Letters 16 (6) (1991) 387–389.  N. Park, J.W. Dawson, K.J. Vahala, All ﬁber, low threshold, widely tunable single-frequency, erbium-doped ﬁber ring laser with a tandem ﬁber Fabry–Perot ﬁlter, Applied Physics Letters 59 (19) (1991) 2369–2371.  J.L. Zyskind, J.W. Sulhoff, J. Stone, D.J. DiGiovanni, L.W. Stulz, H.M. Presby, A. Piccirilli, P.E. Pramayon, Electrically tunable, diode-pumped erbium-doped ﬁbre ring laser with ﬁbre Fabry–Perot etalon, Electronics Letters 27 (21) (1991) 1950–1951. ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 261  J.L. Zyskind, J.W. Sulhoff, Y. Sun, J. Stone, L.W. Stulz, G.T. Harvey, D.J. DiGiovanni, H.M. Presby, A. Piccirilli, U. Koren, R.M. Jopson, Singlemode diode-pumped tunable erbium-doped ﬁbre laser with linewidth less than 5.5 kHz, Electronics Letters 27 (23) (1991) 2148–2149.  H. Schmuck, T. Pfeiffer, G. Veith, Widely tunable narrow linewidth erbium doped ﬁbre ring laser, Electronics Letters 27 (23) (1991) 2117–2119. ! Cochl!ain, R.J. Mears, Broadband tunable single frequency diode-pumped erbium doped  C.R.o. ﬁbre laser, Electronics Letters 28 (2) (1992) 124–126.  H. Schmuck, T. Pfeiffer, H. Bulow, . Design optimisation of erbium ring laser regarding output power and spectral properties, Electronics Letters 28 (17) (1992) 1637–1639.  P.D. Humphrey, J.E. Bowers, Fiber-birefringence tuning technique for an erbium-doped ﬁber ring laser, IEEE Photonics Technology Letters 5 (1) (1993) 32–34.  S. Th!eriault, Utilisation de composantes en ﬁbres optiques dans la r!ealisation d’un laser a" spectre e! troit, Ph.D.Thesis, Universit!e Laval, Sainte-Foy, Canada, 1993, 196pp.  O.G. Okhotnikov, A.B. Lobo Ribeiro, J.R. Salcedo, All-ﬁber travelling-wave laser with nonreciprocal ring conﬁguration, Applied Physics Letters 63 (1993) 2726–2728.  Y.T. Chieng, R.A. Minasian, Tunable erbium-doped ﬁber laser with a reﬂection Mach-Zehnder interferometer, IEEE Photonics Technology Letters 6 (2) (1994) 153–155.  Y. Cheng, J.T. Kringleboth, W.H. Loh, R.I. Laming, D.N. Payne, Stable single-frequency travelingwave ﬁber loop laser with integral saturable-absorber-based tracking narrow-band ﬁlter, Optics Letters 20 (8) (1995) 875–877.  T. Komukai, M. Nakazawa, Tunable single frequency erbium doped ﬁber ring lasers using grating etalons, Japanese Journal of Applied Physics Part 2 34 (6A) (1995) 679–680.  J.J. Pan, Y. Shi, Tunable Er3+-doped ﬁbre ring laser using grating incorporated by optical circulator or ﬁber coupler, Electronics Letters 31 (14) (1995) 1164–1165.  Y. Shi, M. Sejka, O. Poulsen, A tunable quasi-unidirectional Er3+-doped ﬁber ring laser without isolator, IEEE Photonics Technology Letters 7 (3) (1995) 290–292.  A. Gloag, N. Langford, K. McCallion, W. Johnstone, Tunable, single frequency erbium ﬁber laser using an overlay bandpass ﬁlter, Applied Physics Letters 66 (24) (1995) 3263–3265.  Y. Hua, J. Conradi, Single-polarization wavelength-tunable ﬁber laser with a nonrecoprocal cavity, Journal of Lightwave Technology 13 (9) (1995) 1913–1918.  M.C. Farries, D.C. Reid, L. Zhang, I. Bennion, Fibre ring laser with ﬁbre grating transmission ﬁlter, Proceedings of Photosensitivity and quadratic nonlinearity in glass waveguides—fundamentals and applications, OSA 22, paper PMD4, 1995, pp. 265–268.  C.-X. Shi, A novel twisted Er-doped ﬁber ring laser: proposal, theory, and experiment, Optics Communications 125 (1996) 349–358.  M.C. Parker, R.J. Mears, Digitally tunable wavelength ﬁlter and laser, IEEE Photonics Technology Letters 8 (8) (1996) 1007–1008.  F. Chollet, J.-P. Goedgebuer, H. Porte, A. Hamel, Electrooptic narrow linewidth wavelength tuning and intensity modulation of an erbium ﬁber ring laser, IEEE Photonics Technology Letters 8 (8) (1996) 1009–1011.  R.J. Foster, N. Langford, A. Gloag, L. Zhang, J.A.R. Williams, I. Bennion, Longitudinal mode control of an erbium ring ﬁbre laser containing an intracavity chirped Bragg grating etalon, Optics Communications 141 (1997) 283–287.  A. Bellemare, J.-F. Lemieux, M. T#etu, S. LaRochelle, Erbium-doped ﬁber ring lasers step-tunable to exact multiples of 100 GHz (ITU-grid) using periodic ﬁlters, Technical Digest of ECOC’98, paper TuA13, 1998, pp. 153–154.  A. Bellemare, J.-F. Lemieux, M. T#etu, S. LaRochelle, J. Martin, Erbium-doped ﬁber ring laser steptunable to exact multiples of 100 GHz (ITU-grid) using a sampled Bragg grating, Proceedings of ICAPT’98, SPIE 3491 (1998) 623–627.  T. Haber, K. Hsu, C. Miller, Y. Bao, Tunable erbium-doped ﬁber ring laser precisely locked to the 50-GHz ITU frequency grid, IEEE Photonics Technology Letters 12 (11) (2000) 1456–1458. ARTICLE IN PRESS 262 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266  A. Bellemare, M. Kar!asek, Ch. Riviere, F. Babin, G. He, G.W. Schinn, Broadly tunable erbiumdoped ﬁber ring laser: experimentation and modeling, IEEE Journal of Selected Topics in Quantum Electronics 7 (1) (2001) 22–29.  S. Yamashita, M. Nishihara, Widely tunable erbium-doped ﬁber ring laser covering both C-band and L-band, IEEE Journal of Selected Topics in Quantum Electronics 7 (1) (2001) 41–43.  Y. Yu, L. Lui, H. Tam, W. Chung, Fiber-laser-based wavelength-division multiplexed ﬁber Bragg grating sensor system, IEEE Photonics Technology Letters 13 (7) (2001) 702–704.  Y.W. Song, S.A. Havstad, D. Starodubov, Y. Xie, A.E. Willner, J. Feinberg, 40-nm-wide tunable ﬁber ring laser with single-mode operation using a highly stretchable FBG, IEEE Photonics Technology Letters 13 (11) (2001) 1167–1169.  N.J.C. Libatique, R.K. Jain, A broadly tunable wavelength-selectable WDM source using a ﬁber Sagnac loop ﬁlter, IEEE Photonics Technology Letters 13 (12) (2001) 1283–1285.  B.-O. Guan, H.-Y. Tam, H.L.W. Chan, X.-Y. Dong, C.-L. Choy, M.S. Demokan, Temperaturetuned erbium-doped ﬁber ring laser with polymer-coated ﬁber grating, Optics Communications 202 (2002) 331–334.  J.M. Oh, H.B. Choi, D. Lee, S.J. Ahn, Incorporation of a ﬁber Bragg grating to improve the efﬁciency of a 1580-nm-band tunable ﬁber ring laser, Optics Letters 27 (8) (2002) 589–591.  T. Georges, E. Delevaque, Analytical modeling of high-gain erbium-doped ﬁber ampliﬁers, Optics Letters 17 (1992) 1113–1115.  J. Zhang, C.-Y. Yue, G.W. Schinn, W.R.L. Clements, J.W.Y. Lit, Stable single-mode compoundring erbium-doped ﬁber laser, Journal of Lightwave Technology 14 (1) (1996) 104–109.  J. Zhang, J.W.Y. Lit, G.W. Schinn, Cancellation of associated frequency dithering in a singlefrequency compound-ring erbium-doped ﬁber laser, IEEE Photonics Technology Letters 8 (12) (1996) 1621–1623.  A. Gloag, N. Langford, K. McCallion, W. Johnstone, Continuously tunable single-frequency erbium ring ﬁber laser, Journal of the Optical Society of America B 13 (5) (1996) 921–925.  R.J. Foster, N. Langford, Longitudinal mode control of a narrow-linewidth ﬁber laser by use of the intrinsic birefringence of the ﬁber laser, Optics Letters 21 (20) (1996) 1679–1681.  S.L. Gilbert, Frequency stabilization of a tunable erbium-doped ﬁber laser, Optics Letters 16 (3) (1991) 150–152.  S.V. Chernikov, J.R. Taylor, R. Kashyap, Coupled-cavity erbium ﬁber lasers incorporating ﬁber grating reﬂectors, Optics Letters 18 (23) (1993) 2023–2025.  L.W. Liou, M. Yu, T. Yoshino, G.P. Agrawal, Mutual injection locking of a ﬁbre laser and a DFB semiconductor laser, Electronics Letters 31 (1) (1995) 41–42.  K. Hsu, C.M. Miller, J.T. Kringlebotn, D.N. Payne, Continuous and discrete wavelength tuning in Er:Yb ﬁber Fabry–Perot lasers, Optics Letters 20 (4) (1995) 377–379.  K.-Y. Liou, U. Koren, C. Chen, E.C. Burrows, K. Dreyer, J.W. Sulhoff, A 24-channel wavelengthselectable Er-ﬁber ring laser with intracavity waveguide-grating-router and semiconductor FabryPerot ﬁlter, IEEE Photonics Technology Letters 10 (11) (1998) 1787–1789.  J.J. Pan, Y. Shi, 166-mW single-frequency output power interactive ﬁber lasers with low noise, IEEE Photonics Technology Letters 11 (1) (1999) 36–38.  J.J. Pan, Y. Shi, Continuously tunable high power ﬁber lasers with 11 nm tunability, Technical Digest of OFC’99, 2, paper WM2, 1999, pp. 199–201.  M. Zirngibl, C.H. Joyner, L.W. Stulz, C. Dragone, H.M. Presby, I.P. Kaminow, LARNet, a local access router network, IEEE Photonics Technology Letters 7 (2) (1995) 215–217.  International Telecommunication Union (ITU-T), Optical interfaces for multichannel systems with optical ampliﬁers, recommendation G.692, 1998, 40pp.  M.G. Young, U. Koren, B.I. Miller, M.A. Newkirk, M. Chien, M. Zirngibl, C. Dragone, B. Tell, H.M. Presby, G. Raybon, A 16 1 wavelength division multiplexer with integrated distributed Bragg reector lasers and electroabsorption modulators, IEEE Photonics Technology Letters 5 (8) (1993) 908–910.  M.C. Farries, A.C. Carter, G.G. Jones, I. Bennion, Tuneable multiwavelength laser with single ﬁbre output, Electronics Letters 27 (17) (1991) 1498–1499. ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 263  K.R. Poguntke, J.B.D. Soole, A. Scherer, H.P. LeBlanc, C. Caneau, R. Bhat, M.A. Koza, Simultaneous multiple wavelength operation of a multistripe array grating integrated cavity laser, Applied Physics Letters 62 (37) (1993) 2024–2026.  M. Zirngibl, C.H. Joyner, A 12-frequency WDM laser source based on a transmissive waveguide grating router, OFC’94 Post-deadline Papers, paper PD16, 1994.  M. Zirngibl, C.H. Joyner, C.R. Doerr, L.W. Stulz, H.M. Presby, An 18-channel multifrequency laser, IEEE Photonics Technology Letters 8 (7) (1996) 870–872.  H. Takahashi, H. Toba, Y. Inoue, Multiwavelength ring laser composed of EDFAs and an arrayedwaveguide wavelength multiplexer, Electronics Letters 30 (1) (1994) 44–45.  T. Miyazaki, N. Edagawa, S. Yamamoto, S. Akiba, A multiwavelength ﬁber ring-laser employing a pair of silica-based arrayed-waveguide-gratings, IEEE Photonics Technology Letters 9 (7) (1997) 910–912.  K.-H. Kim, H.-K. Lee, S.-Y. Park, E.-H. Lee, Wavelength-varying multi-wavelength optical ﬁlter laser using a single pump light source, United States Patent No. 5524118, 1996, 6pp.  H. Schmuck, T. Pfeiffer, Fibre-pigtailed Fabry-Perot ﬁlter used as tuning element and for comb generation in an erbium doped ﬁbre ring laser, Technical Digest of ECOC’91, paper TuB3, 1991, pp. 145–148.  N. Park, J.W. Dawson, K.J. Vahala, Multiple wavelength operation of an erbium-doped ﬁber laser, IEEE Photonics Technology Letters 4 (6) (1992) 540–542.  J. Hubner, . P. Varming, M. Kristensen, Five wavelength DFB ﬁbre laser source for WDM systems, Electronics Letters 33 (2) (1997) 139–140.  O. Graydon, W.H. Loh, R.I. Laming, L. Dong, Triple-frequency operation of an Er-doped twincore ﬁber loop laser, IEEE Photonics Technology Letters 8 (1) (1996) 63–65.  A.J. Poustie, N. Finlayson, P. Harper, Multiwavelength ﬁber laser using a spatial mode beating ﬁlter, Optics Letters 19 (10) (1994) 716–718.  D. Abraham, R. Nagar, M.N. Ruberto, G. Eisenstein, J.L. Zyskind, D. DiGiovanni, U. Koren, G. Raybon, Intracavity-diode-pumped erbium doped ﬁbre laser, Electronics Letters 28 (19) (1992) 1830–1832.  Y. Zhao, C. Shu, S.P. Li, H. Ding, K.T. Chang, Multiple wavelength operation of a unidirectional Er-doped ﬁber ring laser with optical feedback, Technical Digest of CLEO’97, paper CThL65, 1997, p. 396.  H. Sasamori, K. Isshiki, H. Watanabe, K. Kasahara, Multi-wavelength erbium-doped ring light source with ﬁber grating ﬁlter, Technical Digest of Optical Ampliﬁers and Their Applications (OAA’97), paper WC3, 1997, pp. 235–238.  A. Bellemare, M. Rochette, M. T#etu, S. LaRochelle, Multifrequency erbium-doped ﬁber ring lasers anchored on the ITU frequency grid, Technical Digest of OFC’99, paper TuB5, 1999, pp. 16–18.  M. Kar!asek, A. Bellemare, Numerical analysis of multifrequency erbium-doped ﬁber ring laser employing a periodic ﬁlter and a frequency shifter, IEE Proceedings-Optoelectronics 147 (2) (2000) 115–119.  A. Bellemare, M. Kar!asek, M. Rochette, S. LaRochelle, M. T#etu, Room temperature multifrequency erbium-doped ﬁber ring lasers anchored on the ITU frequency grid, Journal of Lightwave technology 18 (6) (2000) 825–831.  J. Chow, G. Town, B. Eggleton, M. Ibsen, K. Sugden, I. Bennion, Multiwavelength generation in an erbium-doped ﬁber laser using in-ﬁber comb ﬁlters, IEEE Photonics Technology Letters 8 (1) (1996) 60–62.  G.E. Town, K. Sudgen, J.A.R. Williams, I. Bennion, S.B. Poole, Wide-band Fabry-Perot-like ﬁlters in optical ﬁbers, IEEE Photonics Technology Letters 7 (1) (1995) 78–80.  S. Yamashita, K. Hotate, Multiwavelength erbium-doped ﬁber laser using intracavity etalon and cooled by liquid nitrogen, Electronics Letters 32 (14) (1996) 1298–1299.  N. Park, P.F. Wysocki, 24-line multiwavelength operation of erbium-doped ﬁber-ring laser, IEEE Photonics Technology Letters 8 (11) (1996) 1459–1461.  S. Yamashita, K. Hsu, W.H. Loh, Miniature Erbium:Ytterbium ﬁber Fabry-Perot multiwavelength lasers, IEEE Journal of Selected Topics in Quantum Electronics 3 (4) (1997) 1058–1064. ARTICLE IN PRESS 264 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266  A.T. Alavie, S.E. Karr, A. Othonos, R.M. Measures, Amultiplexed Bragg grating ﬁber laser sensor system, IEEE Photonics Technology Letters 5 (9) (1993) 1112–1114.  J.J. Pan, X.L. Jing, Y. Shi, High performance tunable and multiwavelength laser sources for phased array antennas, Digest of Antennas and Propagation Society International Symposium, vol. 2, paper 36.5, 1997, pp. 747–750.  M. Ibsen, S.-U. Alam, M.N. Zervas, A.B. Grudinin, D.N. Payne, 8- and 16-channel all-ﬁber DFB laser WDM transmitters with integrated pump redundancy, IEEE Photonics Technology Letters 11 (9) (1999) 1114–1116.  H.L. An, X.Z. Lin, E.Y.B. Pun, H.D. Liu, Multi-wavelength operation of an erbium-doped ﬁber ring laser using a dual-pass Mach-Zehnder comb ﬁlter, Optics Communications 169 (1999) 159–165.  J. Sun, J. Qiu, D. Huang, Multiwavelength erbium-doped ﬁber lasers exploiting polarization hole burning, Optics Communications 182 (2000) 193–197.  X. Shu, S. Jiang, D. Huang, Fiber grating loop and its multiwavelength-laser application, IEEE Photonics Technology Letters 12 (8) (2000) 980–982.  D. Wei, T. Li, Y. Zhao, S. Jian, Multiwavelength erbium-doped ﬁber ring lasers with overlap-written ﬁber Bragg gratings, Optics Letters 25 (16) (2000) 1150–1152.  X.P. Dong, S. Li, K.S. Chiang, M.N. Ng, B.C.B. Chu, Multiwavelength erbium-doped ﬁbre laser based on a high-birefringence ﬁbre loop mirror, Electronics Letters 36 (19) (2000) 1609–1610.  S. Jarabo, Experimental study of a multiwavelength erbium-doped ﬁber ring laser incorporating a spatial mode beating ﬁlter, Fiber and Integrated Optics 20 (4) (2001) 325–339.  S.K. Kim, M.J. Chu, J.H. Lee, Wideband multiwavelength erbium-doped ﬁber ring laser with frequency shifted feedback, Optics Communications 190 (2001) 291–302. !  L. Talaverano, S. Abad, S. Jarabo, M. Lopez-Amo, Multiwavelength ﬁber laser sources with Bragggrating sensor multiplexing capability, Journal of Lightwave Technology 19 (4) (2001) 553–558.  S. Yamashita, T. Baba, Spacing-tunable multiwavelength ﬁbre laser, Electronics Letters 37 (16) (2001) 1015–1017.  Q. Mao, J.W.Y. Lit, L-band multiwavelength oscillation in erbium-doped ﬁber ring laser, Microwave and Optical Technology Letters 32 (2) (2002) 88–91.  R. Slavik, S. LaRochelle, Multiwavelength ‘single-mode’ erbium doped ﬁber laser for FFHOCDMA testing, Technical Digest of OFC2002, paper WJ3, 2002.  C.-L. Zhao, S. Yang, H. Meng, Z. Li, S. Yuan, K. Guiyun, X. Dong, Efﬁcient multi-wavelength ﬁber laser operating in L-band, Optics Communications 204 (2002) 323–326.  G. Das, J.W.Y. Lit, L-band multiwavelength ﬁber laser using an elliptical ﬁber, IEEE Photonics Technology Letters 14 (5) (2002) 606–608.  Q. Mao, J.W.Y. Lit, Switchable multiwavelength erbium-doped ﬁber laser with cascaded ﬁber grating cavities, IEEE Photonics Technology Letters 14 (5) (2002) 612–614.  K. Takada, M. Shimizu, M. Yamada, M. Horiguchi, A. Himeno, K. Yukimatsu, Ultrahighsensitivity low coherence OTDR using Er3+-doped superﬂuorescent ﬁbre source, Electronics Letters 28 (1) (1992) 29–31.  K. Takada, T. Kitagawa, M. Shimizu, M. Horiguchi, High-sensitivity low coherence reﬂectometer using erbium-doped superﬂuorescent ﬁbre source and erbium-doped power ampliﬁer, Electronics Letters 29 (4) (1993) 365–367.  K. Takada, M. Yamada, S. Mitachi, Tunable narrow-band light source using two optical circulators, IEEE Photonics Technology Letters 9 (1) (1997) 91–93.  D.D. Yang, High power broadband source with stable and equalized spectrum output, United States Patent No. 6172995, 2001, 8pp.  T.A. Berkoff, A.D. Kersey, Fiber Bragg grating array sensor system using a bandpass wavelength division multiplexer and interferometric detection, IEEE Photonics Technology Letters 8 (11) (1996) 1522–1524.  J.S. Lee, Y.C. Chung, D.J. DiGiovanni, Spectrum-sliced ﬁber ampliﬁer light source for multichannel WDM applications, IEEE Photonics Technology Letters 5 (12) (1993) 1458–1461.  D.R. Huber, Narrow band incoherent optical carrier generator, United States Patent No. 5191586, 1993, 7pp. ARTICLE IN PRESS A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266 265  D.D. Sampson, W.T. Holloway, 100 mW spectrally uniform broadband ASE source for spectrumsliced WDM systems, Electronics Letters 30 (19) (1994) 1611–1612.  H. Masuda, S. Kawai, K.-I. Suzuki, K. Aida, 1.65-mm band ﬁbre Raman ampliﬁer pumped by wavelength-tunable broad-linewidth light source, Technical Digest of ECOC’98, 1998, pp. 139–140.  K. Iwatsuki, Long-term bias stability of all-panda ﬁber gyroscope with Er-doped superﬂuorescent ﬁbre laser, Electronics Letters 27 (12) (1991) 1092–1093.  E. Desurvire, J.R. Simpson, Ampliﬁcation of spontaneous emission in erbium-doped single-mode ﬁbers, Journal of Lightwave Technology 7 (5) (1989) 835–845.  P.F. Wysocki, M.J.F. Digonnet, B.Y. Kim, Spectral characteristics of high-power 1.5 mm broadband superluminescent ﬁber sources, IEEE Photonics Technology Letters 2 (3) (1990) 178–180.  P.F. Wysocki, M.J.F. Digonnet, B.Y. Kim, H.J. Shaw, Characteristics of erbium-doped superﬂuorescent ﬁber sources for interferometric sensor applications, Journal of Lightwave Technology 12 (3) (1994) 550–567.  P.R. Morkel, R.I. Laming, D.N. Payne, Noise characteristics of high-power doped-ﬁbre superluminescent sources, Electronics Letters 26 (2) (1990) 96–98.  K. Iwatsuki, Excess noise reduction in ﬁber gyroscope using broader spectrum linewidth Er-doped superﬂuorescent ﬁber laser, IEEE Photonics Technology Letters 3 (3) (1991) 281–283.  P.F. Wysocki, M.J.F. Digonnet, B.Y. Kim, Wavelength stability of a high-output, broadband, Er-doped superﬂuorescent ﬁber source pumped near 980 nm, Optics Letters 16 (12) (1991) 961–963.  J.L. Wagener, M.J.F. Digonnet, H.J. Shaw, A high-stability ﬁber ampliﬁer source for ﬁber optic gyroscope, Journal of Lightwave Technology 15 (9) (1997) 1689–1694.  D.G. Falquier, M.J.F. Digonnet, H.J. Shaw, A depolarized Er-doped superﬂuorescent ﬁber source with improved long-term polarization stability, IEEE Photonics Technology Letters 13 (1) (2001) 25–27.  D.C. Hall, W.K. Burns, Wavelength stability optimisation in Er3+-doped superﬂuorescent ﬁbre sources, Electronics Letters 30 (8) (1994) 653–654.  D.C. Hall, W.K. Burns, R.P. Moeller, High-stability Er3+-doped superﬂuorescent ﬁber sources, Journal of Lightwave Technology 13 (7) (1995) 1452–1460.  L.A. Wang, C.D. Su, Modeling of a double-pass backward Er-doped superﬂuorescent ﬁber source for ﬁber-optic gyroscope applications, Journal of Lightwave Technology 17 (11) (1999) 2307–2315.  L.A. Wang, C.D. Chen, Stable and broadband Er-doped superﬂuorescent ﬁbre sources using double-pass backward conﬁguration, Electronics Letters 32 (19) (1996) 1815–1817.  S. Gray, J.D. Minelly, A.B. Grudin, J.E. Caplen, 1 Watt Er/Yb singlemode superﬂuorescent optical ﬁbre source, Electronics Letters 33 (16) (1997) 1382–1383.  R.P. Espindola, G. Ales, J. Park, T.A. Strasser, 80 nm spectrally ﬂattened, high power erbium ampliﬁed spontaneous emission ﬁbre source, Electronics Letters 36 (15) (2000) 1263–1265.  K. Iwatsuki, Er-doped superﬂuorescent ﬁber laser pumped by 1.48 mm laser diode, IEEE Photonics Technology Letters 2 (4) (1990) 237–238.  H. Fevrier, J.F. Marcerou, P. Bousselet, J. Auge, M. Jurczyszyn, High power, compact 1.48 mm diodepumped broadband superﬂuorescent ﬁbre source at 1.55 mm, Electronics Letters 27 (3) (1991) 261–263.  N.S. Kwong, High-power, broad-band 1550 nm light source by tandem combination of a superluminescent diode and an Er-doped ﬁber ampliﬁer, IEEE Photonics Technology Letters 4 (9) (1992) 996–999.  R. Kashyap, R. Wyatt, R.J. Campbell, Wideband gain ﬂattened erbium ﬁbre ampliﬁer using a photosensitive ﬁbre blazed grating, Electronics Letters 29 (2) (1993) 154–156.  C.W. Hodgson, A.M. Vengsarkar, Spectrally shaped high-power ampliﬁed spontaneous emission sources incorporating long-period gratings, Technical Digest of OFC’96, paper TuG3, 1996, pp. 29–30.  D.G. Falquier, J.L. Wagener, M.J.F. Digonnet, H.J. Shaw, Polarized superﬂuorescent ﬁber source, Optics Letters 22 (3) (1997) 160–162.  L. Goldberg, R.P. Moeller, W.K. Burns, High-power 1.5-mm superﬂuorescent source for ﬁber-optic gyroscope, Technical Digest of OFC’97, paper TuH1, 1997, pp. 28–29.  L.A. Wang, C.D. Chen, Comparison of efﬁciency and output power of optimal Er-doped superﬂuorescent ﬁbre sources in different conﬁgurations, Electronics Letters 33 (8) (1997) 703–704. ARTICLE IN PRESS 266 A. Bellemare / Progress in Quantum Electronics 27 (2003) 211–266  O.G. Okhotnikov, J.M. Sousa, High power superﬂuorescent source with stable single-transversemode output using a multimode Er-doped ﬁbre, Electronics Letters 33 (20) (1997) 1727–1729.  S.D. Dyer, K.B. Rochford, Spectral tailoring of erbium superﬂuorescent ﬁbre source, Electronics Letters 34 (11) (1998) 1137–1139.  S.P. Parry, R. Di Muro, K.J. Cordina, A. Robinson, S.J. Wilson, N.E. Jolley, High power, 80 nm optical noise source using a single Erbium doped silica ﬁbre, Technical Digest of Optical Ampliﬁers and their Applications (OAA’98), paper TuD3, 1998, pp. 128–131.  C.D. Sue, L.A. Wang, Linewidth broadening of Er-doped superﬂuorescent ﬁber source using longperiod grating, Electronics Letters 35 (4) (1999) 331–332.  J.H. Lee, U.-C. Ryu, N. Park, Passive erbium-doped ﬁber seed photon generator for high-power Er3+-doped ﬁber ﬂuorescent sources with an 80-nm bandwidth, Optics Letters 24 (5) (1999) 279–281.  H. Jeong, K. Oh, U.C. Paek, A new broadband ampliﬁed spontaneous emission light source from an Er3+/Tm3+co-doped silica ﬁber, Proceedings of APCC/OECC’99 2 (1999) 1527–1529.  D.M. Dagenais, L. Goldberg, R.P. Moeller, W.K. Burns, Wavelength stability characteristics of a high-power, ampliﬁed superﬂuorescent source, Journal of Lightwave Technology 17 (8) (1999) 1415–1422.  J. Koplow, L. Goldberg, D.A.V. Kliner, R.P. Moeller, High power PM ﬁber ampliﬁer and broadband source, Technical Digest of OFC2000, paper WA5, 2000, pp. 12–13.  D. Guillaumond, J.-P. Meunier, Comparison of two ﬂattening techniques on a double-pass erbiumdoped superﬂuorescent ﬁber source for ﬁber-optic gyroscope, IEEE Journal of Selected Topics in Quantum Electronics 7 (1) (2001) 17–21.  P. Kornreich, N.-S. Cheng, L.M. Wu, J.-T. Tung, R. Boncek, M. Krol, J. Stacy, E. Donkor, Semiconductor cylinder ﬁbers, Journal of Lightwave Technology 14 (7) (1996) 1674–1676.  J.F. Dove, H. Russell, J.-S. Kim, N. Nivartvong, J. Flattery, D. Keller, P. Kornreich, Light ampliﬁcation by Cd3P2 cylinder ﬁber, Optical Devices for Fiber Communication II (SPIE 4216), paper 35, 2001, pp. 62–66.  S.B. Poole, D.N. Payne, M.E. Fermann, Fabrication of lowloss optical ﬁbres containing rare-earth ions, Electronics Letters 22 (17) (1985) 737–738.  L. Reekie, I.M. Jauncey, S.B. Poole, D.N. Payne, Diode-laser-pumped operation of an Er3+-doped single-mode ﬁbre laser, Electronics Letters 23 (20) (1987) 1076–1078.  M.J. Guy, J.R. Taylor, R. Kashyap, Single-frequency erbium ﬁbre ring laser with intracavity phaseshifted ﬁbre Bragg grating narrowband ﬁlter, Electronics Letters 31 (22) (1995) 1924–1925.  A. Gloag, N. Langford, I. Bennion, L. Zhang, Single-frequency travelling-wave erbium doped ﬁbre laser incorporating a ﬁbre Bragg grating, Optics Communications 123 (1996) 553–557.