Review 2003

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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: antoine.bellemare@phy.ulaval.ca (A. Bellemare).
0079-6727/03/$ - see front matter r 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0079-6727(02)00025-3
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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 . . .
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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 . . . . . . .
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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 [1] in 1958 and the first demonstration of the laser effect by Maiman [2] in
1960. In January 1961, Snitzer [3] proposed to use an optical fibre as the gain
medium and resonant cavity, and later that year he demonstrated it experimentally
[4] using a neodymium-doped barium crown glass fibre. In 1963, Prokhorov [5]
modelled amplification in an optical fibre. That same year, Wolff et al. [6]
demonstrated the first plastic fibre laser. In 1964, Koester et al. [7] 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 [8], who in 1966
first discussed the telecommunication potential of optical fibres [9]. 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 [12]. A breakthrough in the modified chemical
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vapour deposition (MCVD) optical fibre fabrication process allowed the incorporation of rare-earth ions in the core of a preform [254], 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 [13]. 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 [14]:
*
*
*
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.
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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. [15]. 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) [16]. 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) [17]
and Raman fibre amplifiers [18]. 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 [19], 0.8 mm
[20,21], 0.98 mm [22], 1.48 mm [23] and 1.53 mm [24]. 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.
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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 [14].
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 [25].
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 [14]. 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. [25].
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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 [26]. 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.
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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
[27], 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 [28] while ESA is considered by
the inclusion of the excited state pump absorption coefficient aESA :
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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 [28], 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. [28] 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 [29] 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.
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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
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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 [30]. 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 [31].
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 [28]. 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 [14].
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
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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
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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 [32] 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 [33].
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,
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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 [34]. 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%
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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.
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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.
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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 [37].
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,
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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 [38].
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 [39]. 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 [28]. 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
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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 [28]. 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 [40] and can be used in spectroscopy [25,41]. Fibre laser transmitters
have been studied in direct detection digital [42–50] and analog [51] fibre-optic
communication systems. Narrow line width lasers have been used in receivers for
coherent communication system experiments [52]. Tuneable EDFL can also be
applied to the characterisation of WDM fibre-optic components [35,53]. High power
double-clad fibre lasers [54] 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 [55]. Visible fibre lasers [56] can serve in displays or data storage
applications.
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The historical development of erbium-doped fibre lasers (EDFL), especially those
related to optical fibre telecommunication, can be traced as follows. Mears et al. [15],
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. [255] 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. [57].
In 1988, the first coupled-cavity EDFL has been made by Barnsley et al. [58].
Using a Fox–Smith resonator, they demonstrated the first single-longitudinal mode
fibre laser. A year later, Scrivener and his co-worker [59] 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. [62] 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 [62], circularly polarised [63] or to have different optical
frequencies [64]. Finally in 1994, it is Ball et al. [65] 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) [66] 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 [38]. 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.
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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. [65], 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 [74].
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.
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Fig. 15. Block diagram depicting a continuously tuned EDFL in MOPA configuration. Taken from
Ref. [65].
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 [25].
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 [114] or Yao’s controller [115], 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
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Table 4
Principal results published on linear cavity EDFL
Ref.
Tuning
(nm)
Linewidth
Power
(mW)
Pump source
Remarks
[75]
[76]
[77]
[78]
[79]
[80]
[81]
[82]
[83]
[84]
[85]
[86]
[87]
[64]
[88]
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
[89]
[90]
[65]
[91]
[92]
[93]
[94]
[95]
[96]
[97]
[98]
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
[99]
[100]
Possible
Possible
?
25 kHz
3
9.8
924 nm, 110 mW
980 nm, 110 mW
[101]
[102]
Possible
Possible
260 kHz
?
17
0.06
524 nm, 190 mW
980 nm, 25 mW
[103]
[104]
[105]
[48]
[106]
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
[107]
[108]
[109]
[110]
[111]
[112]
[113]
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
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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. [149]. 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 [156]. 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
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Table 5
Principal results published on ring cavity EDFL
Ref.
Tuning
(nm)
Linewidth
Power
(mW)
Pump source
Remarks
[119]
[120]
[121]
[122]
[123]
[124]
[125]
[126]
[127]
[128]
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
[129]
[130]
[131]
[60]
[132]
[133]
[134]
[135]
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
[137]
[136]
[138]
[256]
[139]
[140]
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
[141]
[142]
[143]
[144]
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
[145]
0 possible ?
170 kHz
1.6
1480 nm, 70 mW
[146]
47
6 GHz
3
980 nm, 100 mW
[147]
12.1
8 kHz
0.7
980 nm, 100 mW
[148]
[149]
50
112
o126 kHz
1.2 GHz
7
6
972 nm, 85 mW
980 nm, 180 mW
[150]
[151]
[152]
[153]
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
[154]
[155]
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
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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. [149].
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. [149].
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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. [149].
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 [58] 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 [57].
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Fig. 21. Schematic diagram of a fibre Fox-Smith resonator laser. Taken from Ref. [58].
Table 6
Principal results published on coupled cavity EDFL
Ref.
Tuning
(nm)
Linewidth
Power
(mW)
Pump source
Remarks
[58]
[161]
Possible
60
o8.5 MHz
o1.6 MHz
0.08
?
514 nm, 400 mW
529 nm
[162]
[163]
[164]
[157]
[257]
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
[159]
[160]
[165]
8
Possible
38.4
68 kHz
240 kHz
Singlemode
5.2
5
1
1480 nm, 80 mW
1480 nm, 80 mW
?
[166]
Possible
200 kHz
166
[167]
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.
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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) [168]. 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 [169].
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. [170] 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
[171] 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. [172] 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. [173] 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. [174]
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
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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
3.2.2.1. 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. [175] 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 [176]
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. [177] 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.
3.2.2.2. 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. [180] 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. [181] 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.
[182] 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. [183] 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. [184] 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. [185]. They used an
acousto-optic modulator to prevent the laser from reaching steady-state operation
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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. [177].
(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. [189] 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 [190], and five laser peaks spaced by 1.8 nm with a sampled
fibre Bragg grating (see Fig. 24).
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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. [185].
Fig. 24. Schematic diagram of a nitrogen-cooled multifrequency EDFL. Taken from Ref. [189].
That same year, Yamashita et al. [191] 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. [192] using controlled
polarisation evolution in a ring cavity and liquid nitrogen cooling to enhance
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Table 7
Principal results published on multi-frequency EDFL
Ref.
Range (nm)
(# of lines)
Power/ch. (dB m)
(flatness (dB))
Pump
Cooled/
uncooled
Remarks
[178]
21 (5)
22 (3.3)
Uncooled
GRIN-lens F–P filter
[179]
[183]
29 (6)
2.2 (6)
11 (5.7)
? (1)
Uncooled
Uncooled
[194]
4.5 (3)
? (1.7)
Grating WDM filter
Semiconductor
intracavity F–P etalon
serial FBG linear cavity
[182]
6.4 (4)
12.7 (4.0)
980 mm,
o5 mW
980 nm
980 nm,
31 mW
980 nm,
30 mW
1480 nm
[175]
5.3 (4)
5.6 (5.2)
[181]
5.3 (8)
12 (5.9)
[189]
6.5 (11)
14.1 (10.4)
[191]
13 (17)
21.7 (16.4)
[192]
17.6 (17)
16 (4.2)
[193]
14.8 (29)
23.3 (13.3)
[176]
[180]
22.4 (15)
4.2 (5)
+5 (0.7)
14.8 (7.9)
[184]
18.0 (3)
13.6 (0.9)
[185]
12.6 (4)
+13 (0.2)
[195]
7.0 (4)
7 (o1)
[196]
6.0 (16)
12 (4)
[197]
12.4 (9)
2.7 (9.5)
[198]
9.2 (11)
15.2 (1.6)
[188]
10.4 (14)
3.7 (14.6)
[199]
0.7 (3)
9.5 (o2)
[200]
11.2 (8)
23.5 (5.6)
[201]
11.8 (16)
? (5.4)
[202]
27.2 (6)
11.7 (3.3)
[203]
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
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Table 7 (continued)
Ref.
Range (nm)
(# of lines)
Power/ch. (dB m)
(flatness (dB))
Pump
Cooled/
uncooled
Remarks
[204]
18.1 (4)
5 (0.5)
Uncooled
FBG tree filter
[205]
11.7 (9)
16.0 (6)
980 m,
90 mW
1480 nm
Cooled
[206]
6.4 (6)
20.8 (11)
Uncooled
Spacing-tuneable comb
filter based on PMF
L-band operation
[207]
11.2 (15)
1.8 (13)
Cooled
Single-mode ring EDFL
[208]
30 (4)
2.8 (o1)
Uncooled
L-band operation
[209]
2.1 (3)
7.7 (1.5)
Uncooled
Elliptical core EDF
[210]
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. [193] 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
[215], and in optical sensors [215]. In telecommunication, SFS have been used as
transmitters in spectrum-sliced WDM systems [216–218] and as pump sources in
Raman fibre amplifiers [219]. They are also the preferred source for navigational
grade fibre-optic gyroscopes (FOG) [220]. 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) [221]. The ASE
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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 [222]. 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 [223]. Finally, an effective design
for high output power generation is the double-stage configuration [212] 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
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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. [224]:
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 [224] 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 [225]
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 [226]:
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 [227]. 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 [228]. The other
fluctuation effects are related to temperature and can be expressed by the following
relation [226]:
dn%
qn%
qn%
qPpump
qn%
qnpump
¼
þ
þ
:
dT qT
qPpump
qnpump
qT
qT
O¼
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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 [223].
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.
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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 [223].
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.
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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
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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) [226]
— [223]
7.8 m [223]
10.3 m [223]
7.5 m [231]
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
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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 [229]
36.5 m [227]
—
—
13.0 m [232]
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
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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.
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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 [230]. 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. [233]. 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. [219] 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. [233].
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Fig. 36. Wavelength-tuneable high power SFS for Raman fibre amplifier applications. Taken from
Ref. [219].
Fig. 37. Spectrally flattened broadband SFS. Taken from Ref. [234].
In 2000, Espindola et al. [234] 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.
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Table 10
Principal results published on SFS (* indicates a FWHM bandwidth measurements)
Ref.
Bandwidth (nm)
Power (mW)
Pump
Remarks
[221]
[120]
[222]
[235]
[236]
[225]
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
[226]
[220]
[211]
[84]
[237]
[238]
>20
16*
24
10*
21*
35*
[212]
[223]
[229]
1.4
—
18.8
150
123
26
[218]
[230]
[239]
[232]
[240]
[241]
[242]
[232]
[243]
22*
36
36*
27
—
11
27
4*
36*
110
27
6
18.6
12.5
100
26
1030
770
[244]
[245]
27
83.4*
[219]
B1
[246]
[247]
39
B25–30
[248]
[249]
90*
20
—
180
980 nm, 100 mW
980 nm, 100 mW
[250]
[234]
[251]
[214]
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
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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].
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