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Cladding Pumped Multi-core Fiber Amplifiers for
Space Division Multiplexing
Sophie LaRochelle
Center for Optics, Photonics and
Lasers (COPL), Department of
Electrical and Computer
Engineering, Université Laval
Québec, Canada
[email protected]
Cang Jin
Specialty Photonics Division
OFS Fitel
Somerset, NJ, USA
[email protected]
Charles Matte-Breton
Center for Optics, Photonics and
Lasers (COPL), Department of
Electrical and Computer
Engineering, Université Laval
Québec, Canada
[email protected]
Younès Messaddeq
Center for Optics, Photonics and
Lasers (COPL), Department of
Physics, Optics and Engineering
Physics, Université Laval
Québec, Canada
[e-mail]
Abstract—We review the challenges of cladding pumped
multi-core optical fiber amplifiers for application to space
division multiplexing (SDM). Through numerical simulations, we
investigate two fiber designs: the first one with a uniform
cladding and the second one with an annular cladding to guide
the pump. We compare the multi-core amplifier gain, noise
figure and pumping efficiency in a WDM scenario. We present
the fabricated fibers and summarize experimentally measured
performance that shows gain>20 dB and NF<6 dB over the whole
C-band. Finally, we examine scalability of the annular cladding
design.
Keywords—Erbium-doped fiber amplifier (EDFA), optical
amplifier, space division multiplexing (SDM), multi-core amplifier,
spatially integrated amplifier, optical fiber, optical communications
I. INTRODUCTION
Nowadays, optical fiber communication systems are facing an
enormous challenge as the needed capacity increases
exponentially putting pressure not only on long-haul systems
but also on access networks and data communication links.
This high bandwidth demand is driven by the unstoppable
thirst for high definition video streaming, cloud computing and
various big data applications processed in data centers.
Wavelength division multiplexing (WDM) and polarization
division multiplexing (PDM), as well as advanced modulation
formats, were introduced to significantly improve single mode
fiber (SMF) capacity. Despite these efforts, it is now forecasted
that this capacity will saturate in the forthcoming years [1, 2].
The capacity limits of SMF, estimated from Shannon’s
information theory [3], is approximately 35-70 Tb/s for a
waveband of 1530 nm to 1625 nm, i.e. C-band plus L-band.
The calculation takes into account a spectral efficiency limit of
3-6 bit/Hz due to nonlinear effects [4]. Consequently, there is a
growing gap between the data transmission demand and the
capacity offered by state-of-the-art technologies, which
motivates the development of new multiplexing strategies [1].
Inspired by wireless communications, space division
multiplexing (SDM) is now making its way in optical fiber
communications as a means to scale its capacity. SDM is
implemented using either multi-core fiber (MCF) [5], few-
mode fiber (FMF) [6], and even multi-core few-mode fiber [7].
In this paper, we consider single mode multi-core fibers that
straightforwardly multiply the capacity by the number of cores
provided that inter-core cross-talk is negligible. To this end,
transmission fibers are usually designed with a core-to-core
spacing of tens of m and dissimilar cores to reduce coupling.
Another approach is to use an index trench to further confine
the modes. SDM has thus been successfully demonstrated in
combination with WDM, PDM and advanced modulation
formats to produce impressive aggregate transmission rate
exceeding 10 Pb/s in a single fiber strand [8]. However, most
transmission results were obtained over short distances
corresponding to one span because of the lack of suitable
multi-core erbium-doped fiber amplifiers (MC-EDFAs) or by
demultiplexing spatial channels before amplification [9]. An
efficient in-line MC-EDFA is undoubtedly critically needed in
order for SDM to find application in future optical network.
The first MC-EDFA, demonstrated by Abedin et al.,
consisted of seven cores each pumped by a 980 nm laser [10].
Following this work, cladding pumping was introduced in
order to reduce the complexity of the amplifier
implementation. Figure 1 shows pumping configurations with
in-core pumping, edge-coupled cladding pumping and side-
coupled cladding pumping. In Fig. 1a, the multiple lasers could
be replaced by a single pump laser with a power splitter to
share the pump between all branches. However, only cladding
pumping allows the use of a low-cost high-power multimode
laser diode, and only side-coupled cladding pumping allow in-
line amplification without spatial demultiplexers/multiplexers
at the MC-EDFA input/output. Since these components
typically introduce extra cross-talk and degrade noise figure,
side-coupled cladding pumping therefore appears as the
preferred solution.
Fig. 1 Pumping configurations of multi-core erbium-doped fiber amplifiers
showing a) in-core pumping, b) edge-coupled cladding pumping, and c) side-
coupled cladding pumping.

A well-known limitation of cladding pumped amplifiers is
the low pumping efficiency resulting from the small pump
mode field overlap with the doped cores. Many studies on
cladding pumped amplifiers are therefore presently directed at
improving pump utilization while simultaneously optimizing
amplifier configuration to reduce complexity and related
devices costs [12]. Low pump absorption results from the small
aggregate core area with respect to the large pump cladding
area (assuming a uniformly distributed pump power). Fig. 2a
shows such a configuration with a uniform cladding and six-
doped cores. In Fig. 2b, we show the proposed configuration
that has an annular cladding to confine the pump in a smaller
area and in which signal cores are also enlarged. Alternate
proposal to improve pump efficiency include, for example, the
introduction of erbium-ytterbium which does however limit the
gain in the short wavelength side of the C-band [13]
Fig. 2 Multi-core amplifier, with six-doped cores, cladding pumped in a) a
standard uniform double cladding and b) an annular cladding with a depressed
refractive index central region (inner cladding) and enlarged signal cores. The
outer cladding is made of a low index polymer.
In this paper, we discuss MC-EDFA designs optimized
through numerical simulations and compare the performance of
three cladding configurations. We summarize previously
published characterization results of gain and noise figure (NF)
across the C-band. Finally, we discuss the scalability in terms
of number of cores in the proposed annular cladding
configuration.
II. MODELING AND SIMULATIONS
The MC-EDFA, with co-propagation cladding pumping at 980
nm, is modeled using standard rate and propagation equations
as described in [11, 14, 16]. Signal cores are assumed to be
independent and, in comparison to a single core amplifier, the
only modification is the pump propagation equation:
p
1 a,p p
core core
clad
with .
p
p
dP z N P z
dz
N A
A
In (1), Pp is the pump power, z the propagation distance, N1 the
erbium ion population of the ground state, and a,p the
absorption cross-section at 980 nm. The pump overlap factor,
p, takes into account the absorption by all cores and,
therefore, is function of the number of cores Ncore, the area of
the individual cores Acore and the area of the cladding, Aclad, in
which the pump is confined. When neglecting amplified
spontaneous emission and assuming no input signal, the
inversion of the erbium doped fiber, , can be estimated from
p
2
p p a,p
clad
clad
P A
N
P A h
where N2 is the erbium ion population of the metastable level,
is its lifetime, and hp the energy of a pump photon. With the
values shown in Table 1 and a uniform cladding having a
110 m diameter, requires Pp1.0 W
while requires Pp8.5 W. Both (1) and (2) therefore
show the importance to reduce Aclad and increase Acore in order
to limit pump power requirements.
TABLE I. FIBER AND SIMULATIONS PARAMETERS
Description Symbol Value
Core-to-core pitch
Number of cores
Refractive index of the pump
cladding (silica)
Refractive index of the double
cladding (polymer)
Numerical aperture of the pump
waveguide
Pump absorption cross-section
Lifetime of metastable level
Pump wavelength
Central signal wavelength
Number of channels
Frequency spacing of channels
Signal power
Total signal power
Ncore
nclad
npoly
NApump
a,p
p
s
N
s
Ps
P
s
,tot
40 m
6
1.444
1.37
0.46
2.18 10-25 m2
10 ms
980 nm
1550 nm
8
500 GHz
-12 dBm
-3 dBm
A. Signal core optimization
Table 1 shows the parameters used for the MC-EDFA
optimization. The geometry is a double cladding fiber with six
cores placed along a ring with a 40 m pitch. The silica
cladding is surrounded by a low index polymer resulting in a
pump waveguide with 0.46 numerical aperture (NApump). The
fiber parameters that need to be determined are the core radius
(rcore), and numerical aperture (NAcore), as well as the erbium
ion concentration ( ) and fiber length (L). The erbium ion
doping is assumed to be uniform in the core and the absorption
and emission cross-sections are presented in Fig. 3. The MC-
EDFA gain is calculated using the WDM scenario detailed in
Table 1: eight CW wavelength channels, spaced by 500 GHz
and with input power of -12 dBm/channels. Figure 4 shows the
average gain of the wavelength channels as a function of rcore
and NAcore. The calculations are done for a = 2.5x1025 m-3,
L=2 m and Pp=15 W. The single-mode region is limited by the
dotted red line labeled 2.405, which is the cut-off condition of
the LP11 mode. The star corresponds to the chosen design with
rcore=5 m and NAcore=0.105. Fig. 5 shows the amplifier gain as
a function of and L where it can be shown that shorter fibers
require higher erbium concentrations to maximize gain. A
concentration of = 2.5x1025 m-3 was chosen to keep fiber
length short and minimize cross-talk in future amplifier designs
that would have a large number of cores and reduced core-to-
core pitch.

Fig. 3 Absorption and emission cross-sections in the C-band of an erbium-
doped fiber used for the MC-EDFA simulations.
Fig. 4 Simulated average gain of the eight channels as a function of core
radius and core numerical aperture for a fiber length of 2 m, an erbium ion
concentration of 2.5x1025 m-3, and an injected pump power in the cladding of
15 W.
B. Cladding design
In this work, we consider three different fibers: Fiber 1 with a
uniform cladding (Fig. 1a), Fiber 2 that has an inner cladding
of a lower refractive index than the silica cladding and Fiber 3
that has air as the inner cladding. As represented in Fig. 2b,
both Fiber 2 and Fiber 3 create an annular cladding that
increases p. A key parameter in the design of these annular
cladding is the distance, dmin, between the core boundary and
the silica cladding-to-polymer interface (outer cladding in Fig.
2). If the cores are too close to this interface, there can be
confinement or leakage loss to the polymer coating. This
problem was investigated in [15] for multi-core transmission
fibers. In the present case, since the doped fiber is only a few
tens of meters long and the outer polymer coating has a lower
index than the silica cladding, this loss is expected to be
negligible. Nonetheless, the proximity of the cladding can also
cause the waveguide to become birefringent. Figure 6 shows
the birefringence of the fundamental mode (calculated using
COMSOL) as a function of the core position in a 110 m
diameter silica cladding of for Fiber 1, and with an inner
cladding diameter of 50 m for Fiber 2 and Fiber 3.
Considering that fabrication constraints limit dmin to 10 m, the
calculated birefringence is lower than 1 10-6 in all cases.
Fig. 5 Simulated average gain of the eight channels plotted for different fiber
lengths as a function of the erbium ion concentration, for rcore=5 m,
NAcore=0.105 and Pp=15 W.
Fig. 6 Fiber birefringence calculated as a function of the core position in the
silica cladding assuming a uniform cladding (Fiber 1, black), a solid glass
annular cladding (Fiber 2, red) and an annular cladding with a hollow center
(Fiber 3, blue). The x-axis is the radial distance from the fiber axis.
TABLE II. FIBER DESIGN
Description symbol value
Core radius
Core numerical aperture
Pump cladding diameter
Core-cladding distance
Inner cladding diameter
Erbium ion concentration
Erbium doped fiber length
rcore
NAcore
dclad
dmin
dinner
L
5 m
0.105
110 m
10 m
50 m
2.5x1025 m-3
2 m
The gain and noise figure (NF) of the amplifier were calculated
using the design summarized in Table 2 and the initial
parameters indicated in Table 1. The pump power is assumed
to be uniformly distributed in the whole cladding area for Fiber
1, and limited to the annular cladding region for Fiber 2 and
Fiber 3. The results, shown in Figure 7, indicate that the
average signal gain and NF of the eight WDM channels are
respectively >24 dB and <3.5 dB when a pump power greater
than 8.5 W is injected in the cladding.

Fig. 7 Simulated a) gain and b) noise figure. Values are the average of the eight
channels as a function of the injected pump power in the cladding for Fiber 1
(uniform cladding, red), as well as Fiber 2 and Fiber 3 (annular cladding,
black).
III. EXPERIMENTAL RESULTS
For each multi-core erbium doped fibers, three preforms were
fabricated by modified chemical vapor deposition (MCVD).
The erbium ions were inserted by solution doping. Etching of
the cladding was then performed and the resulting thinned rods
were assembled in a stack (Fig. 8a) that was later drawn into an
optical fiber. Before assembly, each preform was cut in half to
make two cores. Scanning electron microscope images of Fiber
1, Fiber 2 and Fiber 3 are shown in Fig. 8b-d. Fiber
characterization showed a good correspondence with design
values, except for the inner cladding diameter that was slightly
larger at 60 m. It is apparent in Fig. 8c and Fig. 8d that the
cores are closer to the inner cladding boundary. Also, the
erbium ion concentration was slightly higher at 2.8x1025 m3.
The pump overlap was p=0.05 for Fiber 1 and p=0.07 for
Fiber 3. For Fiber 2, p is difficult to evaluate as some pump
power is leaking into the inner cladding.
Following fiber fabrication, MC-EDFAs were assembled
and characterized by injecting the pump light in the cladding
through a tapered coreless fiber wrapped around the cladding
[17]. The gain and noise figure were measured by injecting
eight CW wavelength channels spaced by approximately by
Fig. 8 a) Stack assembly for the fabrication of Fiber 2 and scanning electron
microscope image of b) Fiber 1, b) Fiber 2, and c) Fiber 3.
4 nm and covering the C-band. Isolators were placed at the
fiber input and output. One core was characterized at the time
using translation stages to butt coupled the comb source at the
input and send the output to an optical spectrum analyzer. Due
to the large pump power remaining after the 2 m amplifier
length, a short section of the cladding, near the fiber output,
was placed in an etching paste to extract the pump from the
cladding and to send it to a pump dump. The pump input and
output coupling schemes resulted in short sections of the multi-
core erbium-doped fibers being left unpumped, which most
probably degraded the performance.
Fig. 9 Gain and noise figure measurement for the eight channels in the cores of
the three fibers. Measurements are performed one core at the time and the data
shows the average gain (symbol) and the core-to-core variations (minimum and
maximum values defined by the error bars).
Figure 9 summarizes the characterization results while the
detailed measurements can be found in [18,19]. For
characterization of Fiber 1, the signal power was -11
dBm/channel (total input power of -2 dBm), while for Fiber 2
and 3 it was – 12 dBm/channel (-3 dBm total) (due to
experimental constraints as these experiments were performed
several months apart). The pump power was also slightly
different at 13 W of pump power injected in the cladding for
Fiber 1 and 15 watt for Fiber 2 and Fiber 3. In Figure 9, the
data indicate the average gain and NF of the six cores of each

fiber, while the error bars indicate the core-to-core variations
(minimum to maximum values). Results show a 3 dB gain
flatness over the C-band with less than 2 dB core-to-core gain
variations. The core-to-core variation of the NF is less than 1
dB and ranges from 5.5 dB to 3.5 dB over the C-band.
IV. DISCUSSION AND CONCLUSION
Multi-core cladding-pumped erbium doped fibers can be
designed to meet system requirements. Cladding pumping
reduces the component count and allow the use of low-cost
high power laser source. However, efficient use of pump
power requires careful cladding and core design to maximize
pump overlap. In core-pumped erbium-doped fiber designs,
the core area is typically reduced to increase signal and pump
overlap with the doped region. However, in the present case of
cladding pumping, the signal core area should be enlarged as
much as possible while remaining in the single mode regime.
Large cores contribute to pumping absorption and reduced
amplifier saturation by lowering signal intensity. Erbium
doping could further be placed at the edge of the core, or even
in the cladding in order to reduce saturation [20, 21]. Annular
cladding is a solution that reduces cladding area while
managing cross-talk since each cores only has two neighbors.
Although in this design we eliminate the central core, that is
present in many designs, such an annular cladding with a 170
m external diameter would allow the placement of 16 cores
along a ring with core-to-core pitch close to 30 m. With a 25
m annular cladding thickness, the pump overlap factor would
be approximately p=0.1. Each of these cores would have only
two next nearest neighbor. The worst case cross-talk between
the cores can be calculated using coupled-mode equations
describing coupling between identical cores. With identical
cores and 30 m core pitch, there could be significant cross
talk. However, the coupling is easily reduced using dissimilar
cores without penalties to the amplification properties. Finally,
the amplifier shows good NF and gain across the C-band. The
performance, particularly the NF, should be improved by
reducing the input coupling loss resulting from the pump
injection technique and eliminating the unpumped fiber
sections at both ends.
ACKNOWLEDGMENT
We acknowledge the help of Mr S. Morency and N.
Grégoire for the optical fiber fabrication. We are also thankful
to H. Chen, N. Fontaine, R.-J. Essiambre, and R. Ryf of Nokia
Bell Labs as well as B. Huang and G. Li of CREOL,
University of Central Florida for their collaboration on the
characterization of the optical amplifier.
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