COL 10(7), 070604(2012) CHINESE OPTICS LETTERS July 10, 2012
Gain equalization of EDFA using a loop filter with
a single polarization controller
Kejiang Zhou (±±±ôôô)
1
, Shuming Pan (ÓÓÓ²²²)
1
, Nam Quoc Ngo
2
, and Xulin Zhang (ÜÜÜRRR)
3∗
1
Department of Electronic Engineering, Zhejiang University, Hangzhou 310027, China
2
School of Electrical and Electronic Engineering, Nanyang Technological University, 639798, Singapore
3
College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, China
∗
Corresp onding author: zxlin@szu.edu.cn
Received November 7, 2011; accepted January 18, 2012; posted online March 28, 2012
A Sagnac lo op filter with two pieces of high birefringence fiber having equal lengths, and spliced together
at a fixed angle of 30
o
displacement between the two principle axes, is proposed in this letter. Gain
equalization of erbium-doped fiber amplifiers (EDFAs) is implemented by tuning only the polarization
controller in the loop filter. Experimental result shows that there remains a deviation of ±1 dB in the
region of the flattened profile with the useful bandwidth of about 23 nm, thereby demonstrating the
effectiveness of the method. An effective mathematical model and mechanism is also given for further
explanation.
OCIS codes: 060.2320, 060.2280, 260.1440.
doi: 10.3788/COL201210.070604.
Gain equalization of erbium-doped fiber amplifiers (ED-
FAs) is a critical issue for wavelength-division multiplex-
ing (WDM) communication systems
[1]
. This is because
gain un-equalization of the amplifier creates undesirable
non-linear effects, such as the cross and self-phase mod-
ulation, wave mixing, Brillouin and Raman scattering,
and signal overloading and crowding out by noise, all
of which can potentially hamper the performance of the
WDM system. In general, the gain profile of EDFAs can
be flattened by modifying the material composition in
the erbium-doped fiber (EDF), or by using optical filters
to compensate for the variations in the gain spectrum.
Various kinds of optical filters are demonstrated for the
application, including the long-period gratings, fiber
acousto-optic tunable filters, and fiber Bragg gratings
(FBGs) technologies
[2−9]
. Different from other interfer-
ometers, such as the Mach-Zehnder, the light intensity
transfer function (TF) of the Sagnac interferometer is in-
dependent of the input polarization state if there are no
non-reciprocity components and polarization-dependent
components used
[10]
. Fang et al.
[11]
have reported a
Sagnac loop with high birefringence (Hi-Bi) fiber, which
can function as a filter that can be used as a wavelength-
division multiplexer. This Sagnac lo op can also be used
in fiber lasers
[12,13]
. Li et al.
[14]
have employed this type
of lo op to the gain equalizer and a circulator in the fiber
amplifier, using two independent polarization controllers
(PCs) and two indep endent segments of Hi-Bi fiber. In
using this type of loop, there are too many structure
parameters that must be adjusted to adapt the demand
of EDFAs with high gain equalization.
In this letter, we demonstrate the construction of a
Sagnac interferometric loop with only one PC and two
segments of Hi-Bi fiber spliced together as an EDFA
gain-flattening filter. The two Hi-Bi fib ers have equal
lengths and are coupled at a fixed angle of 30
◦
displace-
ment between the two principle axes, making them easier
to adjust which, in turn, results in high performance ex-
perimentally. In our experiment, the output beam is
from the transmission port of the loop, instead of the
circulator or isolator. Different from the reflective port
of the loop, the transmission port may not induce the
lasing oscillation without the circulator or isolator.
The portion with the dotted lines shown in Fig. 1(a)
demonstrates the Hi-Bi loop that serves as an optical
filter. The principle of the Hi-Bi fiber loop filter is
as follows. The input beam from a 3-dB 50:50 cou-
pler made from a single-mode fiber (SMF) is split into
two counter-propagating beams, each of which is decom-
posed into four beams after traveling through the two
Hi-Bi fiber sections that are effectively two polarimetric
interferometers connected in a series. The input polar-
ization state of the light into these interferometers can
be controlled by adjusting the PC. The intensity of the
light coupled back to the output depends on the phase
differences among the interfering beams in terms of the-
birefringence, the length of the Hi-Bi fiber sections, and
the couple angle between the two sections of Hi-Bi fiber.
In general, a Hi-Bi fiber loop filter with n-sections can
produce a spectrum of a 2
n
-beam interferometer. When
the lengths of the Hi-Bi fiber are chosen appropriately,
the transmission spectrum of the Hi-Bi fiber loop filter
can be matched with the EDFA gain profile to produce
an overall gain flattening. A photograph of the EDFA
gain equalization experimental set up used in this letter
is shown in Fig. 1(b).
The typical EDFA gain profile generated from a 15-m
EDF pumped at 88 mW was used in the experiments.
The gain peak at the wavelength of around 1 530 nm
had a gain of 12 dB above that of the flat portion from
1 535 to 1 560 nm, which was difficult to flatten. Ide-
ally, if the gain can be flattened, the entire EDFA gain
bandwidth can be utilized to realize light amplification,
and the useful gain bandwidth of the EDFA spectrum
is normally around 36 nm, ranging from 1 523 to 1 559
nm. Figure 2 shows the ideal filter gain profile that
allows a good equalized output over the broad gain spec-
trum of the EDFA. The equation of the gain profile of the
1671-7694/2012/070604(4) 070604-1
c
° 2012 Chinese Optics Letters
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