May 10, 2008 / Vol. 6, No. 5 / CHINESE OPTICS LETTERS 327
Design of Raman-parametric f iber amplifier for
wavelength division multiplex transmission system
Xiaohong Jiang (
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), Chun Jiang (
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), and Xiaoming Zhang (
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State Key Laboratory of Advanced Optical Communication Systems and Networks,
Shanghai Jiao Tong University, Shanghai 200240
Received December 14, 2007
We optimize the novel configuration of a hybrid fiber amplifier — Raman assisted-fiber-based optical
parametric amplifier (R-FOPA), in which the parametric gain and Raman gain profiles are combined to
achieve a flat composite gain profile. The pump powers and the fiber length in the hybrid amplifier are
effectively optimized by genetic algorithm (GA) scheme. The optimization results indicate that the R-
FOPA can achieve a 200-nm flat bandwidth spectrum with the gain of 20 dB and ripple of less than 4 dB.
OCIS codes: 190.4970, 290.5860, 999.9999.
As wide-band optical fiber amplifiers are indispensable
devices for the further development of large-capacity op-
tical netwo rks, various r esearches have been conducted
to propos e ideal amplifier configurations. Fiber Raman
amplifier (FRA) and fiber-based optical parametric am-
plifier (FOPA) have a ttracted much research interest for
their low noise figure and flexible center wavelength
[1,2]
.
However, it is quite complex in allocating wavelength di-
vision multiplex (WDM) pump powers at different wave-
lengths for FRA to obtain a broad bandwidth, and a
single-section FOPA has an apparent gain imbalance
across a large freq uency range. In response to these lim-
itations, a multi-section design has been proposed
[3−5]
,
which can efficiently widen and flatten the gain spectrum
by using several highly nonlinear fibers and ultra-flat dis-
persion fibers with different zero dispersion wavelengths
(ZDWs) and in different lengths.
In this pape r, we optimize the novel design of fiber
amplifier — Raman assisted-FOPA (R-FOPA), in which
the parametric gain and Raman gain profiles are com-
bined to achieve a wide and flat gain spectrum. Genetic
algorithm (GA), which has been prove d s ucc e ssfully in
optimal design of both FRA and FOPA
[4,6]
, is extended
to effectively optimize the pump powers for the hybrid
amplifier to achieve a flat co mposite gain profile.
Based on the four-wave mixing theory, the gain of
FOPA is calculated by
[7]
G
OPA
= 1 +
γP
OPA
g
OPA
sinh (g
OPA
L)
2
, (1)
where γ is the nonlinear coupling coefficient, L is the
fiber length, P
OPA
is the parametric pump power, g
OPA
is the parametric gain coefficient determined by
g
2
OPA
= −∆β ·
∆β
4
+ γP
OPA
, (2)
in which the small propagation mismatch ∆β is expressed
by
∆β = −
2π
λ
2
0
dD
dλ
(λ
p
− λ
0
) (λ
p
− λ
s
)
2
, (3)
where λ
p
, λ
s
, λ
0
are the wavelengthes of pump, signal,
and zero-dispersion respectively, and dD/dλ is the slope
of the dispersion at λ
0
. When we define k = 2π/λ
2
0
·
dD /dλ · (λ
p
− λ
0
), ∆β can be rewritten as k · (λ
p
− λ
s
)
2
.
The gain of FRA is based on the stimulated Raman
scattering (SRS) in the fiber , and the most important of
which, for the purpose of present consideration, are the
pump-to-pump and pump-to-signal SRS. In steady sta te,
this gain profile is described by
[8]
G
Raman
= exp
g
Raman
P
Raman
L
A
eff
, (4)
where P
Raman
is the pump power, A
eff
is the fiber
effective a rea, and g
Raman
is the Raman gain coefficient.
g
Raman
used in the following calculation referres to Fig. 1
in Ref. [8]. The bandwidth for a single FRA is limited by
the Raman shift, which is up to 170 nm for the telluride-
based fiber (maximum g
Raman
= 55 W
−1
·km
−1
) and 100
nm for the silica-bas e d fiber (maximum g
Raman
= 3.5
W
−1
·km
−1
).
Both FOPA and FRA have the advantage that the
center wavelength of the amplification bandwidth is de-
pendent on λ
p
and their ga in shapes can be controlled by
the use of multiple pumps. Mor e over, the gain profiles of
one-pump FOPA and FRA have differe nt peak features,
which c an be utilized to complement each other in a
hybrid structure.
Here we present an optimized des ign of R-FOPA whose
schematic configuration is shown in Fig. 1. The hybrid
amplifier is forward-pump e d by a parametric pump laser
diode (LD) and back-pumped by one-above Raman pump
LDs. The fiber, as the amplification medium, can be
Fig. 1. Schematic configuration of R-FOPA.
1671-7694/2008/050327-04
c
2008 Chinese Optics Letters