COL 9(6), 060604(2011) CHINESE OPTICS LETTERS June 10, 2011
Optimized optical phase conjugation configuration for fiber
nonlinearity compensation in CO-OFDM systems
Lujiao Li (李李李炉炉炉焦焦焦), Yaojun Qiao (乔乔乔耀耀耀军军军)
∗
, and Yuefeng Ji (纪纪纪越越越峰峰峰)
Key Laboratory of Information Photonics and Optical Communications, Ministry of Education,
Beijing University of Posts and Telecommunications, Beijing 100876, China.
∗
Corresp onding author: qiao@bupt.edu.cn
Received November 29, 2010; accepted January 28, 2011; posted online May 12, 2011
The optimized optical phase conjugation (OPC) configuration is proposed for the 40-Gb/s CO-OFDM sys-
tem. The proposed configuration for nonlinear cancellation is systematically depicted in transmission links
with lumped amplification. Numerical simulations are performed to demonstrate effectiveness. Simulation
results show that mid-span spectral inversion (MSSI) can partially compensate for nonlinear distortions.
Moreover, its optimized configuration can further improve system performances and increase nonlinear
comp ensation effectiveness. Compared with MSSI, the maximal Q factor, nonlinear threshold, and trans-
mission distance of optimized OPC configuration increase by over 1.6 dB, 2 dB, and 2 times, respectively.
OCIS codes: 060.2330, 060.4370, 190.3270, 190.5040.
doi: 10.3788/COL201109.060604.
Coherent optical orthogonal frequency division multi-
plexing (CO-OFDM) for long haul and high speed trans-
mission has attracted significant attention
[1,2]
due to its
high spectral efficiency and robust tolerance to linear
impairments, such as chromatic dispersion and polar-
ization mode dispersion
[3,4]
. However, optical OFDM is
highly susceptible to fiber nonlinearity. Several methods
have already been proposed to comp ensate for nonlinear
impairment
[5,6]
, but they are relatively complicated.
The optical phase conjugation (OPC) technology was
proposed by Yariv et al. in 1979 to compensate for
chromatic dispersion
[7]
. Since then, OPC has been
widely used to compensate for nonlinear distortions re-
sulting from the Kerr effect, such as self phase modula-
tion (SPM), intrachannel nonlinear effects, and nonlin-
ear phase noise
[8]
. Simultaneous compensation of non-
linear effects and chromatic dispersion by the OPC has
also been shown in Ref. [9]. All these applications of
OPC are in a single carrier system. Recently, OPC has
been used in OFDM system
[10,11]
. Theoretically, mid-
span spectral inversion (MSSI) can effectively cancel non-
linearity for transmission links with symmetrical power
profile. However, this condition cannot be satisfied in
transmission links using lumped amplification, thus lim-
iting effectiveness of nonlinearity cancellation of MSSI.
In this letter, we propose an optimized OPC
configuration to increase nonlinearity compensation
effectiveness of the OPC in 40-Gb/s CO-OFDM system.
A specific amount of fiber is applied to produce symmet-
rical nonlinear regions distribution with respect to zero
accumulated dispersion. Simulations show that the op-
timized configuration is highly effective in suppressing
nonlinear distortions in 40-Gb/s CO-OFDM system.
Recently, Watanabe et al.
[9]
proposed the conditions
for exact compensation of both chromatic dispersion and
Kerr effect by OPC as follows:
D
s1
L
1
= D
s2
L
2
, (1)
γ
1
P
1
L
1
= γ
2
P
2
L
2
, (2)
where D
sj
(j=1, 2) is the dispersion parameter of stan-
dard single-mode fiber (SSMF) of fiber-j (fiber-1 is the
first fiber placed before OPC and fiber-2 is the second
fiber placed after OPC, as shown in Fig. 1(a)), L
j
is
the length of fiber-j, γ
j
is the nonlinearity coefficient of
fiber-j, and
P
j
is the path-averaged optical power, where
P
1
=
[
∫
0
−L
1
P
1
(z)dz
]
/L
1
and
P
2
=
[
∫
L
2
0
P
2
(z)dz
]
/L
2
and P
j
is the optical power in fiber-j. Equations (1) and
(2) indicate the dispersion and nonlinearity compensa-
tion conditions, respectively
[9]
. However, satisfying the
two conditions in transmission links with odd-span fiber,
lump amplification, and MSSI is difficult. An example
of three-span (SSMF A, B, and C, OPC is placed after
A) is shown in Fig. 1(a). The number of fiber spans,
erbium-doped fiber amplifiers (EDFAs), and the non-
linear regions before OPC are different from those after
OPC. Nonlinear regions are link regions where nonlin-
ear effects become important for pulse evolution. This
corresponds to the nonlinear effective length L
eff
and
rectangles in Fig. 1. Consequently, it is hard to satisfy
Fig. 1. (a) Odd-span transmission link with MSSI; (b) asym-
metrical power profiles with respect to the OPC caused by
p eriodic lump amplification, dashed line shows accumulated
disp ersion distribution along the link; (c) asymmetrical non-
linear regions distribution with respect to zero accumulated
disp ersion D
acc
.
1671-7694/2011/060604(5) 060604-1
c
° 2011 Chinese Optics Letters