COL 9(7), 070604(2011) CHINESE OPTICS LETTERS July 10, 2011
Impact of the distribution pattern of fast polarization
scrambler on the performance of PMD mitigation with FEC
for DQPSK optical system
Dahai Han (韩韩韩大大大海海海)
∗
, Lixia Xi (席席席丽丽丽霞霞霞), Minliang Li (李李李敏敏敏良良良),
Haoran Chen (陈陈陈浩浩浩然然然), and Feifei Liu (刘刘刘菲菲菲菲菲菲)
Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications),
Ministry of Education, Beijing 100876, China
∗
Corresp onding author: dahaihan@gmail.com
Received December 31, 2010; accepted April 11, 2011; posted online June 16, 2011
The combination of fast polarization scrambler (FPS) and forward error correction (FEC) is one of the
metho ds to mitigate the polarization mode dispersion (PMD) in high-speed optical fib er communication
systems. The effect of the different distribution patterns of FPS on PMD mitigation with FEC is investi-
gated. A comparison of the bit error rates (BERs) between two cases where the distributed FPS is absent
and present along the fiber is carried out by simulation. A novel representation called the “ring chart” to
assess the performance of different distribution patterns intuitively is proposed. The results show that the
distribution pattern is an impact factor for this PMD mitigation scheme.
OCIS codes: 060.0060, 230.0230, 250.0250.
doi: 10.3788/COL201109.070604.
The contradiction between the increase in optical trans-
mission rate and the deterioration of the signal caused by
polarization mode dispersion (PMD) has become more
apparent in recent years. To address this challenge, re-
searchers focused on the study of PMD compensation
and mitigation techniques
[1,2]
. The combination of for-
ward error correction (FEC) and distributed fast polar-
ization scramblers (D-FPSs) as one of the PMD mitiga-
tion schemes has been proposed and presents a remark-
able effect
[3,4]
. Compared with general PMD compen-
sation schemes, this combination scheme shows the ad-
vantage of all-channels simultaneous mitigation and re-
quirement for less time owing to the absence of feedback
control
[5]
. Optical system margin can be expanded when
FEC codes are regarded as a portion of it, while PMD
tolerance for a given PMD penalty cannot b e enlarged as
the number of errors induced by PMD is beyond the er-
ror correction ability of the FEC
[6]
. PMD tolerance can
be increased to a certain extent when the FPS is used in
conjunction with FEC
[4,7]
.
The differential group delay (DGD) is changing over
the fiber link because of the stochastic feature of PMD;
therefore, the FPS distribution pattern demonstrated
in previous studies, which is typically uniform distribu-
tion, also has effect on the PMD mitigation performance.
In this letter, various distribution patterns of FPS for
differential quadrature phase-shift-keying (DQPSK) sys-
tem are investigated, and a novel representation is used
to clarify this effect.
The role of FPS in the fiber link is to accelerate the
convergence of DGD’s probability density function to
Maxwellian distribution, instead of decreasing the nu-
meric value of PMD, thus PMD-induced consecutive er-
rors whose length exceeds the burst error correctable
length (BECL) of FEC can be inhibited effectively
[5,8]
.
In our simulation, a structure of λ/4, λ/2, and λ/4 wave
plates cascaded sequentially comprises the FPS, whose
three cells are all angle-adjustable and retardation-fixed
plates
[9]
. The Jones matrices of this FPS can be denoted
as
U
p
= M(λ/4) · M(λ/2) · M(λ/4)
= − cos α · cos β · σ
0
+ i · sin α · cos β · σ
2
− i · sin β · sin γ · σ
3
− i · sin β · cos γ · σ
1
, (1)
where M(λ/4) and M(λ/2) are the Jones matrices of the
quarter wave plate and the half wave plate, respectively;
σ
i
(i = 0,1,2,3) is the Pauli spin matrices, as defined in
Ref. [10]; α, β, and γ are donated as
α = θ
1
− θ
3
, β = 2θ
2
− (θ
1
+ θ
3
), γ = θ
1
+ θ
3
, (2)
where θ
1
, θ
2
, and θ
3
are the azimuths of three wave plates,
respectively. Each azimuth varies sinusoidally, and there
is a slight distinction between each two frequencies of the
drive voltages so the FPS can scramble over the entire
Poincar´e sphere.
θ
1
=
π
2
sin (2πf · t) , (3a)
θ
2
=
π
2
sin [2π (f + ∆f) · t] , (3b)
θ
3
=
π
2
sin [2π (f + 2∆f) · t] , (3c)
where f is the fundamental frequency for the three wave
plates and ∆f is the unit distinction between each two
frequencies.
Scrambling speed is a key factor for the improvement
of the system performance. Optimal scrambling speed
should be chosen to achieve moderate PMD tolerance.
According to the recommendation in Ref. [5], the ap-
propriate scrambling speed ranges from B/BECL to
B/(8·d), where B is the bit rate, and d is the depth of
interleaving. As Reed-Solomon (RS) encoder (255, 239)
is used in our simulation and the interleaving depth is
16 symbols (i.e., 128 bits), the BECL equals 1,024 bits;
1671-7694/2011/070604(4) 070604-1
c
° 2011 Chinese Optics Letters