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COL 10(Suppl.), S10609(2012) CHINESE OPTICS LETTERS June 30, 2012
Mth-power-law method to compensate laser linewidth of
100 Gb/s PDM-CO-OFDM systems
Xuejun Liu (
444
ÆÆÆ
)
1∗
, Haiying Liang (
ùùù
°°°
===
)
2
, Bo Dai (
ÅÅÅ
)
1
, and Bo Lan (
777
ÅÅÅ
)
1
1
Information Engineering Institute, Beijing Institute of Petrochemical Technology, Beijing 102617, China
2
Beijing Research Institute of Automation for Machinery Industry, Beijing 100120, China
∗
Corresponding author: liuxuejun.china@gmail.com
Received December 31, 2011; accepted March 2, 2012; posted online June 20, 2012
Laser linewidth is the important determinant of applying coherent optical orthogonal frequency division
multiplexing (CO-OFDM) in optical transmission systems. The laser linewidth impairments in 100-Gb/s
polarization division multiplexing CO-OFDM (PDM-CO-OFDM) system without optical dispersion com-
pensation are compensated by the phase drift compensator (PDC) based on M th-power-law method located
at the receiver. PDC is more effective to compensate the phase drift due to laser linewidth. Simulating
results show that the maximum Q factor can be increased by almost >10.0 dB for back-to-back (BtoB).
For the 100-kHz linewidth system of 800-km system, a benefit of about 4.9 dB is possible for the maximum
Q factor.
OCIS codes: 060.4510, 030.1640.
doi: 10.3788/COL201210.S10609.
Orthogonal frequency division multiplexing (OFDM) is a
multicarrier transmission technique where a data stream
is carried with many lower-rate subcarrier tones. Co-
herent optical OFDM (CO-OFDM) brings to optical
communication, which is the combination of two pow-
erful techniques, coherent optical detection and OFDM
technique
[1]
. Together with digital coherent detection,
CO-OFDM brings similar benefits such as high spec-
tral efficiency and high receiver sensitivity as coher-
ent transmission
[2]
. Polarization division multiplexing
(PDM) is a very effective method for doubling spec-
trum efficiency of a transmission system. However, it has
been shown that direct-detected PDM has a reduced tol-
erance to polarization mode dispersion (PMD), because
the polarization demultiplexing introduces cross talk be-
tween the polarization tributaries
[3]
. An elegant way to
overcome this PMD sensitivity is by using polarization
diverse coherent detection with digital equalization in-
stead of direct detection
[3]
. PDM-CO-OFDM has en-
abled the demonstration of various single-carrier multi-
ple input multiple output (MIMO) experiments with bit
rates up to 111 Gb/s and a superior tolerance toward
chromatic dispersion and PMD
[4]
. In contrast to the
conventional design, the CO-OFDM systems do not use
any dispersion compensation fiber
[5]
, therefore 100-Gb/s
PDM-CO-OFDM without optical dispersion compensa-
tion is simulated on a long-haul transmission link in this
letter.
Because coherent detection is employed in PDM-CO-
OFDM, it is important to investigate the optical phase
noise impact due to laser linewidth on system perfor-
mance and explore the optimal approach to estimate
phase evolution of OFDM signals
[6]
. In contrast to
conventional single carrier optical transmission systems,
OFDM systems transmit a data stream over a number
of lower rate subcarriers simultaneously. This opens an
opportunity to employ advanced phase estimation tech-
niques. Recently some studies, such as M th-power-law
method, have been done on laser linewidth phase estima-
tion and compensation at the receiver of 8
[6]
and 10-Gb/s
CO-OFDM
[7]
.
In this letter, we conduct simulation analysis on the
phase drift compensator (PDC) based on Mth-power-law
method at the receiver to compensate for phase impair-
ments originated from laser linewidth on 100-Gb/s PMD-
CO-OFDM systems. We systematically study PDC
against laser linewidth and the simulation results how
that the PDC is very effective to compensate those phase
drift in 100-Gb/s systems.
Phase noise upon optical OFDM signal reception af-
ter fast Fourier transform (FFT) consists of two com-
ponents: a random noise term that can be modeled as
additive Gaussian noise with zero-mean, and a common
deterministic term originated from laser phase drift that
affects all the subcarriers uniformly
[6]
. The thrust of
phase compensator for laser linewidth is to extract laser
phase drift from noisy received signals. Since the OFDM
symbol used has tens of nano second and the laser sources
both have generally about 100-kHz linewidth, which is
close to the value achieved with commercially available
external-cavity semiconductor lasers
[5]
, the phase drift
due to linewidth within one OFDM symbol can be con-
sidered as constant and common to all the subcarriers
[6]
.
The channel model for the kth subcarrier in the ith
symbol in PDM-CO-OFDM systems is given by
[8]
~
r
′
ik
= e
jφ
D
(k)
· e
jφ
k
· H
k
·
~
t
ik
+
~
n
ik
, (1)
where
~
t
ik
= (t
x
ik
t
y
ik
)
t
and
~
t
′
ik
= (r
′x
ik
r
′ y
ik
)
t
are the trans-
mitted and received information symbol in the form of
the Jones vector for the kth subcarrier in the ith OFDM
symbol,
~
n
k
(n
x
ik
n
y
ik
)
t
is the noise including two polariza-
tion components. H
k
is the 2 × 2Jones matrix for the
fiber link representing linear channel effects, φ
D
(f
k
) is
the phase dispersion owing to the fiber CD. φ
k
is the
OFDM symbol phase noise expressed as
φ
k
= φ
G
+ φ
D
, (2)
1671-7694/2012/S10609(4) S10609-1
c
2012 Chinese Optics Letters