194 CHINESE OPTICS LETTERS / Vol. 7, No. 3 / March 10, 2009
Robustness estimation of software-synchronized all-optical
sampling for fiber communication systems
Aiying Yang (
OOO
===
)
∗
, Xiangyu Wu (
ÇÇÇ
), and Yu’nan Sun (
HHH
)
Department of Opto-Electronic Engineering, Beijing Institute of Technology, Beijing 100081
∗
E-mail: yangaiying@googlemail.com
Received August 27, 2008
The robustness of the software-synchronized all-optical sampling for optical performance monitoring is
estimated for 10-Gb/s fiber communication systems. It reveals that the software-synchronized algorithm
is sensitive to the signal degradation caused by chromatic dispersion and nonlinearity in optical fibers.
The influence of timing jitter and amplitude fluctuation of the sampling pulses is also investigated. It
is found that stringent requirements are imposed on the quality of the sampling pulse and the tolerance
of 1-dB Q penalty is measured. Considering the practically available optical sampling pulse sources, the
results indicate that the amplitude fluctuation of the sampling pulses has the dominant impacts on the
software-synchronized method.
OCIS codes: 060.0060, 060.2400, 070.4560.
doi: 10.3788/COL20090703.0194.
As the bit rate per wavelength division multiplexing
(WDM) channel increases up to 40 Gb/s or beyond, the
electronic access to the data signal for channel monitor-
ing becomes expensive or even impossible. All-optical
solution for performance monitoring therefore becomes
promising. With the optical sampling technique and em-
ploying extremely short sampling pulses, a resolution of
less than 1 ps can be achieved. The optical sampling
system has been proven to be a very powerful tool for
real-time system optimization and signal monitoring
[1−5]
.
In contrast to asynchronous or synchronous all-optical
sampling systems, the software-synchronized all-optical
sampling system can provide Q values, synchronized
eye diagrams, and data patterns in real time, but re-
quires no clock recovery circuit for the data signal or
for the sampling pulse to retrieve the required synchro-
nization information
[2−10]
. A software algorithm based
on the Fourier transform of the sampled data, as well
as the eye-diagram timing drift, was proposed by West-
lund et al. to provide the required synchronization
information
[8−10]
. The optical performance monitoring
requires that the software can monitor all kinds of sig-
nals regardless of the signal degradation, which means
that the software-synchronized optical sampling should
be signal quality independent. The robustness of the soft-
ware algorithm against the signal degradation is there-
fore necessary to be evaluated. In this letter, the ro-
bustness against the signal degradation induced by chro-
matic dispersion and fiber nonlinearity is evaluated for
the software-synchronized all-optical sampling for fiber
communication systems. The influence of the timing jit-
ter and amplitude fluctuation of the sampling pulse on
the performance of monitoring system is also researched.
The system setup for software-synchronized all-optical
sampling is shown in Fig. 1. The 10-Gb/s non-return-
to-zero (NRZ) optical data signal propagated through
the fiber, and then was sampled in a periodically poled
lithium niobate (PPLN) chip by optical sampling pulses
generated from a mode-locked fiber ring laser (MLFRL).
The repetition rate of the sampling pulse was 499.99
MHz, and the pulse width was about 5.7 ps. The optical
sampling process was accomplished by the sum-frequency
generation (SFG) in PPLN. The length of the PPLN chip
was 40 mm, and the single grating period was 30.2 µm.
The working temperature was 300 K. The wavelengths
of the optical data signal and the sampling pulse were
1.550 and 1.572 µm, respectively. The sampling pulse
was filtered out by an optical bandpass filter (BPF),
and a photodiode was used to detect the sampling pulse.
Then a data acquisition (DAQ) card collected the elec-
trical signal. Finally, a computer processed the electrical
signal by the software-synchronized algorithm, by which
the eye-diagram of the optical data signal was recovered
and the Q value was computed.
Firstly, the robustness of the software-synchronized
method against the chromatic dispersion was measured.
The fiber for the experiment was G.652 standard single
mode fiber (SSMF), the chromatic dispersion of which
was 17.0 ps/(km·nm), and the nonlinear coefficient was
2.6 km
−1
·W
−1
. To investigate the robustness against
the chromatic dispersion, the power of the data sig-
nal launched into the fiber was 0.0 dBm for no sig-
nificant nonlinearity in SSMF, and the signal transmitted
in different lengths of SSMF without chromatic
Fig. 1. System setup for software-synchronized optical sampling.
1671-7694/2009/030194-04
c
2009 Chinese Optics Letters