642 CHINESE OPTICS LETTERS / Vol. 8, No. 7 / July 10, 2010
Applications of optical duobinary in optical carrier
suppression and separation labeling
Zhenghao Long (ÓÓÓ)
1∗
, Xiangjun Xin ($$$)
2
, Rui Zhou (±±± bbb)
1
,
Zixing Zhang (ÜÜÜfff,,,)
1
, and Daxiong Xu (MMM<<<)
1
1
Institute of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications,
Beijing 100876, China
2
School of Electronic Engineering, Beijing University of Posts and Telecommunications, Beijing 1000876, China
∗
E-mail: zilong98@gmail.com
Received January 13, 2010
A novel approach is used to implement optical carrier suppression and separation (OCSS) labeling. Then,
the performance of 10/40-Gb/s duobinary payload with 2.5-Gb/s amplitude shift keying (ASK) or duobi-
nary label by numerical simulations is studied. Influencing factors, such as demultiplexer bandwidth and
fib er Bragg grating (FBG) filter bandwidth, are investigated. Simulation result shows that the received
sensitivity of ASK label is higher than that of the duobinary label, while the received sensitivity of duobi-
nary payload with duobinary label is higher than that with ASK label.
OCIS co des: 060.2330, 060.4080.
doi: 10.3788/COL20100807.0642.
All-optical label switching is a promising approach in
switching and routing packets in optical layers for next-
generation optical packet switching networks
[1−5]
. In op-
tical labeling, the optical carrier suppression and separa-
tion (OCSS) technique has been proposed as a prevailing
approach due to several reasons: 1) absence of extinction
ratio (ER) limitation for generating payload or label;
2) narrow bandwidth for transporting both label and
payload, thus allowing better spectral efficiency; 3) easy
separation of payload and label by filtering. However, we
have noticed that the OCSS schemes previously reported
mostly used differential phase-shift-keying (DPSK)
[6−8]
or differential quadrature phase-shift-keying (DQPSK)
[9]
modulated payload. The typical DPSK receiver struc-
ture is based on interferometric-detection where one-bit
period Mach-Zehnder delay line interferometer (DLI) is
difficult to fabricate and needs very tight temperature
control in order to provide exactly one-bit period de-
lay. Thus, the transmission cost and system difficulty
increase for DPSK modulation. For DQPSK and some
other advanced modulation formats, the receivers are
more complicated. Therefore, cost-effective solutions for
OCSS implementation are highly desirable. Consider-
able effort has been devoted on the study of possible
replacements
[10]
.
Optical duobinary format is considered attractive be-
cause of its low spectral occupancy and high tolerance to
residual chromatic dispersion
[11−13]
. More importantly,
it can utilize simple and cost-effective direct detection
(DD) receivers, similar to that in on-off keying (OOK)
transmission. These make it one of the most promising
solutions for upgrading existing 10-Gb/s wavelength di-
vision multiplexing (WDM) systems, i.e., through simple
replacement of a 10-Gb/s channel with a 40-Gb/s one
without changing the design of the transmitter, receiver,
or the transmission line. However, to our knowledge,
applications of duobinary in OCSS system for either
payload or label have not been conducted. In this let-
ter, a novel approach for OCSS labeling is proposed.
In addition, the performance of duobinary modulated
payload with amplitude shift keying (ASK) or duobi-
nary label is studied using numerical simulations. The
duobinary payload is 10 or 40 Gb/s, while both ASK
and duobinary labels are 2.5 Gb/s. Influencing factors
like demultiplexer bandwidth and the fiber Bragg grating
(FBG) filter bandwidth are investigated. Results show
that for 10-Gb/s payload, the impact of FBG filter is
negligible, but it is significant for 40-Gb/s payload.
The proposed system using optical duobinary for OCSS
labeling is shown in Fig. 1. Operating principle of the
optical carrier suppression is described as follows. The
carrier frequency of the continuous wave (CW) laser was
set to 193.1 THz and the laser output was injected to a
dual-drive LiNbO
3
Mach-Zehnder modulator (MZM1).
The two parameters of MZM1, which are required for
π phase difference, namely V
π DC
(direct current (DC)
voltage at both split electrodes) and V
π RF
(radio fre-
quency (RF) voltage at both split electrodes), were both
set to 5.0 V. Compared with the conventional method in
implementing OCSS to drive the upper and lower arms
of MZM1 using a pair of complementary RF sinusoidal
clock signals, as shown in Ref. [8], both upper and lower
arms of MZM1 in this letter were driven by the same
20-GHz RF sinusoidal clo ck signal. The amplitude was
set to 1.25 V (V
πRF
/4). Both upper and lower arms of
MZM1 were biased by the same DC source at the mini-
mal intensity point by setting both upper and lower bias
voltages to 2.5 V (V
π DC
/2). CW light was separated
into two longitudinal modes with a fixed frequency spac-
ing equal to twice the clock frequency (Fig. 2(b)). By
varying RF sinusoidal frequency to 10 and 30 GHz, the
spacing between the two longitudinal modes changed to
20 (Fig. 2(a)) and 60 GHz (Fig. 2(c)), respectively.
After carrier suppression, a demultiplexer (DEMUX)
was employed to separate the two longitudinal modes.
We used a single-arm MZM2 to modulate the lower
1671-7694/2010/070642-05
c
° 2010 Chinese Optics Letters