May 10, 2010 / Vol. 8, No. 5 / CHINESE OPTICS LETTERS 449
Novel 64 × 2.5 Gb/s all-optical OFDM symbol generator
based on triangle waveform driving-LiNbO
3
modulators
Yuan Li (ooo )
1,2
, Wei Li (ooo )
1∗
, Xiaojun Liang (ùùù¡¡¡)
1
,
Kecheng Yang (¤¤¤)
1
, and Yaojun Qiao (zzz)
3
1
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China
2
Department of Computer Science and Technology, Central China Normal University, Wuhan 430079, China
3
Key Laboratory of Information Photonics and Optical Communications, Ministry of Education,
Beijing University of Posts and Telecommunications, Beijing 100876, China
∗
E-mail: weilee@hust.edu.cn
Received October 30, 2009
We propose a novel and simple all-optical 160-Gb/s orthogonal frequency division multiplexing (OFDM)
symbol generator which is based on discrete triangle waveform driving-LiNbO
3
mo dulators to realize large-
range linear optical shift. The entire system needs 64 discrete modulators: at the transmitter, a 2.5-Gb/s
optical duobinary (ODB) modulator for data modulation and a 2.5-Gb/s triangle waveform driving-LiNbO
3
phase modulator for phase shift to generate each subcarrier; and at the receiver, a 2.5-GHz optical band
pass filter (OBPF) using Faraday anomalous dispersion optical effect to separate them. Excellent bit error
rate (BER) is observed after 1060 km of transmission without any dispersion compensation.
OCIS codes: 060.0060, 070.0070.
doi: 10.3788/COL20100805.0449.
Optical orthogonal frequency division multiplexing
(OFDM) has become a promising technique in long-haul
and high-speed optical transmission systems because of
its high spectral efficiency, relatively low signal bit rate,
and advanced robustness against chromatic dispersion
and polarization mode dispersion (PMD)
[1−4]
. There are
two kinds of optical OFDM (OOFDM) coherent
[1]
and
all-optical
[2,3]
.
All-optical OFDM systems utilize all-optical trans-
mitters, thereby eliminating the speed limitation set by
electronics. In this kind of system, the key technology
is the optical Fourier transformation (OFT), which has
two kinds: continuous and discrete. In Ref. [2], Yang et
al. used–time lenses to realize a continuous OFT, which
caused it to lose phase information in the OFT process-
ing. Thus, it is not suitable for phase modulation system
such as quadrature phase-shift keying (QPSK). On the
other hand, the discrete OOFDM system is more popu-
lar. Its key technology is the optical discrete phase shift
at the transmitter and optical filter (both time and fre-
quency) at the receiver
[3,4]
. Many researchers have tried
to come up with practical ways to realize this. Three
kinds of physically feasible discrete all-optical OFDM
systems have been reported to date. One was proposed
by Yu et al.
[3]
In his paper, the phase shift was achieved
by the LiNbO
3
phase modulator driving the sinusoidal
waveform. Four subchannels generated by the phase
modulator (one cascaded intensity modulator was used
to equalize the optical power of each peak) were used.
As the sinusoidal waveform driving leads to a nonlinear
phase shift, proper driving voltage is needed and the
phase shift range is limited. Another system required
many phase shifts and delay lines that are not practical
for many subcarriers
[4]
. In Ref. [5], we followed Ref.
[4] in proposing a planar lightwave circuit (PLC) based
integrated optical discrete Fourier transfermation device
based on the thermo-optic effect of SiO
2
in one silica
chip. However, it is still not suitable for many subcar-
riers as the size of a silica chip is limited. Nevertheless,
there are still some other ways to realize discrete optical
phase shifts
[6]
.
Based on the concept presented in Ref. [3], triangle
waveform LiNbO
3
modulators can bring about a lin-
ear large-range phase shift. Thus, the realization of
all-optical OFDM may be simple. Moreover, if we use
discrete phase modulators, many subcarriers can be re-
alized, eliminating the integration limitation imposed
on the number of subcarriers. The proposed symbol
generator would make it physically realizable to build
all-optical OFDM systems with a large number of sub-
carriers. A 160-Gb/s all-optical OFDM transmission
system is designed by employing 64 symbol generators.
Simulations show advanced chromatic dispersion toler-
ance and spectral efficiency. A performance comparison
with a 160-Gb/s optical inverse discrete Fourier trans-
fermation (OIDFT) based all-optical OFDM system is
given. This letter presents the design concepts of the
proposed all-optical OFDM symbol generator and an all-
optical OFDM system design example.
The power spectrum of OFDM signals has perfect
orthogonality, as shown in Fig. 1. This spectral orthogo-
nality can b e obtained by LiNbO
3
phase modulators and
a wavelength division multiplexer (WDM)
[3]
. Figure 1
depicts the operating principle of the prop osed all-optical
OFDM symbol generator. The left part of Fig. 1 depicts
the power spectrum of signals on a subcarrier. The hori-
zontal axis indicates the frequency relative to the optical
carrier frequency f
c
. The unit of the horizontal axis ∆f
is the frequency spacing between neighboring subcarriers,
which is equal to the signal bit rate on each subcarrier.
The vertical axis indicates the normalized power, while
f
c
is modulated by N different parallel data streams.
1671-7694/2010/050449-05
c
° 2010 Chinese Optics Letters
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