COL 13(1), 010605(2015) CHINESE OPTICS LETTERS January 10, 2015
1671-7694/2015/010605(6) 010605-1 © 2015 Chinese Optics Letters
Scheme for generation of at top and high
signal-to-noise ratio optical frequency comb
Jianrui Li (李建蕊), Jiachuan Lin (林嘉川), Xiaoguang Zhang (张晓光)
*
,
Lixia Xi (席丽霞), Xianfeng Tang (唐先锋), and Yaojun Qiao (乔耀军)
State Key Laboratory of Information Photonics and Optical Communications, Beijing
University of Posts and Telecommunications, Beijing 100876, China
*
Corresponding author: xgzhang@bupt.edu.cn
Received August 12, 2014; accepted November 14, 2014; posted online January 5, 2015
We demonstrate a new scheme for generation of optical frequency comb (OFC) based on cascade modulators,
23 comb lines within 0.5 dB spectral power variation are obtained. An optical nite impulse response (FIR)
lter is introduced for suppression of amplied spontaneous emission noise. It is shown that carrier-to-noise-ratio
of the OFC generated by this scheme can be as high as 38.8 dB with 12 dB improvement by using a
16-tap FIR lter, and the error vector magnitude performances of loaded Nyquist-16 quadrature amplitude
modulation signal is improved from 14.20% to 7.44%.
OCIS codes: 060.0060, 060.5060, 350.2460.
doi: 10.3788/COL201513.010605.
Optical frequency comb (OFC), which is also called
multi-carrier, is a series of equally spaced spectral comb
lines
[1]
. With the development of technique, its applica-
tion has been extended to many aspects, such as ultra-
short pulse generation
[2]
, arbitrary waveform generation
[3]
,
and dense wavelength division multiplexing
[4]
. In par-
ticular, the combination usage of OFC technology and
high-order modulation pattern as a solution of tera-
bit transmission has become the development direc-
tion of next-generation ber-optic communication
[5,6]
.
Many endeavors have been made on generation of OFC
including exploiting ber loop modulation
[7]
, mode-locked
lasers
[8]
, re-circulating frequency shifter
[9–11]
and cascaded
modulators
[12–14]
. Among these technologies, cascaded
modulators to generate OFC have attracted intensive
attention due to its phase-correlated, frequency spacing
tunability and large bandwidth with a relatively low-
amplitude radio frequency (RF) driving signal. This
kind of scheme is based on the principle that includes
two key procedures to obtain a high-quality OFC
[12]
. At
rst, a series of at-topped pulses are generated in time
domain, and then are converted from time-to-frequency
domain which is equivalent to mapping the intensity
shape of a single pulse into the frequency domain. The
function of the former can be implemented by an inten-
sity modulation, whereas the function of the latter can
be implemented by a parabolic phase modulation. Non-
at spectral envelope of the comb may limit some of its
applications
[15,16]
. Several eicient solutions have been
proposed and demonstrated to improve the degree of
atness. Wu et al.
[17]
reported 38 comb lines within 1 dB
spectral power variation. They used cascaded intensity
modulator (IM) and phase modulator (PM). The IM
generated the at-topped pulses which realized the rst
procedure of the abovementioned principle, and the PM
was driven by specially tailored RF waveform which
induced an approximation to parabolic temporal phase
in the second procedure. Apparently, the complicated
RF devices might be needed in the second procedure,
which makes the scheme diicult to be implemented.
Dou et al.
[18]
reported another improvement of atness
of OFC based on nonlinear eect of IM, whose simula-
tion results display that with the resolution of optical
spectral analyzer 1.25 GHz, the spectral power varia-
tion is about 1 dB, but with higher resolution such as
24.3 MHz, the spectral power variation expands to
about 4 dB, which is not at enough for application.
Therefore, the generation of a suicient at optical
comb is still a challenging task which is worth trying
for further improvement. At the same time, the scheme
in Ref. [18] generates a dozen or even dozens of comb
lines at one time which leads to the input power allo-
cated to many subcarriers, the signal-to-noise ratio
(SNR) of each carrier is not high enough for its appli-
cation, such as loading higher order modulation signal.
Liu et al.
[5,6]
proposed using erbium-doped ber ampli-
er (EDFA) to amplify the optical comb before loading
the signal to extend transmission distance; however, the
amplied spontaneous emission (ASE) noise will also
be amplied which will result in a deterioration of the
SNR of optical comb.
In this letter, we propose a new modied scheme for
optical comb generation based on cascade modulators
with higher atness which relies on a trade-o of the
good features that the abovementioned two procedures
would seek. Furthermore, an improved conguration
with ASE noise suppression scheme using optical nite
impulse response (FIR) lter is demonstrated
[19]
. By
applying this scheme, an obvious carrier-to-noise-ratio
(CNR) performance improvement is achieved.
As mentioned above, in order to get a large number
of at and high-quality OFC, the conventional scheme
using cascaded modulators is based on the principle
that includes the two key procedures. At rst, at-top