A full-duplex WDM-RoF system based on tunable optical frequency
comb generator
Chan Zhang
a,b
, TiGang Ning
a,b
, Jing Li
a,b
, Li Pei
a,b,
n
, Chao Li
a,b
, Shaoshuo Ma
a,b
a
Key Lab of All Optical Network & Advanced Telecommunication Network of EMC, Beijing Jiaotong University, Beijing 100044, China
b
Institute of Lightwave Technology, Beijing Jiaotong University, Beijing 100044, China
article info
Article history:
Received 20 October 2014
Received in revised form
11 January 2015
Accepted 12 January 2015
Available online 14 January 2015
Keywords:
WDM-RoF
Optical comb
Full-duplex
Optical-wireless
abstract
A prototype of full-duplex wavelength-division-multiplexing-based radio-over-fiber (WDM-RoF) system
using a single continuous wave (CW) laser at central station (CS) is proposed and investigated. In the
proposal, the tunable optical frequency comb (OFC) generator is performed by cascading an electro-
absorption modulator (EAM), a phase modulator (PM) and a Mach–Zehnder intensity modulator (IM).
Then the proposed OFC is used as the source of WDM-RoF network. The base station (BS) is simplified by
using central carrier along with downlink multi-frequency millimeter-wave signals (MMWs). In the
scheme, we obtain 21 comb lines within 0.2 dB power deviation with an interval about 10 GHz. Then we
apply it to a bidirectional 1:7 WDM-RoF cellular network. For the downlink, three MMWs with fre-
quencies of 40 GHz, 60 GHz and 80 GHz are obtained. For the uplink, 7 WDM channels carrying upstream
signal are converted simultaneously. Both down and uplink delivery over 20 km transmission with less
than 1 dB power penalty, which shows high spectral-efficiency and good performance over long-distance
delivery.
& 2015 Elsevier B.V. All rights reserved.
1. Introduction
RoF technology has become an attractive solution for increasing
the capacity and speed of wireless communication [1]. Generation
of low-cost and frequency-tunable MMWs is a key technology to
RoF systems practical for numerous applications [2–4]. What's
more, in order to further increase the bandwidth and the capacity
of wireless system, the WDM technique has been employed in ROF
systems (WDM-RoF) [5–7]. In most cases, mode-locked laser, a
DFB array or a super-continuum light source is employed as the
WDM optical source to provide multiple WDM channels [8–11].
However, the mode-locked laser and super-continuum light source
are very expensive compared with a CW laser. Also, the overall
cost increases a lot as the required sources increase. The number of
WDM channels is limited due to the difficulty of managing many
independent lasers which is too complicated and inevitably re-
duces the scalability and reconfigurability of the WDM system.
Considering these facts, a BS should also be simple and cost-ef-
fective without using expensive devices. OFC is a promising WDM
optical source to provide multiple channels for WDM-RoF system,
which can greatly increase the capacity and mobility of RoF
networks. Various methods have been reported to generate OFC,
which are usually realized by mode-locked lasers [12–14] or fiber
nonlinearities [15,16]. However, the overall cost increases when
the optical sources increase, which inevitably reduces the com-
plexity and reconfigurability of the whole system. Additionally,
optical combs from these methods lack flatness and stability that
cannot be easily applied in WDM systems. Optical modulation is
another attractive approach due to its advantages in terms of
system simplicity, frequency tunability and stability [17,18]. The
major limitation of this approach is that the amplitudes and the
frequencies of the two RF signals have to be precisely controlled.
Later then, more than 30 comb lines were obtained using two
cascaded IMs and a PM [19–21]. But the number of the generated
comb lines is mostly limited due to the inherent amplitude lim-
itation of the driven RF signals, which limits the suitability for
practical applications. Recently, we propose a unidirectional THz
wireless transmission system using an optical comb with 1 dB
power deviation [22]. After downlink data modulation, MMW is
generated by two beating carriers, while other left idle frequency
comb is a big waste which may result in low spectral-efficiency for
the whole system. To meet the ever-increasing demands in future
communication systems, bidirectional fiber-optic transmission in
RoF link is also a promising solution. Some methods have been
proposed to realize the bidirectional transmission [3,23]. Among
the aforementioned schemes, systems are required to have a
multiple side modes injection-locked distributed feedback (DFB)
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/optcom
Optics Communications
http://dx.doi.org/10.1016/j.optcom.2015.01.038
0030-4018/& 2015 Elsevier B.V. All rights reserved.
n
Corresponding author at: Institute of Lightwave Technology, Key Lab of All
Optical Network & Advanced Telecommunication Network of EMC, Beijing Jiaotong
University, Beijing 100044, China.
E-mail address: lipei@bjtu.edu.cn (L. Pei).
Optics Communications 344 (2015) 65–70