1928 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 18, SEPTEMBER 15, 2016
Photonic Generation of Microwave
Frequency Shift Keying Signals
Long Huang, Student Member, IEEE, Peng Wang, Peng Xiang, Dalei Chen, Yiyun Zhang,
Ji Tao, Tao Pu, Member, IEEE, and Xiangfei Chen, Senior Member, IEEE
Abstract—A novel approach to generating microwave
frequency shift keying (FSK) signals using a polarization
modulator (PolM) and a dual-polarization Mach–Zehnder modu-
lator (DPol-MZM) is proposed and experimentally demonstrated.
In the proposed system, the PolM is employed to modulate
the polarization state of an input linearly polarized lightwave
to switch between two orthogonal directions, leading to the
generation of a polarization shift keying (PolSK) signal. Then
the PolSK signal is sent to a DPol-MZM, which is made up
of two polarization multiplexed sub-MZMs by a polarization
controller (PC). Two microwave signals with different frequencies
are applied to the sub-MZMs. After aligning the orthogonal
directions of the PolSK signal with the axes of the DPol-MZM
by the PC, the PolSK signal can be converted into a microwave
FSK signal. A proof-of-concept experiment is carried out to verify
the proposed system. When the sub-MZMs in the DPol-MZM are
biased at a quadrature point, a double sideband microwave
FSK signal at 3/6.5 GHz with a bit rate of 1.25 Gb/s is generated
and transmitted over 10 km of single-mode fiber. When the
sub-MZMs are biased at minimum point, an optical carrier
suppression microwave FSK is achieved at frequency-doubled
6/14 GHz with a bit rate of 2.5 Gb/s.
Index Terms—Microwave photonics, microwave signal
generation.
I. INTRODUCTION
I
N RECENT years, photonic generation of microwave
signals has attracted great research interest in many
applications, such as radio-over-fiber (RoF) systems and radar
systems. Due to the intrinsic advantages of low loss, high
bandwidth, low weight, and immunity to electromagnetic
interference provided by optical fiber and related optical
devices, the photonic approach facilitates the generation and
transmission of broadband microwave signals at high fre-
quency [1], [2]. So far, a variety of schemes for the gen-
eration of microwave signals with binary modulation have
Manuscript received March 23, 2016; revised May 28, 2016; accepted
May 30, 2016. Date of publication June 7, 2016; date of current version
July 1, 2016. This work was supported in part by the Jiangsu Province
Natural Science Foundation under Grant BK20141168, Grant BK20140069,
and Grant BK2012058, in part by the National Natural Science Foundation
of China under Grant 61435014, Grant 61475193, Grant 61306068,
Grant 61032005, and Grant 61177065, and in part by the Technology Support
Program of Jiangsu Province under Grant BE2012157.
L. Huang, P. Wang, Y. Zhang, J. Tao, and X. Chen are with the Microwave-
Photonics Technology Laboratory and the Nanjing National Laboratory
of Microstructures, School of Engineering and Applied Sciences, Nanjing
University, Nanjing 210093, China (e-mail: chenxf@nju.edu.cn).
P. Xiang, D. Chen, and T. Pu are with the PLA University of Science and
Technology, Nanjing 210007, China.
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2016.2576639
been implemented in the optical domain, such as amplitude
shift keying (ASK) [3], [4], phase shift keying (PSK) [5]–[8]
and frequency shift keying (FSK) [9]–[13]. Microwave ASK
signals using double-sideband (DSB), single-sideband (SSB)
and optical carrier suppression (OCS) were generated and
transmitted in RoF systems in [3]–[5], respectively. On the
other hand, photonic generation of microwave PSK signals
is used in radar systems to enhance the pulse compres-
sion ratio (PCR) [6]–[9]. FSK signals which have better
performance than ASK signals in terms of wireless chan-
nel fading [10] have also been generated in the optical
domain. In [11], an 800 Mb/s FSK signal at 2.37/3.48 GHz
was generated based on a self-pulsating compact-disk laser
diode (LD). The self-pulsating frequency of the LD followed
a linear dependence on the bias current. By modulating the
bias current, a FSK signal can be generated. In this scheme,
the linewidth of the generated microwave was very wide,
and the bit rate was also limited since the LD was directly
modulated by a baseband signal. An optical injected LD can
also be used to generate microwave FSK signals [10]. When
the injected LD is operated in a dynamic state, where its
intensity oscillates at a microwave frequency that varies with
the injection strength, the injected optical ASK signal can be
converted to a microwave FSK signal. The experiment in [10]
demonstrated a FSK signal with 622 Mb/s at 15.23/16.4 GHz.
Although the frequency of the generated microwave signal can
be very high, this scheme also suffered from a bad quality of
the microwave subcarrier whose linewidth was measured to
be an order of 10 MHz. Moreover, the frequency deviation
of the FSK signal cannot be flexibly tuned. In [12], another
scheme was proposed by switching between two groups of
optical frequency combs which were shaped by a line-by-
line shaper. This scheme employed many wavelengths which
were not suitable for RoF systems. In [13], a novel scheme
was proposed based on polarization switching and frequency-
to-time mapping (FTTM). However, to effectively induce
FTTM, an expensive optical source generating narrow pulses
should be employed, and in this letter only numeral simulation
was presented without experimental verification. Microwave
FSK signals can also be obtained by a Mach-Zehnder
modulator (MZM) which was switched between quadra-
ture point and minimum point [14]. The frequency shift
was achieved at fundamental and second harmonic of the
microwave signal applied to the MZM. As a result, microwave
subcarriers of the FSK signal cannot be independently tuned.
In this letter, we propose and experimentally demonstrate
a scheme of generating microwave FSK signals based on
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