Frequency-Doubling Optoelectronic

Oscillator Using DSB-SC Modulation

and Carrier Recovery Based on
Stimulated Brillouin Scattering
Volume 5, Number 2, April 2013

Xinkai Liu
Wei Pan
Xihua Zou
Di Zheng
Lianshan Yan
Bin Luo

DOI: 10.1109/JPHOT.2013.2245880
1943-0655/$31.00 Ó2013 IEEE
IEEE Photonics Journal Frequency-Doubling Optoelectronic Oscillator

Frequency-Doubling Optoelectronic
Oscillator Using DSB-SC Modulation
and Carrier Recovery Based on
Stimulated Brillouin Scattering
Xinkai Liu, Wei Pan, Xihua Zou, Di Zheng, Lianshan Yan, and Bin Luo

Center for Information Photonics & Communications, School of Information Science and Technology,
Southwest Jiaotong University, Chengdu 610031, China

DOI: 10.1109/JPHOT.2013.2245880
1943-0655/$31.00 Ó2013 IEEE

Manuscript received December 13, 2012; revised January 25, 2013; accepted January 27, 2013. Date
of publication February 7, 2013; date of current version March 18, 2013. The work was supported
in part by Research Fund for the Doctoral Program of Higher Education of China under Grant
20110184130003, the National Natural Science Foundation of China under Grant 61101053, the B973[
Project under Grant 2012CB315704, the Program for New Century Excellent Talents in University of
China (NCET-12-0940), the Fok Ying-Tong Education Foundation under Grant 132033, the 2013
Doctoral Innovation Funds of Southwest Jiaotong University, and the Fundamental Research Funds for
the Central Universities. Corresponding authors: X. Zou and X. Liu (e-mail: zouxihua@swjtu.edu.cn;
liuxk20040706@163.com).

Abstract: A tunable frequency-doubling optoelectronic oscillator (OEO) is proposed and

experimentally demonstrated, of which the novelty lies in the conjunction of the double-
sideband suppressed carrier (DSB-SC) modulation and the carrier recovery based on
stimulated Brillouin scattering (SBS) effect. Frequency-doubled signals are generated via
the DSB-SC modulation, which is realized by using a polarization modulator (PolM) in
combination with an optical polarizer. Then, the gain provided by the SBS effect is used to
recover the suppressed optical carrier, such that a fundamental-frequency oscillating
required in the OEO loop is maintained. In the experiment, frequency-doubled microwave
signals at 6.1 and 20 GHz are generated and analyzed. Meanwhile, the stability of the
generated signals is also investigated.

Index Terms: Optoelectronic oscillator (OEO), frequency doubling, stimulated Brillouin

scattering (SBS), double-sideband suppressed carrier (DSB-SC), optical carrier recovery.

1. Introduction
Optoelectronic oscillator (OEO), a significant source for generating high spectral purity microwave
and millimeter-wave signals, has attracted extensively attention [1]–[3]. Due to the advantages of
stability and low phase noise, the OEO has been widely employed in the fields of signal
processing, radars, and wireless communication [4]–[7]. Usually, the frequencies of the generated
signals are limited by the bandwidth of the electrical devices in the OEO. To cover higher frequency
band, several approaches for frequency-doubling OEOs have been proposed [8]–[12]. For
instance, a frequency-doubling OEO using a LiNbO3 Mach–Zehnder modulator (MZM) was
proposed in [9], where the MZM was biased at the minimum transmission point to realize double-
sideband suppressed carrier (DSB-SC) modulation. To further extend frequency band of the
generated microwave signals, a polarization modulator (PolM) assisted with two polarizers is
applied [10]. Outside the loop, the DSB-SC modulation was implemented for frequency doubling
by one polarizer, while a normal double-sideband (DSB) modulation was performed to keep

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IEEE Photonics Journal Frequency-Doubling Optoelectronic Oscillator

Fig. 1. Schematic of the proposed frequency-doubling OEO. (OC, optical coupler; PC, polarization
controller; Pol, polarizer; EDFA, Erbium doped fiber amplifier; ISO, isolator; PD, photodetector; EBPF,
electrical bandpass filter).

oscillating inside the loop using another polarizer. Besides, a frequency-doubling OEO can also be
realized by using a phase modulator, a phase-shifted fiber Bragg grating, and an optical filter with
a fixed central wavelength [11]. An optical notch filter was used to remove the optical carrier. The
central wavelength of the notch filter should be simultaneously adjusted to match the carrier,
when the wavelength of optical carrier changes. Lately, a frequency-doubling OEO without optical
notch filter was proposed by employing a dual-parallel (DP) MZM [12]. Six dc biases applied to
the DP-MZM should be accurately controlled to maintain the carrier phase-shifted (CPS) DSB
modulation.
In this paper, a novel tunable frequency-doubling OEO is proposed by using DSB-SC modulation
and carrier recovery based on stimulated Brillouin scattering (SBS). The key contributions are the
implementation of DSB-SC modulation without any optical filter or bias controller and the accurate
carrier recovery using SBS. In the proposed OEO, a PolM and a polarizer inside the loop enable a
stable DSB-SC modulation for realizing frequency doubling, which removes additional dc bias of
the modulator and other optical element outside OEO loop, such as a notch filter. Therefore, the
system can operate stably even when the wavelength of optical carrier changes. Meanwhile, to
sustain the oscillating operation, the suppressed carrier is then recovered via the amplification
effect of SBS, such that the DSB-SC modulation is converted into a normal DSB modulation, with
the required fundamental-frequency signal generated. In addition, a section of highly nonlinear fiber
(HNLF) serves as the Brillouin-active element, which can increase the Q value of the OEO. In the
experiment, the generation of frequency-doubled microwave signals is verified.

2. Principle
The schematic of the proposed frequency-doubling OEO is shown in Fig. 1. The light wave at fc
from a laser diode (LD) is divided into two paths via an optical coupler (OC1). In one path, the light
wave is frequency shifted to a higher frequency to act as the Brillouin pump light SBS, which can be
realized by using single-sideband suppressed carrier (SSB-SC) modulation. The frequency-shifted
light wave at fc þ vB is amplified by an Erbium-doped fiber amplifier (EDFA2) and then injected into

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IEEE Photonics Journal Frequency-Doubling Optoelectronic Oscillator

Fig. 2. Optical spectra of (a) DSB-SC modulation with suppressed carrier and (b) normal DSB
modulation with recovered carrier when the EBPF at 10 GHz is employed.

an HNLF to stimulate the SBS effect, where vB is the backward direction Brillouin frequency shift. In
the other path, the light wave is modulated by an oscillating signal, where the DSB-SC modulation is
realized by the use of a PolM and a polarizer [10]. The output of the polarizer can be written as
E0 / Ei ½expðj!c t þ j sin
t Þ  expðj!c t  j sin
tÞ (1)
where Ei and !c ¼ 2fc are the output field and the angular frequency of LD, respectively,  is the
phase modulation index, and
¼ 2fm is the angular frequency of the oscillating microwave signal.
After passing through another optical coupler (OC2), the DSB-SC modulated light wave is divided
into two parts. One part is sent to the photodetector (PD2) to generate a frequency-doubled
microwave signal. In the case of the small-signal modulation, the generated microwave signal can
be expressed as

I0 / <jE0 j2 ¼ <J12 ðÞ  cosð2

t Þ (2)
where < is the responsivity of PD, and J1 ðÞ is the 1st-order Bessel function of the first kind. The
other part of the DSB-SC modulated light wave is sent to the HNLF via an isolator. Meanwhile, the
SBS pump source, i.e., the frequency-shifted light wave, is injected into the HNLF in the opposite
direction. Therefore, SBS gain acts on recovering the carrier that is suppressed in the DSB-SC
modulation. In this way, the DSB-SC modulation is converted into a normal DSB modulation, which
leads to the generation of the fundamental-frequency signal after the optical-to-electrical
conversion by using PD1. The achieved fundamental-frequency signal is fed back to the RF port
of the PolM, to keep the oscillating in the OEO loop. In addition, an electronic amplifier (EA)
provides suitable gain and an electrical narrow-band bandpass filter (EBPF) performs the oscillation
frequency selection.

3. Experiments
According to the setup shown in the Fig. 1, an experiment is carried out by using a PolM
(Versawave Technologies) with a 40-GHz bandwidth and a 1-km HNLF with a nonlinear coefficient
of 30 ðW  kmÞ1 and a chromatic dispersion of 0.1 ps/nm/km at 1550 nm.
When the EBPF with a center frequency of 10 GHz is adopted, the optical carrier is effectively
suppressed and the spectrum of DSB-SC modulated light before the PD2 is measured. As shown in
Fig. 2(a), a suppressed ratio higher than 20 dB is achieved. Due to the SBS gain, the optical carrier
can be recovered at the input of PD1, as shown in Fig. 2(b) which is the spectrum of a normal DSB
modulated light wave. It also can be seen that, as the 1st-order sidebands fall outside the SBS
gain spectrum, only the carrier is amplified by the SBS effect.
The DSB-SC modulated light wave and the normal DSB modulated light wave are sent to PD2
and PD1, respectively; frequency-doubled and fundamental-frequency microwave signals are
generated, respectively. The electrical spectrum of frequency-doubled signal at 20 GHz is shown in
Fig. 3(a). It is clearly seen that the power level of the frequency-doubled signal is 31.04 dB higher

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IEEE Photonics Journal Frequency-Doubling Optoelectronic Oscillator

Fig. 3. Electrical spectra of (a) frequency-doubled microwave signal at 20 GHz and (b) fundamental-
frequency microwave signal at 10 GHz when the EBPF at 10 GHz is employed.

Fig. 4. Characteristics of the frequency-doubling OEO when the EBPF at 3.05 GHz is employed.
Electrical spectra of (a) fundamental-frequency microwave signal and (b) frequency-doubled microwave
signal. (Insets: Span ¼ 100 kHz, Resolution Bandwidth ¼ 1 kHz). Temporal waveform of (c) fundamental-
frequency microwave signal and (d) frequency-doubled microwave signal.

than that of the fundamental one. In addition, Fig. 3(b) shows the fundamental-frequency signal,
which is generated to maintain the oscillating in the OEO.
The reconfigurability of the proposed frequency-doubling OEO is investigated by changing the
central frequency of the EBPF. By using an EBPF with a 3.05-GHz central frequency, a
fundamental-frequency microwave signal at 3.05 GHz and a frequency-doubled microwave signal
at 6.1 GHz are generated simultaneously. The electrical spectra of the two signals are shown in
Fig. 4(a) and (b), respectively. A power difference up to 42.33 dB is obtained between the
fundamental-frequency and frequency-doubled signals, indicating the excellent quality of the
frequency-doubled signal. Moreover, the temporal waveforms of two signals are measured to
simply demonstrate their qualities, as shown in Fig. 4(c) and (d). By a comparison between the two
temporal waveforms, the quality of the frequency-doubled signal is almost the same as that of the
fundamental-frequency signal. Thus, little distortion is introduced for the frequency doubling. In

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Fig. 5. Frequency stability and power stability of (a) fundamental-frequency signal at 3.05 GHz and
(b) frequency-doubled signal at 6.1 GHz.

addition, the phase noise of the frequency-doubling microwave signal at 6.1 GHz is measured to be
103.19 dBc/Hz at 10 kHz, revealing low phase noise for the frequency-doubling signal generated
by the proposed OEO. The proposed approach has the potential for the continuous tunability by
using photonics microwave filter to replace the EBPF, as shown in [13]–[15].
Due to the limit on the external trigger of the employed oscilloscope (3.2 GHz, Agilent 86100C),
only the frequency-doubled microwave signal at 6.1 GHz can be measured by setting the
fundamental-frequency at 3.05 GHz as the trigger.
The stability of the frequency-doubling OEO has also been investigated. Within the time duration
of 3 min, the electrical spectra of the frequency-doubled and fundamental-frequency signals were
measured every 15 s. Fig. 5(a) shows the stability of the fundamental-frequency signal at 3.05 GHz,
where the triangles describe the power drift which is about 0.17 dB and the circulars describe the
frequency drift, which is estimated to be 1.23 kHz. The results of the frequency-doubled signal at
6.1 GHz are shown in Fig. 5(b). The power drift and the frequency drift are estimated as 0.07 dB
and 2.05 kHz, respectively. In addition, the proposed OEO operates stably for 30 min in the room-
temperature environment, and no significant frequency changes are observed from the electrical
spectra and the waveforms. All these results indicate a stable operation of the proposed OEO.

4. Conclusion
A frequency-doubling OEO using DSB-SC modulation and carrier recovery has been proposed and
experimentally demonstrated. The key features of the proposed frequency-doubling OEO have
been listed as follows. First, no operation is needed outside the OEO loop, such as the use of
optical notch filter to remove the optical carrier. Especially, this feature make frequency-doubling
still working even when the wavelength of optical carrier changes. Next, no bias is needed, which
circumvents the bias drift by using a MZM or a DP-MZM. Moreover, an accurate carrier recovery
has been performed using SBS gain, such that the fundamental-frequency signal is generated from
the normal DSB modulation to sustain the oscillating operation. In the experiments, the frequency-
doubled signal at 6.1 or 20 GHz has been generated, with the electronic spectrum and temporal
waveform measured and analyzed. Meanwhile, the stability of the generated microwave signal has
been evaluated.

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