Silicon high-speed binary phase-shift keying
modulator with a single-drive push–pull
high-speed traveling wave electrode
Jinting Wang,
1
Linjie Zhou,
1,
* Haike Zhu,
1
Rui Yang,
1
Yanyang Zhou,
1
Lei Liu,
2
Tao Wang,
2
and Jianping Chen
1
1
State Key Laboratory of Advanced Optical Communication Systems and Networks, Department of
Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
2
Transmission Technology Research Department, Huawei Technologies Co. Ltd., Shenzhen 518129, China
*Corresponding author: ljzhou@sjtu.edu.cn
Received November 28, 2014; revised February 12, 2015; accepted February 12, 2015;
posted February 13, 2015 (Doc. ID 228707); published April 6, 2015
We demonstrate binary phase shift keying (BPSK) modulation using a silicon Mach–Zehnder modulator with a
π-phase-shift voltage (V
π
)of−4.5 V. The single-drive push–pull traveling wave electrode has been optimized using
numerical simulations with a 3 dB electro-optic bandwidth of 35 GHz. The 32 Gb/s BPSK constellation diagram is
measured with an error vector magnitude of 18.9%. © 2015 Chinese Laser Press
OCIS codes: (230.3120) Integrated optics devices; (250.3140) Integrated optoelectronic circuits; (250.7360)
Waveguide modulators.
http://dx.doi.org/10.1364/PRJ.3.000058
With the number of processing cores increasing in central
processing units, the electrical interconnects gradually be-
come the bottleneck for the further improvement of comput-
ing performances due to their large delay, low bandwidth, and
high power consumption [
1]. Optical interconnects based on
silicon photonics have been regarded as a promising solution
to this problem. Silicon optical modulators, which are used to
encode electrical signals onto optical beams, are a major
workhorse in optical interconnects [
2,3]. Till now, the re-
ported high-speed silicon modulators are dominantly based
on two optical structures: the Mach–Zehnder interferometer
(MZI) [
4–7] and the resonant cavity [8–10]. Silicon modulators
based on microrings and microdisks have the merits of small
size, low modulation power, and high modulation speed of
up to 50 Gb/s [
11]. However, they also suffer from certain
limitations, such as narrow optical bandwidth and high tem-
perature sensitivity, which greatly limit their practical applica-
tions. Silicon modulators based on MZIs, on the other hand,
have broad optical bandwidth and high tolerance to temper-
ature fluctuations. Modulation speeds up to 70 Gb/s have also
been recently demonstrated in MZI modulators (MZMs) inte-
grated with reverse-biased p − n junctions [
12]. A higher
modulation speed can be expected by reducing the radio-
frequency (RF) loss of transmission lines [
13], as the modula-
tion bandwidth is partially limited by the attenuation of RF
drive signals.
Advanced modulation formats are necessary to increase
the aggregation data rate within a limited spectral bandwidth.
As the basis for advanced modulation formats, binary phase-
shift keying (BPSK) modulation based on a silicon MZM has
been reported [
14]. In their design, separate traveling wave
electrodes (TWEs) on the two MZI arms are used, requiring
differential RF signals [
15].
In this paper, we demonstrate a silicon BPSK modulator us-
ing a MZI structure driven by a single-drive push–pull TWE.
The TWE has been optimized with numerical simulations.
32 Gb/s BPSK modulation is achieved with an error vector
magnitude (EVM) of 18.9%.
To design a high-speed modulator, we consider three main
factors that determine the modulation bandwidth. First of all,
the characteristic impedance (Z
0
) of the TWE needs to ap-
proach 50 Ω in order to avoid RF signal backreflection from
the ends of the TWE. Second, the RF attenuation of the TWE
should be small, since the electro-optic (EO) 3 dB bandwidth
is related to the RF electro-electro (EE) 6.4 dB bandwidth [
16].
Third, the electrical and optical waves need to be synchron-
ized to provide the highest modulation efficiency [
17]. For a
lossless modulator with impedance-matched load and gener-
ator, its EO 3 dB bandwidth is determined by the velocity mis-
match between the RF and optical signals and is given by [
18]
f
3 dB
0.18
n
eff
− n
0
l
; (1)
where l is the electrode length, n
eff
is the microwave refractive
index, and n
0
is the group refractive index of the waveguide
optical mode. To increase the modulation bandwidth, n
eff
needs to approach n
0
.
Figure
1(a) shows the schematic structure of our BPSK
modulator consisting of an asymmetric MZI integrated with
a single-drive TWE. We adopt the asymmetric MZI structure
because the bias point (π-phase difference) needed for BPSK
can be conveniently obtained by tuning the input laser wave-
length. In principle, a symmetric MZI structure can be used to
provide a large optical bandwidth, but a phase shifter (e.g.,
based on a microheater) is also needed to set the bias point.
58 Photon. Res. / Vol. 3, No. 3 / June 2015 Wang et al.
2327-9125/15/030058-05 © 2015 Chinese Laser Press