Silicon PAM-4 optical modulator driven by
two binary electrical signals with different
peak-to-peak voltages
LINGCHEN ZHENG,
1,2
JIANFENG DING,
1
SIZHU SHAO,
1,2
LEI ZHANG,
1
AND LIN YANG
1,2,
*
1
State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
2
College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 101408, China
*Corresponding author: oip@semi.ac.cn
Received 4 April 2017; revised 11 May 2017; accepted 11 May 2017; posted 12 May 2017 (Doc. ID 291973); published 1 June 2017
We demonstrate a silicon PAM-4 optical modulator, which is
based on a symmetric Mach–Zehnder interferometer. Two
uncorrelated binary electrical signals with different peak-
to-peak voltages are applied to the phase shifters of the silicon
optical modulator. Accordingly, two different phase shifts are
generated in the two arms. After the permutation, there are
totally four phase differences between the two arms and the
output optical power has four levels. The device can work at
32 Gbaud in the wavelength range from 1525 to 1565 nm,
which is promising for the next-generation high-speed silicon
optical link.
© 2017 Optical Society of America
OCIS codes: (250.3140) Integrated optoelectronic circuits;
(250.7360) Waveguide modulators.
https://doi.org/10.1364/OL.42.002213
The limited bandwidths of the photonic devices and the driving
circuits are becoming the bottlenecks for improving the infor-
mation capacity of the optical communication system [1].
Advanced-format modulation is taken as an effective way to in-
crease the information capacity. In long-haul optical communi-
cation systems, quadrature amplitude modulation has been
widely used to improve the spectral efficiency and increase
the information capacity [2–4]. In short-reach applications, pulse
amplitude modulation (PAM) is becoming popular [5–8].
Moreover, PAM-4 modulation, generating four-level amplitudes,
is a possible standard modulation format in the next-generation
Parallel Single Mode fiber 4-lane (PSM-4) and Coarse
Wavelength Division Multiplexer 4-channel (CWDM-4) optical
modules for data center application. However, it is a challenge to
generate a high-speed PAM-4 electrical signal with large ampli-
tude swing, as the electrical amplifier is required to be high-speed
and linear simultaneously. Compared with the PAM-4 signal
generated in the electrical domain, optical synthesis of the
PAM-4 signal has the advantages of high bandwidth and low
added jitter. An optical digital-to-analog converter has been in-
tensively developed for many years to make an optical arbitrary
waveform generation. Optical PAM-4 generation can be consid-
ered as a case of optical arbitrary waveform generation.
Silicon photonics is taken as a promising technique for the
future optical modules [9–19]. In this Letter, we demonstrate a
silicon PAM-4 optical modulator driven by two binary electri-
cal signals with different peak-to-peak voltages, which can work
at the moderation rate of up to 32 Gbaud in the wavelength
range of 1525–1565 nm. The PAM-4 signal is synthesized in
the optical domain. The modulation scheme does not need the
high-speed linear electrical amplifier, which makes it suitable
for the next-generation 200 Gbps optical modules in data
centers.
The silicon PAM-4 optical modulator is schematically
shown in Fig. 1(a), which is based on a symmetric Mach–
Zehnder interferometer (MZM). Two binary electrical signals
(BES-1 and BES-2) are applied to the device by a GSGSG
(G: ground, S: signal) coplanar wavegu ide (CPW) electrode.
The peak-to-peak voltage of the electrical signal BES-1 is twice
as large as that of the electrical signal BES-2. Two different
phase shifts are generated in the two arms of the silicon
MZM. After the permutation, there are four phase differences
between the two arms and the output optical power has four
levels. The cross section of the PN junction is shown in
Fig. 1(b).
Figure 2 illustrates the transmission function of the MZM,
which indicates the relationship between the output optical
power and the phase difference between the two arms. The
Fig. 1. (a) Illustration of the silicon PAM-4 optical modulator
(BES, binary electrical signal; CW, continuous-wave). (b) Cross
section of the PN junction.
Letter
Vol. 42, No. 11 / June 1 2017 / Optics Letters 2213
0146-9592/17/112213-04 Journal © 2017 Optical Society of America
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