Silicon optical modulator with integrated
grating couplers based on 0.18-μm
complementary metal oxide
semiconductor technology
Haihua Xu
Zhiyong Li
Yu Zhu
Yuntao Li
Yude Yu
Jinzhong Yu
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Optical Engineering 50(4), 044001 (April 2011)
Silicon optical modulator with integrated grating
couplers based on 0.18-μm complementary metal
oxide semiconductor technology
Haihua Xu
Zhiyong Li
Yu Zhu
Yuntao Li
Yude Yu
Jinzhong Yu
Chinese Academy of Sciences
Institute of Semiconductors
State Key Laboratory on Integrated Optoelectronics
No. A35, Qinghua East Road
Beijing 100083, China
E-mail: xu_haihua@semi.ac.cn
Abstract. A silicon p-i-n diode Mach–Zehnder optical modulator integrated with grating couplers is fabricated in 0.18-μm complementary metal
oxide semiconductor technology. The device has an ultracompact length
of 200 μm. High modulation efficiency with a figure of merit of Vπ L =
0.22 V mm is demonstrated. A novel pre-emphasis technique is introduced to achieve high-speed modulation, and a data transmission rate of
C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).
3 Gbps is present. [DOI: 10.1117/1.3560264]
Subject terms: silicon photonics; modulation; grating coupler; pre-emphasis.
Paper 100521R received Jun. 24, 2010; revised manuscript received Jan. 5, 2011;
accepted for publication Feb. 9, 2011; published online Apr. 1, 2011.
1 Introduction
Silicon photonics has recently been extensively applied in the
integrated optoelectronics and optical interconnection systems for its compatibility with complementary metal oxide
semiconductor (CMOS) processes. A silicon electro-optic
modulator, as the key component of silicon photonic integrated circuits, has been widely studied in the past few
years.1–10 Three different device configurations have been
proposed: MOS capacitor types,1, 2 p + -i-n + (PIN) types
based on carrier injection,3–6 and p + -n + (PN) types based
on carrier depletion.7–10 The silicon p-i-n diode modulator,
compared to the other types, has been proven to provide
high modulation efficiency and small device scale. However,
the operation speed of the PIN modulator is limited by the
slow carrier injection time. The pre-emphasis technique is a
waveform distorting method for adding extra high-frequency
components to the initial signal to compensate transmission
losses in interconnect systems,11 and it has been proved to
improve the transmission performance of the silicon optical
modulators.4, 5
The optical modulators mentioned above are all based on
silicon submicron rib waveguides. The large dimension discrepancy between the waveguide and optical fiber will cause
high coupling loss and great alignment difficulty. A grating coupler is a butt coupling approach which can couple
light out of the plane between waveguide and fiber without a cleaved facet.12, 13 It has a dimension of a dozen microns, thus there is a large tolerance range for coupling and
alignment.
In this paper, we demonstrate a silicon forward-biased
Mach–Zehnder interferometer (MZI) modulator with an ultracompact length of 200 μm. The device is integrated with
grating couplers to achieve wafer-scale testing and obtain
high fiber-waveguide coupling efficiency. Meanwhile, we
propose a new pre-emphasis method to improve the modulation performance of the device.
C 2011 SPIE
0091-3286/2011/$25.00 Optical Engineering
2 Device Design and Fabrication
2.1 Waveguide
The silicon MZI modulator is designed using submicron rib
waveguides embedded with p-i-n diodes. The top view and
cross section of the device are shown in Fig. 1 and inset,
respectively. The rib waveguide is 450-nm wide and 340-nm
high, with a 100-nm thick slab layer.
2.2 Grating Coupler
A grating coupler has a basic period structure with a finite number of rectangular grating teeth, which is shown in
Fig. 2(a). The operation principle of the grating coupler is
based on the Bragg diffraction of the grating. The light from
the input fiber is coupled in at a small angle with respect to
the vertical axis above the grating coupler to avoid a second order diffraction, which reflects light back to the input
fiber. In our device, the grating coupler has a small scale
with a footprint of 18×14 μm2 with a uniform period of
600 nm (30 periods) and a filling factor of 0.5. To achieve
process uniformity, the etch depth of the grating couplers is
designed the same as the rib waveguides. The coupling loss
of the grating coupler is measured in the wavelength range
1460 to 1580 nm. Figure 2(b) shows that the grating coupler has a coupling loss of ∼11 and 1 dB optical bandwidth
of 35 nm.
2.3 Mach–Zehnder Interferometer
To optimize the arm length of the Mach–Zehnder interferometer, we first calculated the carrier density change of the
modulator for different bias voltages. Then, we calculated
the refractive index change via the free carrier plasma dispersion effect in silicon,14 which is related with the carrier
density change. Meanwhile, we modeled the optical mode
distribution using the optical simulation tool PhotonDesign
FIMMWAVE/FIMMPROP.15 Combined with the calculated
refractive index change and optical mode distribution, we obtained the effective index change (neff ) by means of overlap
integral methods.16 Finally, we obtained the phase shift φ
using the relationship:
044001-1
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April 2011/Vol. 50(4)
Xu et al.: Silicon optical modulator with integrated grating couplers. . .
Fig. 1 Top view of the silicon MZI modulator, with the inset for cross
section view of the device.
φ =
2π n e f f L
,
λ
(1)
where L is the arm length of the Mach–Zehnder interferometer. Figure 3 shows the voltage required for π phase shift (Vπ )
and a figure of merit for phase modulation efficiency (Vπ L)
as a function of the arm lengths. As the data (black line in
Fig. 3) shows, Vπ is decreasing nonlinearly with the increasing arm lengths, which can be explained that the refractive
index change in silicon is nonlinearly related with the applied voltage.17 As a consequence, Vπ L has a minimum value
at a certain arm length (illustrated by the inverted-trianglemarked line in Fig. 3); the optimum arm length is nearly 200
μm, which is 1 order of magnitude shorter than that of MOS
and PN type MZI modulators1, 2 with arm lengths of several
millimeters. Such ultracompact size can ensure high integration in on-chip interconnect systems, where a large number
of modulators will be used. To characterize modulation efficiency of the device, we design an asymmetric MZI with an
arm length difference of 30 μm.
Multimode-interference couplers are added to the input and output of the Mach–Zehnder interferometer as the
waveguide splitter and combiner, respectively.
The device was fabricated on a silicon-on-insulator
substrate with a 1-μm thick buried oxide layer and a
0.34-μm thick top silicon layer in Semiconductor Manufacturing International Corporation’s (SMIC) 0.18-μm
CMOS technology. The phase shifters of the MZI modulator were p + -i-n + diodes embedded in each of the
two arms. The p-doped (anode) and n-doped (cathode)
concentrations were both ∼1×1019 cm − 3 . The doped regions were ∼400 nm away from the edges of the rib
Fig. 3 Voltage required for π phase shift (Vπ ) and a figure of merit
for phase modulation efficiency Vπ L.
waveguide. A thin nickel was evaporated onto the heavilydoped regions and annealed to ensure good ohmic contacts.
The microscope image of the fabricated device is shown
in Fig. 4.
3 Experimental Results
3.1 Static Characteristic
First, we measured the I–V characteristics of the device from
forward bias 0 to 2 V; the result is shown in Fig. 5(a). Current amplitude of nearly 60 mA for forward bias of 2 V
is illustrated, which means low contact resistance between
the electrode and heavily doped regions. Large current can
improve the modulation efficiency due to the optical modulation mechanism of the free carrier plasma dispersion effect
in silicon.14 Meanwhile, a low forward differential resistance
of ∼ 20 is revealed to ensure low power consumption.
To obtain the modulation efficiency of the device with a
length (L) of 200 μm, we characterize the optical spectra
for the different forward biases [shown in Fig. 5(b)]. The
spectra are blueshifted for increased voltages and the voltageinduced phase shift (φ) can be obtained by the following
relationship:17
φ = 2π
λ
,
FSR
(2)
Fig. 2 (a) The schematic of the grating coupler ( is the grating period); (b) the coupling loss of the grating coupler. The minimum loss is 11 dB
and the 1 dB optical bandwidth is 35 nm.
Optical Engineering
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April 2011/Vol. 50(4)
Xu et al.: Silicon optical modulator with integrated grating couplers. . .
Fig. 4 Microscope image of the fabricated silicon MZI modulator.
where λ is the voltage-induced wavelength shift and FSR is
the free spectral range of the MZI modulator. As the optical
spectra show, FSR is ∼20 nm and the wavelength shift at the
forward bias 1.1 V is ∼10 nm. According to Eq. (2), the π
phase shift is obtained at 1.1 V, resulting in a figure of merit
Vπ L = 0.22 V mm.
The extinction ratios (ERs) of the device at 1545 nm wavelength for the different biases are demonstrated in Fig. 5(c).
A large ER of ∼18 dB at 1.1 V bias was obtained to achieve
good data transmission and low bit-error-rate.
3.2 Dynamic Characteristic
As mentioned in Sec. 3.1, the slow carrier diffusion dynamic limits the operation speed of the silicon p-i-n diode
modulators. The pre-emphasis technique has been proved to
improve the operation speed of the injection-based silicon
modulators.3, 4
The pre-emphasis signal generation methods reported in
Refs. 3 and 4 contained an extra impulse generation network which is complicated and difficult to achieve. Here,
we propose a new method to generate pre-emphasis signals; the schematic is shown in Fig. 6(a). Two synchronous
nonreturn-to-zero (NRZ) signals (CH1 and CH2) are first
generated from a pattern generator [shown in Fig. 6(b)]. The
CH2 signal is inverted and delayed by an inverter and a
delay controller, respectively. Both of the signals are then
combined and amplified to obtain the pre-emphasis signal
[shown in Fig. 6(c)]. The detailed information about the
pre-emphasis technique can be referred to another paper
of ours.18
We measured the data transmission performance of the
MZI modulator using a pre-emphasis NRZ data at 3 Gbps.
The eye-diagram is shown in Fig. 7. Jitter and noise are
caused by the discrepant response of the different bits;
one can optimize the pre-emphasis signals to improve the
quality of the eye-diagram. To improve the speed performance, the electrical structure of the PIN diode modulator can be optimized; for example, the distance between
the heavily doped region and rib waveguide can be reduced to enhance the carrier diffusion motion. Meanwhile,
one can design contract resistances carefully to reduce the
RC constant.
In order to estimate data transmission performances of the
modulator under different bias conditions, we calculated the
transient responses of the device for three voltage amplitudes
(Vpp = 1.5, 2, and 2.5 V), which are shown in Fig. 8. The
rise time of the optical output for 1.5 V bias voltage is quite
Optical Engineering
Fig. 5 (a) I–V characteristic curve of the MZI modulator; (b) normalized output spectra for the forward biases of 0, 1.0, and 1.1 V applied
to one of the arms; and (c) normalized optical output for different
forward biases at 1545 nm.
slow [nearly 3 ns, shown in Fig. 8(a)]. When the bias voltage
increases, the rise time will be improved [nearly 1.5 and
0.8 ns for bias voltages 2 and 2.5 V, shown in Figs. 8(b)
and 8(c)]. This can be explained that with the increased bias
voltages, the majority carrier concentration in the PIN diode
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April 2011/Vol. 50(4)
Xu et al.: Silicon optical modulator with integrated grating couplers. . .
Fig. 6 (a) The schematic of the pre-emphasis signal generation
method; the waveform of the NRZ signal (b); and pre-emphasis signal
(c) at data transmission of 3 Gbps.
is also increasing, which means that carrier diffusion motion
has been enhanced, resulting in a faster rise time response.
4 Results
We demonstrate an injection-based silicon MZI modulator
integrated with grating couplers using a 180-nm CMOS
Fig. 8 Simulated transient characteristic for different bias voltages.
(a) Vpp = 1.5 V; (b) Vpp = 2 V; (c) Vpp = 2.5 V.
process. The device has an ultrashort length of 200 μm and
an ultra-low Vπ L of 0.22 V mm. A new pre-emphasis generation method has been proposed to achieve optical modulation
speed at data transmission of 3 Gbps.
Fig. 7 The eye-diagram of the output at 3 Gbps NRZ data.
Optical Engineering
Acknowledgments
The authors are grateful to Xianyong Pu, Zuoya Yang, and
other technicians of SMIC, Shanghai for their helpful discussions and expert technological assistance. This work was
supported by the National Basic Research Program of China
(Grant No. 2006CB302803), the National Natural Science
Foundation of China (Grant No. 60877036), State Key Lab044001-4
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April 2011/Vol. 50(4)
Xu et al.: Silicon optical modulator with integrated grating couplers. . .
oratory of Advanced Optical Communication Systems and
Networks, China (Grant No. 2008SH02), and the Knowledge
Innovation Program of Institute of Semiconductors, Chinese
Academy of Sciences (Grant No. ISCAS2008T10).
Zhiyong Li received his PhD degree in from the graduate school of
Chinese Academy of Sciences in 2007 and is an assistant researcher
in the Institute of Semiconductors, CAS. His major interest is in silicon photonics, especially optical switch, modulator, and slow-light
device.
Yu Zhu received his PhD degree in microelectronics and solid electronics in from the Institute of Semiconductors, Chinese Academy
of Sciences, Beijing, China. His major interest
is in silicon photonics including silicon grating
coupler and other passive devices.
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Haihua Xu received his BS degree in
electronic science and technology from
Huazhong University of Science and Technology, Wuhan, China, in 2006. He is currently working toward his PhD degree in
microelectronics and solid electronics in
the Institute of Semiconductors, Chinese
Academy of Sciences, Beijing, China. His
current research interests include siliconbased optoelectronics integration, measurement, and packaging.
Optical Engineering
Yuntao Li received his PhD degree in from
the graduate school of Chinese academy
of sciences in 2007 and is an assistant researcher in the iInstitute of sSemiconductors,
CAS. His major interest is in silicon photonics, especially optical switch, modulator
and slow-light devices, and biological photonics. His current interests include siliconon-insulator optoelectronic devices for interchip optical interconnection and lab-on-chip
optoelectronics devices for biological detection. He has published more than 30 papers and 4 issued
patents.
Yude Yu graduated from the department of
physics, University of Science and Technology of China, Hefei, China, in 1977. From
1977 to 2003, he worked at the Institute
of Physics, Chinese Academy of Sciences
(CAS), Beijing, China, and his research field
focused on crystal structure analysis by x
ray diffraction, new material exploration, single crystal growth, and material science research under microgravity condition. During
1987 to 1989, he did research work on neutron scattering at Institute fuer Kristallographie, University of Munch,
Germany, as a visiting scholar. In 2003, he transferred to Institute of
Semiconductors, CAS. His present interests include Si-based photonics and material science research under microgravity condition.
He has published over 40 papers.
Jinzhong Yu graduated from the University
of Science and Technology of China, Beijing
in 1965 and received his doctoral degree in
electrical engineering from Osaka University,
Japan in 1991. In 1965 he joined the Institute of Semiconductors, Chinese Academy
of Sciences. Since then, he has engaged in a
study on laser diodes, detectors, and waveguide devices. His present research concentrates on Si-based photonics and integrated
optoelectronics. He has been a professor in
the Institute since 1994. He is a member of the American Society of
Optics and SPIE, a member of Chinese Society of Optics, and a member of Chinese Society of Electronics. He has received awards such
as Chinese Academy of Sciences’ Achievement Award (Grade 2)
(1985 and 1991) and the Chinese National Science Progress Award
(Grade 2) (1986 and 1992). He is an owner of Chinese Government
Supported subsidy since 1992. He has published more than 180
papers in Chinese and English.
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