128-Gb/s DP-QPSK Silicon Optical Modulator Module
Tsung-Yang Liow,1 Xiaoguang Tu,1 Guo-Qiang Lo,1 Dim-Lee Kwong,1
Hiroki Ihihara,2 Yasuhiro Mashiko,2 Kazuhiro Goi,2 and Kensuke Ogawa3
Digital coherent communication systems have been deployed in long-haul optical transport
networks. The systems are now extended to metro-area optical networks, where small form
factor and low-cost optical modules are crucial. Small-footprint PDM IQ optical modulator
based on silicon-photonic platform and low-profile packaging technology for the silicon optical
modulator are developed in Fujikura to meet the requirement of compact digital coherent
transceivers in the metro networks. This review focuses on a monolithic silicon PDM IQ optical
modulator copackaged with high-speed electrical modulator drivers in a small-footprint lowprofile module and its transmission performance in optical-fiber links up to 1000 km in 128-Gb/s
DP-QPSK for digital coherent communication in the metro networks.
1. Introduction
There is growing data transport in mobile communication on account of extended use of smart phones
and tablets dealing with large-size contents. The mobile traffic in various routes is forwarded to long-haul
back-bone optical transport networks, and thus higher
transmission capacity is required to optical-fiber telecommunication. Digital coherent communication, in
which optical signals are multiplexed in advanced
modulation formats such as DP-QPSK, has taken the
place of the conventional binary on-off keying (OOK)
because of its capability of high-speed communication
1) 2)
. DP-QPSK are superior to OOK, because optical
signals are multiplexed into 4 bits per symbol in DPQPSK after 2-bit multiplication in QPSK and further
2-bit multiplication in two orthogonal polarization
states, thereby 100 Gb/s transmission is realized with
25-Gbaud symbol rate. Layouts and configurations of
optical devices and electric circuits for DP-QPSK are
more complex than those for OOK due to the high
multiplicity of transmission data. Therefore, small
form factors and low costs are crucial to the deployment of digital coherent communication systems in
optics networks such as metro-area optical networks
consisting of a number of nodes, where huge number
of optical transceivers in transport equipment are stationed. Digital coherent transceivers in metro-area optical networks are required to be ten times smaller in
footprint according to roadmaps such as for compact
pluggable digital coherent transceivers 3) 4).
Optical modulators designed and fabricated on silicon-photonic platform are most suited to compact
1 Institute of Microelectronics, Singapore
2 Optical Device Research Department
3 Optical Device Research Department, Chief Researcher (Ph. D)
Advanced Technology Laboratory
30
form factor optical transceivers. High-index contrast
optical waveguides formed in SOI wafer and monolithic integration of optical devices using design rules and
CMOS-based fabrication technology lead to small-footprint photonic circuits constituting optical modulators
for DP-QPSK such as photonic circuits for PDM, IQ
optical phase modulation, and monitor photodetectors
(PDs) for performance monitoring. Chip fabrication
on large-scale SOI wafers of diameter as large as 200
mm or more has the advantage of cost reduction in
production. Monolithic silicon PDM IQ optical modulator chips in 5 x 6.5 mm2 in footprint per each chip
have been designed and fabricated in Fujikura, and
128-Gb/s DP-QPSK modulation was proved on chip
characterization 5) 6).
In this review, silicon DP-QPSK optical modulator
module, in which a monolithic silicon PDM IQ optical
modulator chip is copackaged with high-speed electrical modulator drivers in a small-footprint low-profile
ceramic-based metal housing, is presented, and 128Gb/s DP-QPSK modulation in optical-fiber transmission up to 1000 km is proved using the module as a
candidate for compact digital coherent transceiver in
metro-area optical networks.
2. 128-Gb/s silicon DP-QPSK optical modulator
module
Top-view photograph of a silicon PDM IQ optical
modulator chip is presented in Fig. 1 with schematic
layout of building blocks in the chip. The chip was fabricated by CMOS-based fabrication processes on a
200-mm SOI wafer. In DP-QPSK modulation, 4-bit signal is produced per symbol in PDM of IQ modulation
with the layout depicted in Fig. 1. The layout consists
of the following four essential building blocks: (1) an
input waveguide split into two parallel waveguides
RF
input
5.1 mm
IQ2
6.5 mm
IQ1
IQ1
TO phase
controller
IQ2
PR
PR
PBC
Ge PD
RF output
input light
output light
PBC
Ge PD
input light
output light
high-speed modulator driver
Fig. 1. Top-view photograph and layout of silicon PDM IQ
optical modulator chip.
bias tee
modulator chip
RF in
with 1x2 splitter consisting of a multi-mode interferometer, (2) two silicon IQ Mach-Zehnder (MZ) modulators, each of which consists of a silicon nested MZ
interferometer (MZI) incorporating silicon single subMZIs on the both of parent MZI arms and operates in
TE polarization state, (3) a polarization rotator (PR),
with which polarization of light after one of the IQ MZ
modulators is converted to TM polarization and (4) a
polarization beam combiner (PBC) to multiplex TE
and TM optical modulation signals. The all optical
waveguides in the elements of IQ modulators, PR and
PBC were designed under a common design rule,
where all the waveguide sections have the same vertical heights in their rib parts and slab parts, and fabricated in common lithography and etching processes
simultaneously with low process variation in heights.
A Si PDM IQ MZ modulator was monolithically integrated in a footprint as small as 5 x 6.5 mm2 with germanium PDs for performance monitoring and silicon
thermo-optic (TO) phase controllers to sustain phase
separation of p/2 between I and Q optical phase components in each polarization by adjusting electrical
heater currents to the TO phase controllers. The germanium PDs operate in a wide spectral range over C
and L bands 7). The optical phase shifters in the subMZIs are driven with RF signals at a symbol rate of 32
Gbaud including a margin for forward-error-correction
coding bits 8).
Top-view photograph of silicon DP-QPSK optical
modulator module is presented in Fig. 2. A monolithic
silicon PDM IQ optical modulator chip was copackaged with four high-speed electrical drivers, bias tees
and RF terminators in a ceramic-based metal housing
as illustrated schematically in Fig. 2. RF signals for
QPSK components in the two orthogonal polarization
states are fed through RF pins to the modulator drivers. The RF pins are disposed on one of the shorter
side faces of 15-mm width. Input polarization-maintain-
Fujikura Technical Review, 2015
RF terminator
35 mm
4.5 mm
15 mm
optical fiber
Fig. 2. Photograph of silicon DP-QPSK optical modulator
module and illustration of its inside.
ing and output single-mode fiber pigtails are mounted
on the other shorter side face. The ceramic base has
the advantage of thermally stable low-strain assembly
because the linear thermal expansion coefficient of ceramic is close to that of silicon crystal. The module has
been hermetically sealed with a metal lid in a dimension of 15 x 35 mm2 in footprint and 4.5 mm in height,
thereby small-footprint low-profile module has been
realized on the basis of silicon-photonic platform. DC
pins for reverse biases to the silicon optical phase
shifters and heater current to the TO optical phase
controllers are disposed on one of the longer side faces of 35-mm width.
For low-profile packaging, butt coupling with input
and output optical fibers has been adopted instead of
conventional lens coupling 9). A suspended mode-field
converter was designed and fabricated for low-loss
butt coupling between the input and output optical fibers and the waveguides of the silicon PDM IQ optical
modulator chip. The mode-field converter consists of a
suspended silica waveguide, silica taper waveguide
and a silicon nano-taper waveguide in series as illustrated in Fig. 3. Adiabatic mode conversion in the suspended mode-field converter allows optical energy
transfer between the mode fields of 10-mm diameter in
single-mode silica fibers and the mode fields in highindex contrast silicon waveguides of submicron diameter with coupling loss as low as 2.0 dB per facet 10).
31
Bridge
Si
SiO2
Silica
Air groove
SiO2
Air
SiO2
AIr
2.5
Coupling Loss [dB]
Silica
Waveguide
3
Silicon nano-taper
waveguide
Cross section
Silicon
substrare
2
1.5
1
0
1530
Fig. 3. Perspective and coress-section illustrations of suspended
fiber-waveguide coupler.
TE
0.5
TM
1550
1570
1590
Wavelength [nm]
1610
Fig. 4. Coupling losse between suspended coupler and
polarization-maintaining fiber.
32 Gbaud
PPG
(PRBS)
trigger
LO
laser
2x2
coupler
silicon modulator
module
coherent receiver
optical
switch
offline
DSP
fiber loop
BPF
EDFA
100 km
SMF
Fig. 5. Setup for optical-fiber transmission measurement.
Wavelength dependence of coupling loss was measured and is plotted in C and L bands in TE and TM
polarizations in Fig. 4. Polarization-maintaining fiber
was used in the measurements. The coupling loss is
about 2 dB at a wavelength of 1550 nm for the both
polarizations. Polarization dependence of the coupling
loss is less than 0.3 dB over C and L bands. Therefore,
the suspended mode-field converter is suited to lowloss fiber-waveguide coupling with high polarization
diversity required for DP-QPSK modulation in C and L
bands. The total optical insertion loss of the modulator
module excluding modulation loss is 15 dB or lower in
C and L bands. Further design refinements on silicon
optical phase shifters as well as the suspended modefield converter allow further reduction in the total optical insertion loss.
3. DP-QPSK transmission performances
Transmission performances of the silicon DP-QPSK
modulator module have been characterized in the set32
up depicted in Fig. 5. Single-mode continuous-wave
(CW) light at a wavelength of 1550 nm from a laser
with emission linewidth narrower than 100 kHz was
input to the module. The input light was separated
through a fiber splitter and input also to a coherent
receiver as LO light in homodyne coherent detection.
PRBS electrical signals in 231-1 bit length at a symbol
rate of 32 Gbaud were fed to RF input pins of the module. The PRBS electrical signals were amplified with
the high-speed modulator drivers to peak-to-peak voltage amplitude of +/-3.25 V in 50-W impedance matching to generate DP-QPSK optical signals at a bit rate of
128 Gb/s.
The optical signals from the module were loaded
into an optical-fiber link consisting of an optical burst
switch and a fiber loop, which was connected to input
and output optical fibers of the link with a 3-dB 2x2
optical coupler. The fiber loop was constructed with
100-km SMF, an EDFA and a BPF. bust trains of DPQPSK optical signals were generated by switching on/
off the burst optical switch and then input to the fiber
loop. The burst period was determined to avoid contention between optical signals in specified loop turns
and those in unspecified loop turns in the coherent
receiver. Electrical signals after the coherent detection were output from the coherent receiver and input
to offline digital signal processor (DSP). The electrical
signals in the specified turns were selected and demodulated to plot constellation diagrams in the specified transmission distance in the offline DSP using
electrical trigger pulses from the burst optical switch
as a reference time base. An EDFA was inserted to
compensate propagation loss in 100-km SMF and 3-dB
splitter loss at the 2x2 coupler per turn. Amplified
spontaneous emission noise from the EDFA was partially eliminated with a BPF, which has a spectral pass
bandwidth of 1 nm around the peak of the optical spectrum of the DP-QPSK signals. Constellation diagrams
of the DP-QPSK signals were acquired
Constellation diagrams in back-to-back transmission (zero loop turn), 500-km transmission (five loop
turns) and 1000-km transmission (ten turns) in two
orthogonal linear polarization states (X and Y) are plotted in Fig. 6. Four constellation spots corresponding to
four bit states in QPSK are clearly resolved in X- and
Y-polarization states. BER was obtained as <10-5, ~1.1
x10-3 and ~5.3x10-3 in back-to-back, 500-km and 1000km transmission distances, respectively. The BER in
1000-km distance is still lower than the limit of the forward error correction, thereby error-free transmission
up to 1000-km SMF transmission has been proved.
Therefore, the silicon DP-QPSK optical modulator
module presented in this review is a candidate for
small-footprint optical modulators in digital coherent
transceivers of compact form factors to be deployed in
metro-area optical networks.
4. Conclusion
A small-footprint monolithic silicon PDM IQ optical
modulator chip fabricated on a 200-mm SOI wafer was
copackaged with high-speed modulator drivers in a
hermetically sealed ceramic-based metal housing of
DP-QPSK optical modulator module. The module has
a footprint as small as 15x35 mm2 and a height as low
as 4.5 mm. Transmission performances of the module
in 128-Gb/s DP-QPSK were characterized in the constellation measurements using an optical fiber link
with a 100-km fiber loop. BER in the constellation diagrams in 1000-km transmission in the fiber link was
lower than the upper limit of forward error correction,
thereby error-free transmission is assured. The modulator module is a candidate for small-footprint low-profile modulators for compact form factor digital coherent transceivers to be deployed in metro-area optical
networks.
References
1)Y. Miyamoto and I. Morita, “High-capacity optical communication system enhanced by digital signal processing technologies,” The Journal of the Institute of Electronics, Infor-
Back-to-back
500 km
0
–1
0
In-phase
1
0
In-phase
1
–1
–1
0
–1
–1
0
In-phase
BER<10–5
1
0
In-phase
1
1
Quadrature
Quadrature
1
0
0
–1
–1
1
Quadrature
0
–1
–1
Y-pol.
1
Quadrature
1
Quadrature
X-pol.
Quadrature
1
1000 km
0
–1
–1
0
1
In-phase
BER~1.1 x 10–3
–1
0
1
In-phase
BER~5.3 x 10–3
Fig. 6. Constellation diagrams.
Fujikura Technical Review, 2015
33
mation and Communication Engineers, vol. 94, no.2,
pp.72-78, 2011
2)S. Suzuki et al., “R & D on the digital coherent signal processing technology for large-capacity optical communication networks,” The Journal of the Institute of Electronics,
Information and Communication Engineers, vol. 95, no.12,
pp.1100-1116, 2012
3)CFP Multi-Source Agreement Documents, http://www.cfpmsa.org/documents.html
4)C. Cole, “Next Generation CFP Modules,” in Optical Fiber
Communication Conference and Exposition and National
Fiber Optic Engineers Conference 2012 (OFC/NFOEC,
Los Angeles, 2012), NTu1F.1
5)T.-Y. Liow, X. Tu, G.-Q. Lo, D.-L. Kwong, K. Goi, A. Oka, H.
Kusaka, Y. Mashiko and K. Ogawa, “128-Gb/s Monolithic
Silicon Optical Modulator for Digital Coherent Communication”, Fujikura Technical Review vol. 44, pp.1-8, 2015
6)K. Goi, et al, “128-Gb/s DP-QPSK using low-loss monolithic
silicon IQ modulator integrated with partial-rib polarization
34
rotator,” in Optical Fiber Communication Conference and
Exhibition 2014 (OFC, San Francisco, 2014), W1I.2
7)H. Kusaka, et al, “Monolithic Photonic Integrated Circuit
for Optical Performance Monitoring of Silicon MachZehnder Modulator in C and L Bands,” in 18th OptoElectronics and Communications Conference/Photonics in
Switching 2013 (IEICE, Kyoto, 2013), MM1-4
8)K. Onohara, et al, “Soft-Decision-Based Forward Error Correction for 100 Gb/s Transport Systems,” IEEE Journal of
Selected Topics in Quantum Electronics, vol. 16, no.5,
pp.1258-1267, 2010
9)H. Ishihara, et al, “High-On/Off-Contrast 10-Gb/s Silicon
Mach-Zehnder Modulator in High-Speed Low-Loss Package, “ in International Conference on Electronics Packaging
2014 (ICEP, Toyama, 2014), FE2-1
10)Q. Fang, et al, “Mode-size converter with high coupling efficiency and broad bandwidth,” Optics Express, vol. 19,
no.22, pp.21588-21594, 2011