Download Silicon photonics for advanced optical communication

Silicon photonics for advanced optical
communication systems
Zhiping Zhou
Xingjun Wang
Huaxiang Yi
Zhijuan Tu
Wei Tan
Qifeng Long
Mei Yin
Yawen Huang
Optical Engineering 52(4), 045007 (April 2013)
Silicon photonics for advanced optical
communication systems
Zhiping Zhou
Xingjun Wang
Huaxiang Yi
Zhijuan Tu
Wei Tan
Qifeng Long
Mei Yin
Yawen Huang
Peking University
School of Electronics Engineering and Computer
Science
State Key Laboratory of Advanced Optical
Communication Systems and Networks
Beijing 100871, China
E-mail: [email protected]
Abstract. Recent progress on Si-based optical components for advanced
optical communication systems has been demonstrated. The polarization
beam splitter with extinction ratio of more than 20 dB and the optical
90-deg hybrid having phase deviation within 5- deg were obtained
using multimode interference structures. The 12 Gb∕s modulators and
the 20 GHz photodetectors were measured. Benefiting from the unique
properties of silicon modulator, an error-free 80 Km transmission of the
signals generated by our silicon carrier-depletion Mach-Zehnder modulator was also demonstrated at 10 Gb∕s and the power penalty was as low
as 1.15 dB. These results show that silicon photonics has a great potential
in advanced optical communication systems. © 2013 Society of Photo-Optical
Instrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.52.4.045007]
Subject terms: silicon photonics; coherent optical communication; modulator; optical hybrid; germanium photodetector.
Paper 130311P received Feb. 28, 2013; revised manuscript received Mar. 22, 2013;
accepted for publication Mar. 25, 2013; published online Apr. 19, 2013.
1 Introduction
The year 2012 was actually another year for rapid development in information technology. New applications and
services continue to demand higher bandwidth and more
functionalities, which drive advancement of optical communication systems. Among many advanced approaches, coherent optical communication is coming back in a rather strong
fashion.
Coherent optical communications were studied extensively in the 1980s mainly due to the fact that the high
sensitivity of coherent receivers permit much longer transmission distance for the same amount of transmitter
power, and the use of coherent detection allows an efficient
use of the bandwidth. However, their research and development have been interrupted for nearly 20 years behind the
rapid progress in high-capacity wavelength division multiplex (WDM) systems using erbium doped fiber amplifiers
(EDFAs). The advancement in digital signal processing
(DSP), particularly, the demonstration of digital carrier phase
estimation in coherent receivers in 2005, has awoken a widespread interest in coherent optical communications.
Via custom-designed DSP functions for adaptive polarization tracking, chromatic dispersion compensation, and forward error correction (FEC), communication capability was
boosted through coherent optical communication systems.
Commercial coherent systems for fiber-optic networks were
introduced at 40 and 100 Gb∕s in 2008 and 2010, using
polarization-division multiplexed (PDM) quadrature phaseshift keying (QPSK) at 11.5 and 28 Gb∕s.1 In research
labs, single-carrier 640 Gb∕s has been achieved using PDM
quadrature amplitude modulation (QAM) at a symbol rate of
80 Gb∕s.2 However, the complex transmitters and receivers
in these systems suffer from high power consumption, large
volume, and weak stability of the individual components.
To tackle with these problems, one of the best approaches
is the monolithic integration of transmitters and receivers.
0091-3286/2013/$25.00 © 2013 SPIE
Optical Engineering
The use of silicon photonics platform promises a low cost
solution since optical, optoelectronic and electronic components may be integrated onto a single chip through mature
CMOS technology.
2 System Description
A coherent optical communication system includes a transmitter, a receiver and a single local oscillator serving the
receiver. The output of the local oscillator is directly fed to
one input of a coherent receiver. The transmitter and receiver
are the core part of a coherent optical communication system
to generate and receive the optical signal. In 2011, one hundred Gigabit Ethernet standards were established. One of the
most promising technologies for a 100 Gb∕s coherent optical communication system is the combination of dual-polarization quadrature phase-shift keying (DP-QPSK) signals
with a phase and polarization diversity coherent receiver.3,4
For the optical part of transmitter, advanced modulation formats for 100 Gb∕s will be QPSK, requiring half the symbol
rate compared with conventional PSK. Moreover, polarization multiplexing could provide another factor of 2 in reduction of the symbol rate, reaching 25 Gb∕s symbol rate per
optical tributary. Therefore, the optical modulator in the
transmitter needs to have a 25 Gb∕s symbol rate. In addition,
other parameters are also very necessary for 100 Gb∕s optical transmitter, such as above 20 dB On/Off extinction ratio
(ER), 14 dB optical insertion loss and lower drive voltage
(5 V) according to implementation agreement for Integrated
Polarization Multiplexed Quadrature Modulated Transmitters
by Optical Internetworking Forum (OIF). For the optical
coherent receiver, above 20 dB, ER is required to make the
transverse electric (TE) and transverse magnetic (TM) separate. Another key component is the 90-deg hybrid that separates the quadrature components of the incident signal using a
continuous wave local oscillator reference source, the lessthan 5- deg phase error between the in-phase X-polarization
component (XI) and quadrature X-polarization component
(XQ) and between the in-phase Y-polarization component
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(YI) and in-phase Y-polarization component (YQ) were
needed.5 The other important component is a high-speed
photodetector (PD) with high quantum efficiency and highpower handling capability. For above system, 25 Gb∕s,
symbol rate per optical tributary, larger than 0.5 A∕W
responsibility, and less than 150 nA dark current were
required.6 Besides the required high speed operation, energy
consumption and cost also play an important role. The usage
of silicon photonics promises a reduction of costs because
optical, optoelectronic and electronic components can be
integrated on a single chip by self-alignment techniques.
With support from the National High Technology
Research and Development Program (863) of China, we
have been working on a 100 Gb∕s coherent optical communication system on SOI substrate in recent years. It consists
of a multilevel modulated transmitter and coherent receiver.
For the optical transmitter, a multimode interference (MMI)
polarizing beam splitter (PBS) was planned on the SOI substrate, in order to achieve high coupling efficiency and
polarization splitting. The signal was coupled into the waveguide, and divided into two orthogonal polarized beams, TE
and TM. Then one polarized beam entered the upper or lower
arms of the Si based Mach-Zehnder modulator to form a
multilevel, multiphased signal stream. Finally, two orthogonal polarized signal beams were combined into one, which
was transmitted outwards. At the optical coherent receiver
end, the similar MMI-PBS was also designed to achieve
high coupling efficiency and polarization splitting. The signal beam and local oscillation beam were coupled into the
input waveguide of the optical 90-deg hybrid. The optical
90-deg hybrid was designed using a 4 × 4 MMI interferometer structure. Finally the mixed beam was divided into a
2 × 2 two-way balanced Germanium photodetectors to realize coherent detection.
In this paper, we will report our progress in Si based photonic components, which are designed and optimized, in the
optoelectronics domain, for the purpose of the coherent optical communication. The main goals are to contribute in
phase estimation, polarization diversity, linearity, and spectral efficiency.
3 Components
As described above, passive and active components are
required to form a coherent optical communication system,
which are sometimes crucially important to the success of an
advanced system. In this section, we will describe our work
on the key components for the coherent optical communication system, namely the polarization beam splitter, hybrid,
modulator, and the photodetector.
3.1 Polarization Beam Splitter
Many approaches have been proposed to realize polarization
splitting, such as directional couplers,7 Mach-Zehnder interferometers,8 grating couplers,9 and multimode interference
couplers. Among them, we prefer the MMI-based PBS
due to their unique properties, including low crosstalk and
imbalance, large optical bandwidth, ease of fabrication, and
good tolerances. Moreover, a wide variety of materials
have been adopted for making the PBSs, such as silica,10
InGaAsP/InP,11 LiNbO3 ,12,13 polymer materials,14 and siliconon-insulator (SOI).9,15 The SOI is selected for compatibility
with CMOS processing and for compact design resulting
Optical Engineering
from the high refractive index contrast between silicon
(n ¼ 3.45) and SiO2 (n ¼ 1.46). The geometrical birefringence results in a distinct difference in the MMI self-image
length between TE mode and TM mode. Generally, the total
length of the device is the least common multiple of lengths
of the first self-image of TE and TM modes. In our case, the
lengths of the first self-image of TE and TM modes are 517
and 470 μm. Originally, the total length of PBS can be design
as 5170 μm that equals 10 times of 517 μm and 11 times of
470 μm. In our work, the quasi-states16 theory is adopted and
the length of our device, which is 1034 μm described in following principle, is decreased to one-fifth of the general
length of 5170 μm. This compact SOI MMI waveguide outperforms competing polarization splitting behavior, which
enables us to integrate the other components, such as modulators in parallel, into a DP-QPSK system.
This PBS is designed with the self-image principle and
quasi-state (QS) theory. Defined Lπ as the beat length of the
two lowest-order modes,17 the length of the first self-image is
3Lπ , which can be calculated in the equation
Lπ ¼
π
4n W 2
≈ r e;
β0 − β1
3λ0
(1)
where the β is the propagation constants, nr represents the
effective index, W e is the effective width and λ denotes
the wavelength. The design of PBS meets the length,
L ¼ ðm1 ÞðLπTE Þ ¼ ðm2 ÞðLπTM Þ, where m1 and m2 are an
even and odd number or vice versa, which separates the
TE and TM modes into two outputs. Concerning QS theory,
the QS images are formed before or after every normal image
in the propagation direction. The beat length between a
normal-state and a QS images is just 1∕5 of that between the
normal-states. By the approximation of self-imaging principle with power distribution of the lowest five modes, the QS
image is used in TM mode and the normal image is utilized
in TE mode. Combining the self-image principle with the QS
theory, the split length of this MMI device is shortened to be
one-fifth of a normally designed MMI split length. At a
length of device, one of the outputs comes out the TE mode
and the other goes with the TM mode, which results in a PBS
in a multimode interference coupler. Figure 1 shows the
schematic of the cross-section of a SOI waveguide and
the configuration of a MMI coupler of our design.
The waveguides are established on the 340-nm-thick
top silicon layer between the 1-μm-thick insulator layer
and air. The length and width of the multimode interference
region are 1034 and 8 μm. The width of the input and output
straight waveguides is 2.8 μm. In the 3D-BPM simulation, to
keep the extinction ratio more than 15 dB, the theoretical
tolerances of width and length are 0.14 μm and
Fig. 1 Schematic structure of the PBS based on the QS-MMI coupler.
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Fig. 2 Tolerances of the width (a) and the length (b) in the multimode interference region.
35 μm for TM, 0.04 μm and 10 μm for TE, as shown
in Fig. 2.
The scanning electron microscope (SEM) images of the
device are shown in Fig. 3, where the ∼ sign means the omitted part which represents the MMI section.
The device is fabricated using electron beam lithography
and inductively coupled plasma etching. A standard integrated optics setup with a tunable laser, a determined
polarization controller (DPC5500) and a charge coupled
device (CCD) are used to test the device. For different input
polarization states, their output images are shown in Fig. 4.
The bandwidths and the extinction ratios are obtained
using the optical spectrum analyzer, as shown in Fig. 5.
For TE and TM mode, the extinction ratios are 27.3 and
26.6 dB at λ ¼ 1.55 μm, respectively. For the extinction
ratio more than 15 dB, the bandwidths are about 20 nm
in TE polarization and more than 40 nm in TM polarization
with the center wavelength at 1.55 μm.
3.2 Optical 90-deg Hybrid
Fig. 3 SEM image of the device.
Fig. 4 MMI output images on CCD camera: (a) TE input, (b) TM input.
A QPSK system is considered as the most promising
approach to increase the transmission capacity. In order to
demodulate the signal light, which is modulated by the
QPSK system, a multilevel coherent transmission systems
consisting of an optical 90-deg hybrid is prerequisite. So
far, the proposed methods to realize the optical 90-deg hybrid
include star couplers,18,19 array waveguide gratings (AWG),20
and multimode interference couplers.21–24 The disadvantages
of star couplers and array waveguide gratings are low transmission efficiency, complicated design, high imbalance, and
large footprint. However, the MMI-based hybrids have their
unique merits including large optical bandwidth, compact
size, low crosstalk and imbalance, and ease of fabrication,
as shown in Table 1. Then, we will present our design of
the optical 90-deg hybrid using 4 × 4 MMI couplers on silicon-on-insulator (SOI) platform, and show the simulation
and experiment results. The transmission efficiency of our
Fig. 5 Measured output power as a function of wavelength at (a) port bar and (b) port cross.
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Table 1 The advantages and disadvantages of different methods of
the optical 90-deg hybrid.
Transmission
efficiency
Design
Imbalance
Footprint
Star
couplers
Low
Complicated
High
Large
AWG
Low
Complicated
High
Large
MMI
High
Simple
Low
Small
device is over 95% across C-band, whereas that of the star
couplers is only better than 30%.19 The excess loss of our
hybrid is about 1 dB, while that of the array waveguide grating is 9.5 dB.20
The 4 × 4 MMI 90-deg hybrid is based on the self-image
principle, which is due to the interference between a large
numbers of supported modes in multimode waveguides.
The device we designed has six ports with two inputs for
QPSK signal (S) and local oscillator (LO) signal, respectively, and four outputs used for getting a certain phase quadrature relationship for coherent optical detection. The
schematic of the optical 90-deg hybrid is shown in Fig. 6.
Based on the influence of the coupler geometry on imbalance and excess loss, the width of the multimode waveguide
and the access waveguides are 10 and 1 μm, respectively.
And the length of the multimode waveguide can be designed
by Lmmi ¼ neff · W 2eff ∕λ, where W eff and neff are the effective
width and the effective index of the multimode waveguide,
so the optimum coupler length is about 200 μm. We simulate
our devices in 340 nm top silicon SOI wafer with fully etching technology. Since the devices are used for C-band transmission, the 4 × 4 MMI couplers are designed at
wavelength λ ¼ 1550 nm.
An important figure of merit for the 4 × 4 MMI coupler is
phase deviation defined as the phase difference between the
ideal quadrature phase and actual phase, which determines
the system bandwidth. The phase deviation of our 4 × 4
MMI coupler is found below 2.2-deg across C-band in simulation, as shown in Fig. 7. The results, therefore, indicate
satisfactory C-band performance of 4 × 4 waveguide couplers. The transmission efficiency is above 98% at 1550 nm,
and the excess losses of both two input ports are less than
0.5 dB, which means the extinction ratio is larger than
20 dB and fulfils the system demand.
3.3 Modulator
Silicon modulators as essential components of advanced
optical communication systems have been extensively
Fig. 6 Schematic of the optical 90-deg hybrid with balanced PDs.
Optical Engineering
Fig. 7 Phase deviation of the 4 × 4 MMI coupler in simulation.
studied for their capability of small footprints and high
speed, as well as compatibility with CMOS fabrication processes.25–29 There have been several reports on silicon
modulators based on the free-carrier plasma dispersion
effect.25–28 The silicon Mach–Zehnder modulator (MZM)
based on a p-n junction phase shifter operated in reverse
bias shows great potential for wideband high-speed performance. Experimentally, modulation speeds of up to 50 Gb∕s
have been demonstrated with extinction ratio (ER) of 3.1 dB
driven with RF signals of 6.5 Vpp and the total insertion loss
is 7.4 dB.29
In our work, we fabricated a carrier-depletion-type modulator with 15 GHz bandwidth. By designing the PN junction
and MZI device, low absorption loss and high extinction
ratio are obtained to suit better for practical application.
The waveguides are 220 nm high and 600 nm wide for less
sidewall roughness scattering loss. An offset of 100 nm and
an intrinsic gap of 50 nm between n-type and p-type regions
are used to enhance the modulation efficiency and decrease
the capacitance of the diode. The P and N doping concentrations are about 1 × 1017 ∕cm3 for low absorption loss. The
waveguide of phase shifter has a 60 nm slab that is doped to
form the PN diode structure for electrical contacts. The Pþþ
and Nþþ doping concentrations are about 1 × 1020 ∕cm3 and
500 nm away from the edge of the waveguide to minimize
the optical absorption loss. Reversed PN junctions are
applied in both arms of our 2 mm long phase shifter in order
to keep the absorption balance in MZM. Consequently, the
static extinction ratio is about 20 dB driving voltage from
0 to 6 V. We can get the modulation efficiency about
1.52 V·cm and absorption loss about 1 dB∕mm. For
12 Gb∕s operation, an ER of 7.1 dB is achieved, shown
in Fig. 8(a), which is driven with an RF signal of 7Vpp .
A coplanar waveguide structure has been used to drive the
phase shifters, in which the electrodes are 10 μm wide, separated by a 6.4 μm gap with a 30 ohms characteristic impedance. The termination resistance used is 50 ohms and the
resulting 3 dB roll-off frequency is 15 GHz at −4 V bias,
as shown in Fig. 8(b). Thus, low absorption loss and the
large extinction ratio of the Si modulator have been demonstrated in our device, which makes it possible to compete
with commercial modulators to benefit the advanced optical
communication system. The bandwidth improvement will be
presented later by optimizing the RF design.
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Fig. 8 (a) Eye diagram of Si MZI modulator with 7.1 dB ER at 12 Gb∕s. (b) Normalized optical response of Si MZI modulator against RF frequency
at −4 V bias.
As one of most important trend in advanced optical communication systems, coherent optical OFDM (CO-OFDM)
has been proposed and the proof-of-concept transmission
experiments have shown its extreme robustness against
chromatic dispersion and polarization mode dispersion.30
Because the most critical assumption for OFDM is the linearity in modulation, transmission, and demodulation, the
modulator needs to be linear to avoid signal distortions.
Different from conventional LiNbO3 modulator based on
linear electro-optic effect-Pockel effect, silicon modulator
is based on plasma dispersion effect, which is a nonlinear
electron-optic effect in nature. Nonlinearities in the silicon
modulator come from the MZ transfer function and the
PN junction design, but they might cancel each other.31–33
Also the third-order intermodulation distortion (IMD3) for
silicon modulators is a function of the modulator bias
point.34 So there are three ways to improve the linearity
of a silicon modulator: optimizing the embedded diode structure, changing the optical structure and tuning the bias voltage. Recently, some progresses on research of linearity of
silicon modulator have been achieved in theory and experiments34,35 and show that the linearity of an optimized silicon
modulator could greatly exceed that of the ideal, linear (e.g.,
LiNbO3 ) modulator, such as 5.9 dB improvement of
SFDR.31–33 Thus, silicon modulator is the most promising
linear modulator contributing to the CO-OFDM in advanced
optical communication systems.
3.4 Photodetector
In advanced optical communication systems, photodetectors
function in converting incident light into electrical signals,
which can be used to either monitor light intensity variations
or detect high-speed optical signals.36 Due to the intrinsic
bandgap limitation, bulk Si photodetectors cannot meet
the C-band requirements of the wavelength. However, with
an indirect bandgap of 0.67 eV, Germanium can offer much
higher optical absorption in 1.55 μm wavelength. The fabrication process of Germanium is compatible with the
mature CMOS technology. As to the conventional normal
incident photodetector, there exists a trade-off between quantum efficiency and bandwidth. Nevertheless, by using the
Germanium detector as part of a waveguide, we can get
out of the dilemma.37 Since the light propagating direction
is perpendicular to that of the collection of the carriers,
Optical Engineering
high quantum efficiency and high bandwidth can be achieved
at the same time in terms of the waveguide-integrated
photodiodes. In addition, with the incorporation of optical
amplifier in the high-speed digital transmission systems,
photodetectors are desired to be capable of handling high
optical input power without a significant deterioration of
the high frequency signal.38 Conventional normal incident
photodetectors demonstrate limited power handling behavior
for the reason that the generated carriers are located in a relatively small volume. Under the illumination of a high power
light, high carrier density can cause screening effects in the
drift region, which will lead to larger drift time. Since the
light absorption is spread over a lager region, the waveguide-integrated structures can also overcome this limitation.
In order to fabricate a high-speed photodetector for the
integrated coherent receiving system, we design a germanium waveguide photodetector, as shown in Fig. 9. Given
the tolerance of the fabrication process, we propose a compact germanium photodetector which is 1.6 × 10 μm2 in a
top view. The germanium layer is integrated on the top of
a SOI rib waveguide and the light in the silicon rib waveguide is evanescently coupled to the overlying germanium
layer.39
The dark current-voltage (I-V) characteristics of the
photodetector were measured and a low dark current of
0.66 μA at the bias of −1 V was obtained. The 3-dB bandwidth was measured using a vector network analyzer (VNA),
which provided measurement capability of 20 GHz. The normalized optical response as a function of frequency is plotted
in Fig. 10. As can be seen, the 3-dB bandwidth of our photodetector ended at 20 GHz.
Fig. 9 3D schematic structure of the Ge pin waveguide photodetector
integrated on top of an SOI waveguide.
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φ ¼ tan−1
sin Δϕ − 1
;
cos Δϕ
(2)
where Δϕ s the phase change in the phase shifter. Based on
the measured spectra shift to different voltage, the phase
change Δϕ to the driving voltage V is Δϕ ¼ −0:02772þ
0:60119 × V − 0:07639 × V 2 þ 0:00806 × V 3 .
Considering the propagation of Gaussian input pulses in
optical fibers and the initial amplitude modulated through a
silicon Mach-Zendner modulator (Si MZM), we can obtain
the following equation:
1 þ iC t 2
Að0; tÞ ¼ A0 exp −
;
(3)
2
T0
Fig. 10 Normalized optical response versus frequency for the
reported germanium photodetector under 3.8 V reverse bias at
1550 nm wavelength.
In order to check the viability of our photodetector in the
optical communication system, an eye-diagram measurement was undertaken. For the 28 Gb∕s operation, the
eye diagram of the modulated optical signal is shown in
Fig. 11(a). Then the light was fed into the photodetector
under test conditions. We can find from Fig. 11(b) that the
eye diagram is still open, even though the input signal is not
very good.
Overall, the dark current of 0.66 μA and the 28 Gb∕s
operation indicate that our photodetector can make for the
integrated advanced optical communication system. More
precise measurement and further bandwidth improvement
are on the way.
4 Applications
Based on the unique properties of silicon photonics devices,
the applications in advanced optical communication systems
have been started to be implemented recently. Our works are
focused on the silicon modulator in long-haul transmission
taking the advantage of its negative chirping property.
Because of the plasma dispersion effect in silicon material,
the electronic signal drives the optical nonlinearly. As to the
single arm driven silicon MZI modulator, we can get the output phase φ described as below:
where A0 is the peak amplitude, T 0 is the half-width at 1∕eintensity point, and chirp parameter nn C has ðC∕T 20 Þt ¼
−∂φ∕∂t. Using above equations and assuming the T 0 is
about 40 ps, the average chirp parameter has C ∼ 0.8. This
indicates the Si MZM has a negative chirp.
The corresponding pulse broadening factor B is shown in
Fig. 12(a). In the case of C ¼ 0.8, the pulse width initially
decreases and drops to its minimum, and then increase along
with longer transmission distance. The B with no chirp and
positive chirp are also discussed under same parameters. The
pulse widths increase during transmissions and are broaden
with lager positive C. Thus, negative C can compress the
pulse at short transmission length, and lead to some broadening at long transmission length. The corresponding power
penalty is illustrated in Fig. 12(b). Initially, the power penalty
Fig. 12 (a) Pulse broadening factor B versus transmission length with
different chirp parameters; (b) power penalty versus transmission
length.
Fig. 11 (a) Eye diagram of the 28 Gb∕s modulated optical signal. (b) Eye diagram of the corresponding electrical signal from the photodetector at
−4 V bias.
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Fig. 13 Experimental setup diagrams of the Si MZM.
Fig. 14 Experimentally measured system-level performance characterization of varying propagation distances compared Si MZM and LiNbO3
MZM at 10 Gb∕s modulation rate. (a) BER curves as the function of ROP; (b) ROP versus propagation distance under 10−10 BER.
decreases due to the compressed pulse signal under the compensation of the negative chirp effect. Thus, lower power
penalty can be obtained because of the negative chirp of
Si MZM.
Using the devices presented in Sec. 3, the long-haul transmission is demonstrated. Based on the system in Fig. 13,
we demonstrate error-free 80 km transmission by a silicon
carrier-depletion Mach-Zehnder modulator at 10 Gb∕s and
the power penalty is as low as 1.15 dB.
The BER measured data was drawn in Fig. 14(a) and the
curves of ROP as the function of distance is described in
Fig. 14(b). The power penalties are −0.55, −0.4, and
1.15 dB for 26, 53, and 80 km at the BER of 10−10 . It is
smaller than the one of LiNbO3 MZM, which is 1.1, 1.6,
and 3.65 dB, respectively. Up to 80 km transmission, the
Si MZM can reach 10−10 BER with −12.35 dBm ROP
while LiNbO3 MZM requires a higher ROP (−11.75 dBm).
After a system level comparative study between our Si
MZM and a commercial LiNbO3 MZM, the negative chirp
character of the Si MZM is verified in the experiment, which
compensates the dispersion deterioration and leads to a low
power penalty in long-haul transmission. Therefore, Si MZM
is veritably a practical photonic device for future middle- or
long-haul WDM transmission systems.
5 Conclusion
In order to satisfy the accurate phase estimation, polarization
diversity, linearity, and spectral efficiency, high-speed, and
low-cost requirements of coherent optical communication,
Si based optical components have been studied. The passive
components, such as the polarization beam splitter with
extinction ratio of more than 20 dB and the optical 90-deg
Optical Engineering
hybrid having phase deviation within 5 - deg, were optimized and fabricated by using the MMI structure. In active
components, the 12 Gb∕s modulator and the 20 GHz
photodetectors were experimentally measured. Benefiting
from the unique properties of silicon modulator, an errorfree 80 Km transmission of the signals generated by a silicon
carrier-depletion Mach-Zehnder modulator was also demonstrated at 10 Gb∕s and the power penalty was as low as
1.15 dB. After a system level comparative study between
our Si MZM and a commercial LiNbO3 MZM, the negative
chirp character of the Si MZM was verified in the experiment, which compensated the dispersion deterioration and
led to a low power penalty in long-haul transmission. These
results shows silicon photonics have great potential in
advanced optical communication systems.
Acknowledgments
This work is partially supported by National High
Technology Research and Development Program of China
(Grant No. 2011AA010302).
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Biographies and photographs of the authors are not available.
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