Silicon photonics for advanced opticalcommunication systems

Zhiping ZhouXingjun WangHuaxiang YiZhijuan TuWei TanQifeng LongMei YinYawen Huang

Silicon photonics for advanced opticalcommunication systems

Zhiping ZhouXingjun WangHuaxiang YiZhijuan TuWei TanQifeng LongMei YinYawen HuangPeking UniversitySchool of Electronics Engineering and Computer

ScienceState Key Laboratory of Advanced Optical

Communication Systems and NetworksBeijing 100871, ChinaE-mail: zjzhou@pku.edu.cn

Abstract. Recent progress on Si-based optical components for advancedoptical communication systems has been demonstrated. The polarizationbeam splitter with extinction ratio of more than 20 dB and the optical90-deg hybrid having phase deviation within �5- deg were obtainedusing multimode interference structures. The 12 Gb∕s modulators andthe 20 GHz photodetectors were measured. Benefiting from the uniqueproperties of silicon modulator, an error-free 80 Km transmission of thesignals generated by our silicon carrier-depletion Mach-Zehnder modula-tor was also demonstrated at 10 Gb∕s and the power penalty was as lowas 1.15 dB. These results show that silicon photonics has a great potentialin advanced optical communication systems. © 2013 Society of Photo-OpticalInstrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.52.4.045007]

Subject terms: silicon photonics; coherent optical communication; modulator; opti-cal hybrid; germanium photodetector.

Paper 130311P received Feb. 28, 2013; revised manuscript receivedMar. 22, 2013;accepted for publication Mar. 25, 2013; published online Apr. 19, 2013.

1 IntroductionThe year 2012 was actually another year for rapid devel-opment in information technology. New applications andservices continue to demand higher bandwidth and morefunctionalities, which drive advancement of optical commu-nication systems. Among many advanced approaches, coher-ent optical communication is coming back in a rather strongfashion.

Coherent optical communications were studied exten-sively in the 1980s mainly due to the fact that the highsensitivity of coherent receivers permit much longer trans-mission distance for the same amount of transmitterpower, and the use of coherent detection allows an efficientuse of the bandwidth. However, their research and develop-ment have been interrupted for nearly 20 years behind therapid progress in high-capacity wavelength division multi-plex (WDM) systems using erbium doped fiber amplifiers(EDFAs). The advancement in digital signal processing(DSP), particularly, the demonstration of digital carrier phaseestimation in coherent receivers in 2005, has awoken a wide-spread interest in coherent optical communications.

Via custom-designed DSP functions for adaptive polari-zation tracking, chromatic dispersion compensation, and for-ward error correction (FEC), communication capability wasboosted through coherent optical communication systems.Commercial coherent systems for fiber-optic networks wereintroduced at 40 and 100 Gb∕s in 2008 and 2010, usingpolarization-division multiplexed (PDM) quadrature phase-shift keying (QPSK) at 11.5 and 28 Gb∕s.1 In researchlabs, single-carrier 640 Gb∕s has been achieved using PDMquadrature amplitude modulation (QAM) at a symbol rate of80 Gb∕s.2 However, the complex transmitters and receiversin these systems suffer from high power consumption, largevolume, and weak stability of the individual components.

To tackle with these problems, one of the best approachesis the monolithic integration of transmitters and receivers.

The use of silicon photonics platform promises a low costsolution since optical, optoelectronic and electronic compo-nents may be integrated onto a single chip through matureCMOS technology.

2 System DescriptionA coherent optical communication system includes a trans-mitter, a receiver and a single local oscillator serving thereceiver. The output of the local oscillator is directly fed toone input of a coherent receiver. The transmitter and receiverare the core part of a coherent optical communication systemto generate and receive the optical signal. In 2011, one hun-dred Gigabit Ethernet standards were established. One of themost promising technologies for a 100 Gb∕s coherent opti-cal communication system is the combination of dual-polari-zation quadrature phase-shift keying (DP-QPSK) signalswith a phase and polarization diversity coherent receiver.3,4

For the optical part of transmitter, advanced modulation for-mats for 100 Gb∕s will be QPSK, requiring half the symbolrate compared with conventional PSK. Moreover, polariza-tion multiplexing could provide another factor of 2 in reduc-tion of the symbol rate, reaching 25 Gb∕s symbol rate peroptical tributary. Therefore, the optical modulator in thetransmitter needs to have a 25 Gb∕s symbol rate. In addition,other parameters are also very necessary for 100 Gb∕s opti-cal 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 IntegratedPolarization Multiplexed Quadrature Modulated Transmittersby Optical Internetworking Forum (OIF). For the opticalcoherent receiver, above 20 dB, ER is required to make thetransverse electric (TE) and transverse magnetic (TM) sepa-rate. Another key component is the 90-deg hybrid that sepa-rates the quadrature components of the incident signal using acontinuous wave local oscillator reference source, the less-than�5- deg phase error between the in-phase X-polarizationcomponent (XI) and quadrature X-polarization component(XQ) and between the in-phase Y-polarization component0091-3286/2013/$25.00 © 2013 SPIE

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(YI) and in-phase Y-polarization component (YQ) wereneeded.5 The other important component is a high-speedphotodetector (PD) with high quantum efficiency and high-power handling capability. For above system, 25 Gb∕s,symbol rate per optical tributary, larger than 0.5 A∕Wresponsibility, and less than 150 nA dark current wererequired.6 Besides the required high speed operation, energyconsumption and cost also play an important role. The usageof silicon photonics promises a reduction of costs becauseoptical, optoelectronic and electronic components can beintegrated on a single chip by self-alignment techniques.

With support from the National High TechnologyResearch and Development Program (863) of China, wehave been working on a 100 Gb∕s coherent optical commu-nication system on SOI substrate in recent years. It consistsof a multilevel modulated transmitter and coherent receiver.For the optical transmitter, a multimode interference (MMI)polarizing beam splitter (PBS) was planned on the SOI sub-strate, in order to achieve high coupling efficiency andpolarization splitting. The signal was coupled into the wave-guide, and divided into two orthogonal polarized beams, TEand TM. Then one polarized beam entered the upper or lowerarms of the Si based Mach-Zehnder modulator to form amultilevel, multiphased signal stream. Finally, two orthogo-nal polarized signal beams were combined into one, whichwas transmitted outwards. At the optical coherent receiverend, the similar MMI-PBS was also designed to achievehigh coupling efficiency and polarization splitting. The sig-nal beam and local oscillation beam were coupled into theinput waveguide of the optical 90-deg hybrid. The optical90-deg hybrid was designed using a 4 × 4 MMI interferom-eter structure. Finally the mixed beam was divided into a2 × 2 two-way balanced Germanium photodetectors to real-ize coherent detection.

In this paper, we will report our progress in Si based pho-tonic components, which are designed and optimized, in theoptoelectronics domain, for the purpose of the coherent opti-cal communication. The main goals are to contribute inphase estimation, polarization diversity, linearity, and spec-tral efficiency.

3 ComponentsAs described above, passive and active components arerequired to form a coherent optical communication system,which are sometimes crucially important to the success of anadvanced system. In this section, we will describe our workon the key components for the coherent optical communica-tion system, namely the polarization beam splitter, hybrid,modulator, and the photodetector.

3.1 Polarization Beam Splitter

Many approaches have been proposed to realize polarizationsplitting, such as directional couplers,7 Mach-Zehnder inter-ferometers,8 grating couplers,9 and multimode interferencecouplers. Among them, we prefer the MMI-based PBSdue to their unique properties, including low crosstalk andimbalance, large optical bandwidth, ease of fabrication, andgood tolerances. Moreover, a wide variety of materialshave been adopted for making the PBSs, such as silica,10

InGaAsP/InP,11 LiNbO3,12,13 polymer materials,14 and silicon-

on-insulator (SOI).9,15 The SOI is selected for compatibilitywith CMOS processing and for compact design resulting

from the high refractive index contrast between silicon(n ¼ 3.45) and SiO2 (n ¼ 1.46). The geometrical birefrin-gence results in a distinct difference in the MMI self-imagelength between TE mode and TM mode. Generally, the totallength of the device is the least common multiple of lengthsof the first self-image of TE and TM modes. In our case, thelengths of the first self-image of TE and TM modes are 517and 470 μm. Originally, the total length of PBS can be designas 5170 μm that equals 10 times of 517 μm and 11 times of470 μm. In our work, the quasi-states16 theory is adopted andthe length of our device, which is 1034 μm described in fol-lowing principle, is decreased to one-fifth of the generallength of 5170 μm. This compact SOI MMI waveguide out-performs competing polarization splitting behavior, whichenables us to integrate the other components, such as mod-ulators in parallel, into a DP-QPSK system.

This PBS is designed with the self-image principle andquasi-state (QS) theory. Defined Lπ as the beat length of thetwo lowest-order modes,17 the length of the first self-image is3Lπ , which can be calculated in the equation

Lπ ¼π

β0 − β1≈4nrW2

e

3λ0; (1)

where the β is the propagation constants, nr represents theeffective index, We is the effective width and λ denotesthe wavelength. The design of PBS meets the length,L ¼ ðm1ÞðLπTEÞ ¼ ðm2ÞðLπTMÞ, where m1 and m2 are aneven and odd number or vice versa, which separates theTE and TM modes into two outputs. Concerning QS theory,the QS images are formed before or after every normal imagein the propagation direction. The beat length between anormal-state and a QS images is just 1∕5 of that between thenormal-states. By the approximation of self-imaging princi-ple with power distribution of the lowest five modes, the QSimage is used in TM mode and the normal image is utilizedin TE mode. Combining the self-image principle with the QStheory, the split length of this MMI device is shortened to beone-fifth of a normally designed MMI split length. At alength of device, one of the outputs comes out the TE modeand the other goes with the TMmode, which results in a PBSin a multimode interference coupler. Figure 1 shows theschematic of the cross-section of a SOI waveguide andthe configuration of a MMI coupler of our design.

The waveguides are established on the 340-nm-thicktop silicon layer between the 1-μm-thick insulator layerand air. The length and width of the multimode interferenceregion are 1034 and 8 μm. The width of the input and outputstraight waveguides is 2.8 μm. In the 3D-BPM simulation, tokeep the extinction ratio more than 15 dB, the theoreticaltolerances 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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�35 μm for TM, �0.04 μm and �10 μm for TE, as shownin Fig. 2.

The scanning electron microscope (SEM) images of thedevice are shown in Fig. 3, where the ∼ sign means the omit-ted part which represents the MMI section.

The device is fabricated using electron beam lithographyand inductively coupled plasma etching. A standard inte-grated optics setup with a tunable laser, a determined

polarization controller (DPC5500) and a charge coupleddevice (CCD) are used to test the device. For different inputpolarization states, their output images are shown in Fig. 4.

The bandwidths and the extinction ratios are obtainedusing the optical spectrum analyzer, as shown in Fig. 5.For TE and TM mode, the extinction ratios are 27.3 and26.6 dB at λ ¼ 1.55 μm, respectively. For the extinctionratio more than 15 dB, the bandwidths are about 20 nmin TE polarization and more than 40 nm in TM polarizationwith the center wavelength at 1.55 μm.

3.2 Optical 90-deg Hybrid

A QPSK system is considered as the most promisingapproach to increase the transmission capacity. In order todemodulate the signal light, which is modulated by theQPSK system, a multilevel coherent transmission systemsconsisting of an optical 90-deg hybrid is prerequisite. Sofar, the proposed methods to realize the optical 90-deg hybridinclude star couplers,18,19 array waveguide gratings (AWG),20

and multimode interference couplers.21–24 The disadvantagesof star couplers and array waveguide gratings are low trans-mission efficiency, complicated design, high imbalance, andlarge footprint. However, the MMI-based hybrids have theirunique merits including large optical bandwidth, compactsize, low crosstalk and imbalance, and ease of fabrication,as shown in Table 1. Then, we will present our design ofthe optical 90-deg hybrid using 4 × 4 MMI couplers on sil-icon-on-insulator (SOI) platform, and show the simulationand experiment results. The transmission efficiency of our

Fig. 2 Tolerances of the width (a) and the length (b) in the multimode interference region.

Fig. 3 SEM image of the device.

Fig. 4 MMI output images on CCD camera: (a) TE input, (b) TM input.

Fig. 5 Measured output power as a function of wavelength at (a) port bar and (b) port cross.

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device is over 95% across C-band, whereas that of the starcouplers is only better than 30%.19 The excess loss of ourhybrid is about 1 dB, while that of the array waveguide gra-ting is 9.5 dB.20

The 4 × 4 MMI 90-deg hybrid is based on the self-imageprinciple, which is due to the interference between a largenumbers of supported modes in multimode waveguides.The device we designed has six ports with two inputs forQPSK signal (S) and local oscillator (LO) signal, respec-tively, and four outputs used for getting a certain phase quad-rature relationship for coherent optical detection. Theschematic of the optical 90-deg hybrid is shown in Fig. 6.

Based on the influence of the coupler geometry on imbal-ance and excess loss, the width of the multimode waveguideand the access waveguides are 10 and 1 μm, respectively.And the length of the multimode waveguide can be designedby Lmmi ¼ neff · W2

eff∕λ, whereWeff and neff are the effectivewidth and the effective index of the multimode waveguide,so the optimum coupler length is about 200 μm. We simulateour devices in 340 nm top silicon SOI wafer with fully etch-ing technology. Since the devices are used for C-band trans-mission, the 4 × 4 MMI couplers are designed atwavelength λ ¼ 1550 nm.

An important figure of merit for the 4 × 4MMI coupler isphase deviation defined as the phase difference between theideal quadrature phase and actual phase, which determinesthe system bandwidth. The phase deviation of our 4 × 4MMI coupler is found below 2.2-deg across C-band in sim-ulation, as shown in Fig. 7. The results, therefore, indicatesatisfactory C-band performance of 4 × 4 waveguide cou-plers. The transmission efficiency is above 98% at 1550 nm,and the excess losses of both two input ports are less than0.5 dB, which means the extinction ratio is larger than20 dB and fulfils the system demand.

3.3 Modulator

Silicon modulators as essential components of advancedoptical communication systems have been extensively

studied for their capability of small footprints and highspeed, as well as compatibility with CMOS fabrication proc-esses.25–29 There have been several reports on siliconmodulators based on the free-carrier plasma dispersioneffect.25–28 The silicon Mach–Zehnder modulator (MZM)based on a p-n junction phase shifter operated in reversebias shows great potential for wideband high-speed perfor-mance. Experimentally, modulation speeds of up to 50 Gb∕shave been demonstrated with extinction ratio (ER) of 3.1 dBdriven with RF signals of 6.5 Vpp and the total insertion lossis 7.4 dB.29

In our work, we fabricated a carrier-depletion-type modu-lator with 15 GHz bandwidth. By designing the PN junctionand MZI device, low absorption loss and high extinctionratio are obtained to suit better for practical application.The waveguides are 220 nm high and 600 nm wide for lesssidewall roughness scattering loss. An offset of 100 nm andan intrinsic gap of 50 nm between n-type and p-type regionsare used to enhance the modulation efficiency and decreasethe capacitance of the diode. The P and N doping concen-trations are about 1 × 1017∕cm3 for low absorption loss. Thewaveguide of phase shifter has a 60 nm slab that is doped toform the PN diode structure for electrical contacts. The Pþþand Nþþ doping concentrations are about 1 × 1020∕cm3 and500 nm away from the edge of the waveguide to minimizethe optical absorption loss. Reversed PN junctions areapplied in both arms of our 2 mm long phase shifter in orderto keep the absorption balance in MZM. Consequently, thestatic extinction ratio is about 20 dB driving voltage from0 to 6 V. We can get the modulation efficiency about1.52 V·cm and absorption loss about 1 dB∕mm. For12 Gb∕s operation, an ER of 7.1 dB is achieved, shownin Fig. 8(a), which is driven with an RF signal of 7Vpp.A coplanar waveguide structure has been used to drive thephase shifters, in which the electrodes are 10 μm wide, sep-arated by a 6.4 μm gap with a 30 ohms characteristic imped-ance. The termination resistance used is 50 ohms and theresulting 3 dB roll-off frequency is 15 GHz at −4 V bias,as shown in Fig. 8(b). Thus, low absorption loss and thelarge extinction ratio of the Si modulator have been demon-strated in our device, which makes it possible to competewith commercial modulators to benefit the advanced opticalcommunication system. The bandwidth improvement will bepresented later by optimizing the RF design.

Table 1 The advantages and disadvantages of different methods ofthe optical 90-deg hybrid.

Transmissionefficiency Design Imbalance Footprint

Starcouplers

Low Complicated High Large

AWG Low Complicated High Large

MMI High Simple Low Small

Fig. 6 Schematic of the optical 90-deg hybrid with balanced PDs.

Fig. 7 Phase deviation of the 4 × 4 MMI coupler in simulation.

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As one of most important trend in advanced optical com-munication systems, coherent optical OFDM (CO-OFDM)has been proposed and the proof-of-concept transmissionexperiments have shown its extreme robustness againstchromatic dispersion and polarization mode dispersion.30

Because the most critical assumption for OFDM is the lin-earity in modulation, transmission, and demodulation, themodulator needs to be linear to avoid signal distortions.

Different from conventional LiNbO3 modulator based onlinear electro-optic effect-Pockel effect, silicon modulatoris based on plasma dispersion effect, which is a nonlinearelectron-optic effect in nature. Nonlinearities in the siliconmodulator come from the MZ transfer function and thePN junction design, but they might cancel each other.31–33

Also the third-order intermodulation distortion (IMD3) forsilicon modulators is a function of the modulator biaspoint.34 So there are three ways to improve the linearityof a silicon modulator: optimizing the embedded diode struc-ture, changing the optical structure and tuning the bias volt-age. Recently, some progresses on research of linearity ofsilicon modulator have been achieved in theory and experi-ments34,35 and show that the linearity of an optimized siliconmodulator could greatly exceed that of the ideal, linear (e.g.,LiNbO3) modulator, such as 5.9 dB improvement ofSFDR.31–33 Thus, silicon modulator is the most promisinglinear modulator contributing to the CO-OFDM in advancedoptical communication systems.

3.4 Photodetector

In advanced optical communication systems, photodetectorsfunction in converting incident light into electrical signals,which can be used to either monitor light intensity variationsor detect high-speed optical signals.36 Due to the intrinsicbandgap limitation, bulk Si photodetectors cannot meetthe C-band requirements of the wavelength. However, withan indirect bandgap of 0.67 eV, Germanium can offer muchhigher optical absorption in 1.55 μm wavelength. The fab-rication process of Germanium is compatible with themature CMOS technology. As to the conventional normalincident photodetector, there exists a trade-off between quan-tum efficiency and bandwidth. Nevertheless, by using theGermanium detector as part of a waveguide, we can getout of the dilemma.37 Since the light propagating directionis perpendicular to that of the collection of the carriers,

high quantum efficiency and high bandwidth can be achievedat the same time in terms of the waveguide-integratedphotodiodes. In addition, with the incorporation of opticalamplifier in the high-speed digital transmission systems,photodetectors are desired to be capable of handling highoptical input power without a significant deterioration ofthe high frequency signal.38 Conventional normal incidentphotodetectors demonstrate limited power handling behaviorfor the reason that the generated carriers are located in a rel-atively small volume. Under the illumination of a high powerlight, high carrier density can cause screening effects in thedrift region, which will lead to larger drift time. Since thelight absorption is spread over a lager region, the wave-guide-integrated structures can also overcome this limitation.

In order to fabricate a high-speed photodetector for theintegrated coherent receiving system, we design a germa-nium waveguide photodetector, as shown in Fig. 9. Giventhe tolerance of the fabrication process, we propose a com-pact germanium photodetector which is 1.6 × 10 μm2 in atop view. The germanium layer is integrated on the top ofa SOI rib waveguide and the light in the silicon rib wave-guide is evanescently coupled to the overlying germaniumlayer.39

The dark current-voltage (I-V) characteristics of thephotodetector were measured and a low dark current of0.66 μA at the bias of −1 V was obtained. The 3-dB band-width was measured using a vector network analyzer (VNA),which provided measurement capability of 20 GHz. The nor-malized optical response as a function of frequency is plottedin Fig. 10. As can be seen, the 3-dB bandwidth of our photo-detector ended at 20 GHz.

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 frequencyat −4 V bias.

Fig. 9 3D schematic structure of the Ge pin waveguide photodetectorintegrated on top of an SOI waveguide.

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In order to check the viability of our photodetector in theoptical communication system, an eye-diagram measure-ment was undertaken. For the 28 Gb∕s operation, theeye diagram of the modulated optical signal is shown inFig. 11(a). Then the light was fed into the photodetectorunder test conditions. We can find from Fig. 11(b) that theeye diagram is still open, even though the input signal is notvery good.

Overall, the dark current of 0.66 μA and the 28 Gb∕soperation indicate that our photodetector can make for theintegrated advanced optical communication system. Moreprecise measurement and further bandwidth improvementare on the way.

4 ApplicationsBased on the unique properties of silicon photonics devices,the applications in advanced optical communication systemshave been started to be implemented recently. Our works arefocused on the silicon modulator in long-haul transmissiontaking 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 thesingle arm driven silicon MZI modulator, we can get the out-put phase φ described as below:

φ ¼ tan−1�sinΔϕ − 1

cosΔϕ

�; (2)

where Δϕ s the phase change in the phase shifter. Based onthe measured spectra shift to different voltage, the phasechange Δϕ to the driving voltage V is Δϕ ¼ −0:02772þ0:60119 × V − 0:07639 × V2 þ 0:00806 × V3.

Considering the propagation of Gaussian input pulses inoptical fibers and the initial amplitude modulated through asilicon Mach-Zendner modulator (Si MZM), we can obtainthe following equation:

Að0; tÞ ¼ A0 exp

�−1þ iC

2

�tT0

�2�; (3)

where A0 is the peak amplitude, T0 is the half-width at 1∕e-intensity point, and chirp parameter nn C has ðC∕T2

0Þt ¼−∂φ∕∂t. Using above equations and assuming the T0 isabout 40 ps, the average chirp parameter has C ∼ 0.8. Thisindicates the Si MZM has a negative chirp.

The corresponding pulse broadening factor B is shown inFig. 12(a). In the case of C ¼ 0.8, the pulse width initiallydecreases and drops to its minimum, and then increase alongwith longer transmission distance. The B with no chirp andpositive chirp are also discussed under same parameters. Thepulse widths increase during transmissions and are broadenwith lager positive C. Thus, negative C can compress thepulse at short transmission length, and lead to some broad-ening at long transmission length. The corresponding powerpenalty is illustrated in Fig. 12(b). Initially, the power penalty

Fig. 10 Normalized optical response versus frequency for thereported germanium photodetector under 3.8 V reverse bias at1550 nm wavelength.

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.

Fig. 12 (a) Pulse broadening factor B versus transmission length withdifferent chirp parameters; (b) power penalty versus transmissionlength.

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decreases due to the compressed pulse signal under the com-pensation of the negative chirp effect. Thus, lower powerpenalty can be obtained because of the negative chirp ofSi MZM.

Using the devices presented in Sec. 3, the long-haul trans-mission is demonstrated. Based on the system in Fig. 13,we demonstrate error-free 80 km transmission by a siliconcarrier-depletion Mach-Zehnder modulator at 10 Gb∕s andthe power penalty is as low as 1.15 dB.

The BER measured data was drawn in Fig. 14(a) and thecurves of ROP as the function of distance is described inFig. 14(b). The power penalties are −0.55, −0.4, and1.15 dB for 26, 53, and 80 km at the BER of 10−10. It issmaller than the one of LiNbO3 MZM, which is 1.1, 1.6,and 3.65 dB, respectively. Up to 80 km transmission, theSi MZM can reach 10−10 BER with −12.35 dBm ROPwhile LiNbO3 MZM requires a higher ROP (−11.75 dBm).

After a system level comparative study between our SiMZM and a commercial LiNbO3 MZM, the negative chirpcharacter of the Si MZM is verified in the experiment, whichcompensates the dispersion deterioration and leads to a lowpower penalty in long-haul transmission. Therefore, Si MZMis veritably a practical photonic device for future middle- orlong-haul WDM transmission systems.

5 ConclusionIn order to satisfy the accurate phase estimation, polarizationdiversity, linearity, and spectral efficiency, high-speed, andlow-cost requirements of coherent optical communication,Si based optical components have been studied. The passivecomponents, such as the polarization beam splitter withextinction ratio of more than 20 dB and the optical 90-deg

hybrid having phase deviation within 5� - deg, were opti-mized and fabricated by using the MMI structure. In activecomponents, the 12 Gb∕s modulator and the 20 GHzphotodetectors were experimentally measured. Benefitingfrom the unique properties of silicon modulator, an error-free 80 Km transmission of the signals generated by a siliconcarrier-depletion Mach-Zehnder modulator was also demon-strated at 10 Gb∕s and the power penalty was as low as1.15 dB. After a system level comparative study betweenour Si MZM and a commercial LiNbO3 MZM, the negativechirp character of the Si MZM was verified in the experi-ment, which compensated the dispersion deterioration andled to a low power penalty in long-haul transmission. Theseresults shows silicon photonics have great potential inadvanced optical communication systems.

AcknowledgmentsThis work is partially supported by National HighTechnology Research and Development Program of China(Grant No. 2011AA010302).

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Biographies and photographs of the authors are not available.

Optical Engineering 045007-8 April 2013/Vol. 52(4)

Zhou et al.: Silicon photonics for advanced optical communication systems