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High-speed optical modulation based on carrier depletion in a silicon waveguide Ansheng Liu1, Ling Liao1, Doron Rubin2, Hat Nguyen1, Berkehan Ciftcioglu1, Yoel Chetrit2, Nahum Izhaky2, and Mario Paniccia1 1 Intel Corporation, 2200 Mission College Blvd, SC12-326, Santa Clara, CA 95054 2 Intel Corporation, S. B. I. Park Har Hotzvim, Jerusalem, 91031, Israel Ansheng.liu@intel.com Abstract: We present a high-speed and highly scalable silicon optical modulator based on the free carrier plasma dispe
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  High-speed optical modulation based on carrierdepletion in a silicon waveguide Ansheng Liu 1 , Ling Liao 1 , Doron Rubin 2 , Hat Nguyen 1 , Berkehan Ciftcioglu 1 , YoelChetrit 2 , Nahum Izhaky 2 , and Mario Paniccia 1   1  Intel Corporation, 2200 Mission College Blvd, SC12-326, Santa Clara, CA 95054 2  Intel Corporation, S. B. I. Park Har Hotzvim, Jerusalem, 91031, Israel  Ansheng.liu@intel.com Abstract:   We present a high-speed and highly scalable silicon optical modulator based onthe free carrier plasma dispersion effect. The fast refractive index modulation of the device isdue to electric-field-induced carrier depletion in a Silicon-on-Insulator waveguide containing areverse biased pn junction. To achieve high-speed performance, a travelling-wave design isused to allow co-propagation of electrical and optical signals along the waveguide. Wedemonstrate high-frequency modulator optical response with 3 dB bandwidth of ~20 GHz anddata transmission up to 30 Gb/s. Such high-speed data transmission capability will enablesilicon modulators to be one of the key building blocks for integrated silicon photonic chips fornext generation communication networks as well as future high performance computingapplications.   ©2007 Optical Society of America OCIS codes: (250.7360) Waveguide modulators; (060.4080) Modulation; (203.7370)Waveguides; (250.5300) Photonic integrated circuits. References and Links 1.   A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, M. Paniccia, “A high-speedsilicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427 , 615-618 (2004).2.   L. Liao, D. Samara-Rubio, M. Morse, A. Liu, H. Hodge, D. Rubin, U. D. Keil, T. Franck, “High-speedsilicon Mach-Zehnder modulator,”   Opt. Express 13 , 3129-3135 (2005).3.   A. Huang, G. Gunn, G.-L. Li, Y. Liang, S. Mirsaidi, A. Narasimha, T. Pinguet, “A 10 Gb/s photonicmodulator and WDM MUX/DEMUX integrated with electronics in 0.13 µ m SOI CMOS,” in TechnicalDigest of 2006 IEEE International Solid-State Circuits Conference, Session 13/ OpticalCommunication/13.7.4.   S. J. Koester, G. Dehlinger, J. D. Schaub, J. O. Chu, Q. C. Ouyang, A. Grill, “Germanium-on-insulatorphotodetectors,” in Technical Digest of 2005 2 nd IEEE International Conference on Group IV Photonics, pp.171-173.5.   M. Oehme, J. Werner, E. Kasper, M. Jutzi, M. Berroth, “High bandwidth Ge p-i-n photodetector integratedon Si,” Appl. Phys. Lett. 89 , 071117-071117-3 (2006).6.   O. Boyraz, B. Jalali, “Demonstration of a silicon Raman laser,” Opt. Express 12 , 5269-5273 (2004).7.   H. Rong, et. al. “A continuous-wave Raman silicon laser ,” Nature 433 , 725-728 (2005).8.   A. Liu, H. Rong, M. Paniccia, O. Cohen, D. Hak, “Net optical gain in a low loss silicon-on-insulatorwaveguide by stimulated Raman scattering,” Opt. Express 12 , 4261-4268 (2004).9.   O. Boyraz, B. Jalali, “Demonstration of 11 dB fiber-to-fiber gain in a silicon Raman amplifier,” Electron.Express 1 , 429-434 (2004).10.   Q. Xu, V. R. Almeida, M. Lipson, “Demonstration of high Raman gain in a submicrometer-size silicon-on-insulator waveguide,” Opt. Lett. 30 , 35-37 (2005).11.   R. Jones, et. al. “Net continuous-wave optical gain in a low loss silicon-on-insulator waveguide bystimulated Raman scattering,” Opt. Express 13 , 519-525 (2005).12.   R. L. Espinola, J. I. Dadap, R. M. Osgood, Jr., S. J. McNab, Y. A. Vlasov, “C-band wavelength conversionin silicon photonic wire waveguides,” Opt. Express 13 , 4341-4349 (2005).13.   K. Yamada, et. al. “All-optical efficient wavelength conversion using silicon photonic wire waveguide,”IEEE Photon. Technol. Lett. 18 , 1046-1048 (2006).14.   H. Rong, Y. H. Kuo, A. Liu, M. Paniccia, O. Cohen, “High efficiency wavelength conversion of 10 Gb/sdata in silicon waveguides,” Opt. Express 14 , 1182-1188 (2006).15.   A. W. Fang, H. Park, O. Cohen, R. Jones, M. Paniccia, J. E. Bowers, “Electrically pumped hybridAlGaInAs-silicon evanescent laser,” Opt. Express 14 , 9203-9210 (2006).16.   K. Noguchi, O. Mitomi, H. Miyazawa, “Millimeter-wave Ti:LiNbO 3 optical modulators,” J. LightwaveTechnol . 16 , 615-619 (1998).  17.   K. Tsuzuki, T. Ishibashi, T. Ito, S. Oku, Y. Shibata, T. Ito, R. Iga, Y. Kondo, Y. Tohmori, “A 40-Gb/sInGaAlAs-InAlAs MQW n-i-n Mach-Zehnder modulator with a drive voltage of 2.3 V,” IEEE Photon.Technol. Lett. 17 , 46-48 (2005).18.   R. A. Soref, P. J. Lorenzo, “All-silicon active and passive guided-wave components for λ  =1.3 and 1.6 µ m,”IEEE J. Quantum Electron. QE-22 , 873-879 (1986).19.   R. A. Soref, B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron. QE-23 , 123-129(1987).20.   R. S. Jacobsen, et al. “Strained silicon as a new electro-optic material,” Nature 441 , 199-202 (2006).21.   Y.-H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, J. S. Harris, “Strongquantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature 437 , 1334-1336(2005).22.   C. K. Tang, G. T. Reed, “Highly efficient optical phase modulator in SOI waveguides,” Electron. Lett. 31 ,451-452 (1995).23.   F. Y. Gardes, G. T. Reed, N. G. Emerson, C. E. Png, “A sub-micron depletion-type photonic modulator inSilicon on Insulator,” Optics Express 13 , 8845-8853 (2006).24.   Q. Xu, B. Schmidt, S. Pradhan, M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435 ,325-327 (2005).25.   F. Gan, F. X. Kartner, “High-speed silicon electrooptic modulator design,” IEEE Photon. Technol. Lett. 17 ,1007-1009 (2005).26.   A. Alping, X. S. Wu, T. R. Hausken, and L. A. Coldren, “Highly efficient waveguide phase modulator forintegrated optoelectronics,” Appl. Phys. Lett. 48 , 243-245 (1986).27.   J. G. Mendoza-Alvarez, L. A. Coldren, A. Alping, R. H. Yan, T. Hausken, K. Lee, and K. Pedrotti,“Analysis of depletion edge translation lightwave modulators,” IEEE J. Lightwave Technol. 6 , 793-807(1988).28.   R. C. Alferness, “Waveguide electrooptic modulators,” IEEE Trans. Microwave Theory Tech. 30 , 1121-1137 (1982).29.   R. G. Walker, “High-speed III-V semiconductor intensity modulators,” IEEE J. Quantum Electron.   27 , 654-667 (1991).30.   S. L. Chuang, Physics of Optoelectronics Devices. (John Wiley, New York, 1995).31.   G. T. Reed, A. P. Knights, Silicon Photonics: an introduction (John Wiley, Chichester, 2004).32.   K. Tsuzuki, K. Sano, N. Kikuchi, N. Kashio, E. Yamada, Y., Shibata, T. Ishibashi, M. Tokumitsu, and H.Yasaka, “0.3 V pp single-drive push-pull InP Mach-Zehnder modulator module for 43-Gbit/s systems,” inTechnical Digest of 2006 Optical Fiber Communication Conference and National Fiber Optic EngineersConference 5-10 March 2006, p.3.33.   Y. Cui and P. Berini, “Modeling and design of GaAs traveling-wave electrooptic modulators based oncapacitively loaded coplanar strips,” IEEE J. Lightwave Technol. 24 , 544-554 (2006).34.   S. Pae, T. Su, J. P. Denton, G. W. Neudeck, “Multiple layers of silicon-on-insulator islands fabrication byselective epitaxial growth,” IEEE Electron Device Lett. 20 , 194-196 (1999). 1. Introduction Silicon photonics has recently become a subject of intense interest because it offers anopportunity for low cost optoelectronic solutions for applications ranging fromtelecommunications down to chip-to-chip interconnects. In the past few years, there have beensignificant advances in pushing device performance of CMOS-compatible silicon buildingblocks needed for developing silicon integrated photonic circuits. Fast silicon opticalmodulators [1-3], SiGe photo-detectors [4, 5], silicon Raman lasers [6, 7], silicon opticalamplifiers [8-11], silicon wavelength converters [12-14], and hybrid silicon lasers [15] havebeen demonstrated. Nevertheless, the fastest demonstrated data transmission using a siliconmodulator so far is ~10 Gb/s [2, 3]. In order to meet the ever-increasing bandwidth demandof next generation communication networks and future high performance computingapplications, it would be desirable to have significantly faster (>>10 Gb/s) modulation anddata transmission capabilities.Today’s commercially available high-speed optical modulators at >>10 Gb/s are based onelectro-optic materials such as lithium niobate [16] and III-V semiconductors [17]. Thesedevices have demonstrated modulation capability as high as 40 Gb/s. To achieve fastmodulation in silicon is challenging, however, due to the fact that crystalline silicon exhibitsno linear electro-optic (Pockels) coefficient and very weak Franz-Keldysh effect [18, 19].  Although it has been very recently shown that strained silicon possesses the Pockels effect[20], the measured electro-optic coefficient is relatively small (an order of magnitude smallerthan that for LiNbO 3 ). It has also been shown that strained Ge/SiGe quantum well structureshave relatively strong electro-optic absorption due to the quantum-confined Stark effect [21],making it possible for optical modulation. However, critical strain engineering is needed andoptical modulator performance has yet to be demonstrated for the Ge quantum well system.To date, high speed modulation in silicon has only been demonstrated via the free carrierplasma dispersion effect [2, 3].In silicon, free carrier density change results in a change in the refractive index of thematerial; therefore, the modulation speed of a silicon modulator based on the free carrierplasma dispersion effect is determined by how fast the free carriers can be injected orremoved. Three different device configurations, namely, forward biased p-i-n diode [18, 19,22],   MOS capacitor [1, 2], and reverse biased pn junction [3, 23] have been proposed toachieve phase modulation in silicon. The forward biased p-i-n diode approach has beenproven to provide high modulation efficiency (in turn compact device size [24]). However,due to the slow carrier generation and/or recombination processes, the modulation speed isusually limited unless the carrier lifetime can be significantly reduced [25]. While both MOScapacitor and reverse biased pn junction rely on electric-field induced majority carrierdynamics that can potentially achieve >10 Gb/s operation, one would need to adoptappropriate device design to improve the phase modulation efficiency. In addition, appropriateelectrical driving schemes and resistance capacitance (RC) limitations have to be taken intoaccount to reduce the device parasitic effects on the high frequency response of the device.We note that optical modulation based on free carrier effects in III-V semiconductors has alsobeen investigated previously [26, 27]. It was suggested that the device speed for a reverse-biased pn junction is only limited by the RC constant.    1x2 MMI2x1 MMIpnphase shifters AB p-Sin-Sip++p++n++ Traveling-wave electrodes oxideSi substratesignalgroundground waveguideRF sourceinputoutputLoad resistor ~   Phase shifterwaveguide C Metal contact   Fig. 1. ( A ) Top view of an asymmetric Mach-Zehnder interferometer silicon modulator containing two pn junction based phase shifters. The waveguide splitter is an 1x2 multi-mode interference (MMI) coupler.The RF signal is coupled to the travelling wave electrode from the optical input side and termination loadis added to the output side. ( B ) Cross-sectional view of a pn junction waveguide phase shifter in Silicon-On-Insulator. The coplanar waveguide electrode has a signal metal width of ~6 µ m and a signal-groundmetal separation of ~3 µ m. The metal thickness is ~1.5 µ m. The high-frequency characteristic impedanceof the travelling wave electrode is ~20 Ω . ( C ) Scanning electron microscope (SEM) image of a pn diodephase shifter waveguide. 2. Device design and fabrication The high-speed silicon modulator presented here is based on a Mach-Zehnder interferometer(MZI) with a reverse biased pn diode embedded in each of the two arms, as shown in Fig.1(a). To obtain better phase modulation efficiency, we designed and fabricated a sub-micrometer size waveguide. The silicon rib waveguide width is ~0.6 µ m, rib height is ~0.5 µ m, and etch depth is ~0.22 µ m. Both modelling and experiment confirm that the waveguideis a single mode device for wavelengths around 1.55 µ m. The waveguide splitter is an 1x2multi-mode interference (MMI) coupler. The MMI coupler is used because it has a broaderrange of operating wavelengths and larger fabrication tolerance as compared to a directionalcoupler. Also, because of the small waveguide dimensions, a conventional Y-junction would
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