Silicon Photonics - Mellanox Technologies

1 4600 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 12, DECEMBER 2006 Silicon PhotonicsBahram Jalali,Fellow, IEEE, and Sasan Fathpour,Member, IEEEI nvited PaperAbstract After dominating the electronics industry fordecades, Silicon is on the verge of becoming the material of choicefor the Photonics industry: the traditional stronghold of III Vsemiconductors. Stimulated by a series of recent breakthroughsand propelled by increasing investments by governments andthe private sector, Silicon Photonics is now the most activediscipline within the field of integrated optics. This paper providesan overview of the state of the art in Silicon Photonics andoutlines challenges that must be overcome before large-scalecommercialization can occur. In particular, for realization ofintegration with CMOS very large scale integration (VLSI), siliconphotonics must be compatible with the economics of siliconmanufacturing and must operate within thermal constraints ofVLSI chips.

2 The impact of Silicon Photonics will reach beyondoptical communication its traditionally anticipated has excellent linear and nonlinear optical properties inthe midwave infrared (IR) spectrum. These properties, alongwith Silicon s excellent thermal conductivity and optical damagethreshold, open up the possibility for a new class of mid-IRphotonic Terms CMOS, continuum generation, erbium-dopedsilicon, integrated Photonics , nonlinear optics, optical amplifier,optical modulator, photodetector, photovoltaic effects, power dis-sipation, Raman laser, Raman scattering, Silicon laser, Silicon -on-insulator, Silicon Photonics , Silicon -rich oxide, VLSI, INTRODUCTIONTHE ROOTS of Silicon Photonics can be traced back to thepioneering works of Soref and Petermann in the late 1980sand early 1990s [1] [4]. The early work stimulated activitiesthat resulted in substantial progress, mostly in passive devices,in the 1990s [4] [11].

3 The technology boom of the late 1990sand the concomitant abundance of the private capital triggered arapid growth of the field. While the level of private funding diddiminish in the early 2000s, it served as a catalyst by raisingawareness to this new technology. This led to increased levelof investments by large corporations and government agenciesthat have fueled spectacular progress in the last five than attempting to provide a comprehensive historicalreview of Silicon Photonics , this paper offers a sampling of themost recent developments, combined with the authors perspec-tive on the promises of this technology and the challenges thatManuscript received June 8, 2006; revised September 27, 2006. This workwas supported by the Defense Advanced Research Project Agency (DARPA).The authors are with the Department of Electrical Engineering, University ofCalifornia, Los Angeles, CA 90095-1594 versions of Figs.

4 2 6, 8 10, 13 16, 18, and 19 are available online Object Identifier before the benefits can come to fruition. We begin bydescribing the motivation for Silicon Photonics : both the tradi-tional argument that is still valid and the new insight that hasrecently been gained. This will be followed by applications thatare expected to be impacted. Next, this paper discusses recentdevelopments in components ranging from passive devices tomodulators, detectors, and light amplifiers and sources. Thispaper concludes by looking ahead at challenges as well aspotentials of the technology that have not been fully recognizedin the traditional argument in favor of Silicon Photonics isbased on its compatibility with the mature Silicon IC manufac-turing. Silicon wafers have the lowest cost (per unit area) andthe highest crystal quality of any semiconductor material.

5 Theindustry is able to produce microprocessors with hundreds ofmillions of components, all integrated onto a thumb-size chip,and offer them at such a low price that they appear in con-sumer electronics. Silicon manufacturing represents the mostspectacular convergence of technological sophistication andeconomics of low-cost Photonics for mass-market applicationsby exploiting the mighty IC industry has been the traditionalmotivation for Silicon Photonics researchers. Another moti-vation is the availability of high-quality Silicon -on-insulator(SOI) wafers, an ideal platform for creating planar waveguidecircuits. The strong optical confinement offered by the highindex contrast between Silicon (n= )and SiO2(n= )makes it possible to scale photonic devices to the hundredsof nanometer level. Such lateral and vertical dimensions arerequired for true compatibility with IC processing.

6 In addition,the high optical intensity arising from the large index contrast(between Si and SiO2) makes it possible to observe nonlinearoptical interactions, such as Raman and Kerr effects, in chip-scale devices. This fortuitous outcome has enabled opticalamplification, lasing, and wavelength conversion, functions thatuntil recently were perceived to be beyond the reach of above arguments represent the traditional and still validmotivation in favor of Silicon Photonics . However, the case forsilicon Photonics is even stronger. Silicon has excellent materialproperties that are important in photonic devices. These includehigh thermal conductivity ( 10 higher than GaAs), highoptical damage threshold ( 10 higher than GaAs), and highthird-order optical nonlinearities. Kerr effect is 100 times larger,whereas Raman effect is 1000 times stronger than those in silicafiber.

7 Fig. 1 shows the absorption spectrum of Silicon , boastinga low-loss wavelength window extending from to nearly0733-8724/$ 2006 IEEEJALALI AND FATHPOUR: Silicon PHOTONICS4601 Fig. spectrum of Silicon grown by the Czochralski method andmeasured using Fourier transform infrared VOA representing a commercial Silicon photonicsproduct [13]. The device is manufactured by Kotura Corporation and is used forautomatic channel equalization in add/drop multiplexers in Nortel s metropoli-tan area network gear (figure courtesy of A. Martin).7 m [12]. Far from being limited to the near-infrared (IR) datacommunication band of m, Silicon is an excellentmaterial in the midwave IR APPLICATIONSB oasting low-cost substrates and mature manufacturing in-frastructure, Silicon Photonics represents a path toward massmanufacturing of discrete optical components, as well asintegrated transceivers for synchronous optical network, gigabitEthernet, and optical backplane markets.

8 An example of acommercial Silicon photonic component is the eight-channelvariable optical attenuator (VOA) manufactured by KoturaCorporation [13]. The product, as shown in Fig. 2, is used forautomatic channel equalization in add/drop multiplexers andappears in Nortel Corporation s products aimed at metropolitannetworks. Fig. 3 describes what might be a next generation ofsilicon photonic product. The prototype device is a four-channelwavelength-division multiplexing (WDM) transceiver reportedby Luxtera Corporation, Carlsbad, CA [14]. It is fabricated on a90-nm SOI CMOS process and monolithically integrates wave-length filters, photodetectors, electronic amplifiers, and only components that are not monolithically integratedare four InP lasers that are flip-chip bonded onto the second mainstream application envisioned for siliconphotonics is optical interconnects for CMOS electronics [15].

9 Fig. 4 highlights the communication bottleneck in very largescale integration (VLSI) electronics. The Cell processor de-veloped jointly by Sony and IBM is at the heart of Sony sPlaystation 3 game console. The eight-core processor has aninternal computation power of 256 giga floating point opera-tions per second (GFLOPS) and communicates with the periph-eral graphics processor and memory at data rates of 25 Gb/sor higher [16], [17]. Such data rates challenge wisdom holds that optical interconnects aremuch better suited than copper interconnects in handling suchhigh data rates. However, with the use of equalization and othersignal processing techniques, copper interconnects can addresshigher and higher data rates, albeit at the cost of higher powerdissipation. Therefore, for optical interconnects to replace theircopper counterparts, they must provide a lower power to now, the power dissipation of Silicon photonic deviceshas rarely been addressed by the research community.

10 However,given critical importance of power dissipation in integratedsystems, it is a topic that must take central stage in future addition to optical interconnects, several other applica-tions are envisioned for Silicon Photonics . The technology canalso play a role in biosensing applications. A disposable mass-produced sensor would be attractive as it could grow the marketfor biosensors. Sensor applications are somewhat different fromoptical communication as there are other very low cost opticaltechnologies that compete in this space [18]. One likely applica-tion area for Silicon Photonics is the so-called lab-on-a-chip inwhich both reaction and analysis are performed in a single de-vice. In the future, this could be extended to include electronicintelligence and wireless communications key functions thatwill be needed to create intelligent sensor networks for environ-mental example of an advanced biosensor being developed inacademia (at Vanderbilt University) is shown in Fig.