Rai Kou

High-performance silicon photonics technology for telecommunications applications

Abstract By way of a brief review of Si photonics technology, we show that significant improvements in device performance are necessary for practical telecommunications applications. In order to improve device performance in Si photonics, we have developed a Si-Ge-silica monolithic integration platform, on which compact Si-Ge–based modulators/detectors and silica-based high-performance wavelength filters are monolithically integrated. The platform features low-temperature silica film deposition, which cannot damage Si-Ge–based active devices. Using this platform, we have developed various integrated photonic devices for broadband telecommunications applications.

Monolithic Integration of a Silica-Based Arrayed Waveguide Grating Filter and Silicon Variable Optical Attenuators Based on p–i–n Carrier-Injection Structure

We demonstrate a monolithic integration of a silica-based arrayed waveguide grating filter and silicon variable optical attenuators. The integrated device perform wavelength demultiplexing in 16 channels and provide high-speed power-level adjustment with a response of <15 ns in each channel.

Influence of carrier lifetime on performance of silicon p-i-n variable optical attenuators fabricated on submicrometer rib waveguides

We investigated influence of carrier lifetime on performance of silicon (Si) p-i-n variable optical attenuators (VOAs) on submicrometer Si rib waveguides. VOAs were fabricated with and without intentional implantation of lattice defects into their intrinsic region. Carrier lifetime was measured by pulse responses for normal incidence of picosecond laser pulse of 775 nm to the VOA, as approximately 1 ns and approximately 7 ns for the VOAs with and without defects, respectively. Carrier lifetime is determined by the sum of surface recombination and Auger recombination for VOAs without defects, while Schockley-Read-Hall recombination is dominant for the VOA with defects. As a result, attenuation efficiency (dB/mA) is 0.2-0.7 and 0.04-0.1, while 3-dB bandwidth is 40-100 MHz and over 200 MHz for the VOAs with and without defects, respectively. There is a trade-off relation between attenuation and response speed of the VOAs with respect to carrier lifetime i.e., attenuation efficiency is linearly proportional to the carrier lifetime, whereas response speed is inversely proportional to it.

Si-Ge-Silica Monolithic Integration Platform and Its Application to a 22-Gb/s

We describe a Si-Ge-silica monolithic integration platform for telecommunications applications. The monolithic integration process features low-temperature silica film deposition by electron-cyclotron-resonance chemical vapor deposition to prevent thermal damage to Si/Ge active devices. The monolithically integrated Si and SiOx waveguides show propagation losses of 2.8 and 0.9 dB/cm, and the inverse-tapered spot-size converters show a coupling loss of 0.35 dB. We applied the platform to a 22-Gb/s × 16-ch wavelength-division multiplexing receiver, in which a 16-ch SiOx arrayed waveguide grating (AWG) with 1.6-nm channel separation and Ge photodiodes (PDs) are monolithically integrated. The AWG-PD device exhibits fiber-to-PD responsivity of 0.29 A/W and interchannel crosstalk of less than -22 dB and successfully receives 22-Gb/s signal for all 16 channels. In addition, we demonstrate 40-km transmission of 12.5-Gb/s signal and obtain sensitivity of -6.8 dBm at a bit error rate of 10-9 without transimpedance amplifiers.

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Optical absorption efficiency of graphene integrated onto a silicon photonic platform was characterized at around 1.55-µm optical telecommunications wavelength. Micro-Raman spectroscopy performed after the completion of all fabrication processes confirmed that transferred chemical-vapor-deposited graphene is a single layer (>90% coverage) without any significant damage. Absorption efficiencies of the single-layer graphene (SLG) on a silicon wire waveguide, obtained by measuring different lengths (cutback method) of the SLG from 2.5 to 200 µm, were 0.09 and 0.05 dB/µm for TE- and TM-polarized light. The unusual relationship in the polarization dependency can be explained by strong surface-plasmon-porlariton support in the TM mode.

16-ch WDM Receiver

We developed a low-polarization-dependent silica- based waveguide, which can be monolithically integrated with a silicon (Si) waveguide device on a silicon-on-insulator (SOI) substrate. For the monolithic integration, silica-based materials must be deposited at low temperature in order not to damage Si waveguide devices. Due to this low-temperature fabrication method, however, the silica films exhibit high residual stress, resulting in high material birefringence. In order to compensate for this birefringence, we introduce a multi-layer core structure. First, we design the structure taking the monolithic integration with the Si waveguide devices into account. Then, the designed waveguides and arrayed-waveguide gratings (AWGs) are fabricated using low-temperature fabrication processes. Next, we experimentally confirm that the waveguide exhibits low waveguide birefringence. In addition, we monolithically integrate the AWG and Si waveguide devices.

Characterization of optical absorption and polarization dependence of single-layer graphene integrated on a silicon wire waveguide

We experimentally demonstrate a high-quality phase shift keying demodulator based on a silicon photonic wire waveguide. Since the birefringence of the waveguide generates extremely huge differential group delay, an ultra-compact and high-extinction-ratio delay line interferometer is devised in TE and TM modes. We firstly calculated and simulated the requirements for propagation length and waveguides dimensions. Then, we measured the interference spectrum, eye pattern, bit error rate, and temperature dependence to ascertain its feasibility for DPSK demodulation. For a 2.8 cm-long wire waveguide, a free spectral range of 9.6 GHz and an error-free DPSK demodulation around 10 Gb/s are obtained.

Low-Polarization-Dependent Silica Waveguide Monolithically Integrated on SOI Photonic Platform

Selectively patterned graphene is integrated onto a silicon ring resonator to investigate the quality factor (Q factor) variation. The Q factor sharply decreases from 7900 to 1200 as the patterned graphene length increases from 0 to 20 μm. A numerical estimation, which takes into account optical absorption by graphene, shows an exponential damping of the Q factor with increasing graphene length and is consistent with the experimental result. We expect these fundamental characterizations to be helpful in developing graphene-integrated silicon photonics applications.

Single silicon wire waveguide based delay line interferometer for DPSK demodulation

A Mg:LiNbO3-based adhered ridge waveguide was fabricated by dry etching on a silicon platform. Efficient QPM-SHG (normalized efficiency: 450%/W) and -DFG (efficiency: -9.8 dB for pump power 38 mW) were achieved in the telecommunication band.

Influence of graphene on quality factor variation in a silicon ring resonator

Various photonic devices covering passive to active functions have been developed and monolithically-integrated on a silicon wire waveguide platform. Obstacles to practical applications are being eliminated by applying state-of-art fabrication technologies and unique device designs.