Xianshu Luo

Cascaded Microresonator-Based Matrix Switch for Silicon On-Chip Optical Interconnection

This paper reviews developments in cascaded microresonator-based matrix switches for silicon photonic interconnection networks in many-core computing applications. Specifically, we emphasize our recently proposed 5 times 5 matrix switch comprising two-dimensionally cascaded microring resonator-based electrooptic switches coupled to a waveguide cross-grid on a silicon chip. The cross-grid adopts low-loss low-crosstalk multimode-interference-based waveguide crossings. Such a microresonator-based matrix switch offers nonblocking interconnections among multiple inputs and multiple outputs, with the key merits of i) a tens to hundreds of micrometers-scale footprint, ii) gigabit/second-scale data transmission, iii) nanosecond-speed circuit-switching, iv) 100-muW-scale DC power consumption per link, and v) large-scale integration for networks-on-chips applications. We analyze in detail the microring resonator-based cross-grid switch design for high-data-rate signal transmission in the context of our proposed 5 times 5 matrix switch. We also study the feasibility of large-scale integration of the matrix switch. We report proof-of-concept experiments of a single cross-grid switch element and a 2 times 2 matrix switch, propose design guidelines, and discuss future engineering challenges.

Cavity-enhanced photocurrent generation by 1.55 μm wavelengths linear absorption in a p-i-n diode embedded silicon microring resonator

We demonstrate 20-fold cavity-enhanced photocurrent generation in 1.55 μm wavelengths in a p-i-n diode embedded silicon microring resonator with Q factor of 8000. The on-resonance wavelength shows linear responsivity of 0.12 mA/W upon 0 V bias and 0.25 mA/W upon −15 V bias. We attribute the linear absorption to surface-state absorption at the microring waveguide interfaces. Our experiments indicate that the photocurrent generation is linear to the estimated coupled power up to ∼500 μW.

50-Gb/s silicon optical modulator with traveling-wave electrodes

We demonstrate silicon Mach-Zehnder Interferometer (MZI) optical modulator with 50.1-Gb/s data rate and 5.56 dB dynamic extinction ratios. The phase shifter is composed by a 4 mm-long reverse-biased p-n junction with a modulation efficiency (V(π) · L(π)) of ~26.7 V · mm and phase shifter loss of ~1.04 dB/mm at V(bias) = -6 V. The measured electro-optic bandwidth reaches 25.6 GHz at V(bias) = -5 V. Compensation doping method and low loss traveling-wave electrodes are utilized to improve the modulator performance. Measurement result demonstrates that reasonable choosing of working point and doping profile of the silicon optical modulator is critical in order to match the performance requirement of the real application.

Cascaded active silicon microresonator array cross-connect circuits for WDM networks-on-chip

We propose a design of an optical switch on a silicon chip comprising a 5 × 5 array of cascaded waveguide-crossing-coupled microring resonator-based switches for photonic networks-on-chip applications. We adopt our recently demonstrated design of multimode-interference (MMI)-based wire waveguide crossings, instead of conventional plain waveguide crossings, for the merits of low loss and low crosstalk. The microring resonator is integrated with a lateral p-i-n diode for carrier-injection-based GHz-speed on-off switching. All 25 microring resonators are assumed to be identical within a relatively wide resonance line width. The optical circuit switch can employ a single wavelength channel or multiple wavelength channels that are spaced by the microring resonator free spectral range. We analyze the potential performance of the proposed photonic network in terms of (i) light path cross-connections loss budget, and (ii) DC on-off power consumption for establishing a light path. As a proof-of-concept, our initial experiments on cascaded passive silicon MMI-crossing-coupled microring resonators demonstrate 3.6-Gbit/s non-return-to-zero data transmissions at on- and off-resonance wavelengths.

Silicon High-Order Coupled-Microring-Based Electro-Optical Switches for On-Chip Optical Interconnects

We demonstrate an electro-optically (EO) tunable switch using tenth-order coupled-microring resonators in silicon-on-insulator using complementary metal-oxide-semiconductor fabrication technology. The measured drop-port transmission spectra show box-like transmission passband with ~100-GHz bandwidth and ~50-dB extinction ratio. With a DC voltage supply to the integrated p-i-n diodes surrounding the microrings, the optical switch shows, respectively, ~10 and ~45-dB on/off ratios from throughputand drop-port. The measured EO switching times are ~1 ns upon a 1.2-Vpp low-speed driving signal and a DC power consumption of ~37 mW. Up to 30-Gb/s pseudorandom binary sequence (231-1) signal transmissions suggest high-data-rate signal switching capability.

Electrical tracing-assisted dual-microring label‑free optical bio/chemical sensors

We propose and demonstrate a novel electrical tracing-assisted dual-microring resonator-based optical sensor system in silicon-on-insulator substrate. The system comprises one microring resonator-based sensing element and another microring resonator-based tracing element integrated with electrical controller. The resonance wavelength shift of sensing microring induced by the refractive index change is traced and determined by direct voltage supply of the electrical tunable tracing microring. Such optical sensing system eliminates the traditional wavelength-scanning method thus provide a cost effective sensing scheme. Proof-of-principle demonstration by testing polyelectrolyte multilayer shows the sensitivity of ~4.0 mW/ng∙mm-2 and the detection limit of ~5.35 pg/mm2.

Reciprocal transmissions and asymmetric modal distributions in waveguide-coupled spiral-shaped microdisk resonators

We respond to the comment [Opt. Express 16, to be published (2008)] J. Wiersig, on our recent work reporting on the reciprocal transmissions and asymmetric modal distributions in waveguide-coupled spiral-shaped microdisk resonators [J. Y. Lee et al., Opt. Express, 15 (22), 14650 (2007)].

CMOS compatible monolithic multi-layer Si 3 N 4 -on-SOI platform for low-loss high performance silicon photonics dense integration

We demonstrated a low-loss CMOS-compatible multi-layer platform using monolithic back-end-of-line (BEOL) integration. 0.8dB/cm propagation loss is measured for the PECVD Si₃N₄ waveguide at 1580nm wavelength. The loss is further reduced to 0.24dB/cm at 1270nm wavelength, justifying the platforms feasibility for O-band operation. An inter-layer transition coupler is designed, achieving less than 0.2dB/transition loss across 70nm bandwidth. This is the lowest inter-layer transition loss ever reported. A thermally tuned micro-ring filter is also integrated on the platform, with performance comparable to similar device on SOI platform.

Efficient silicon nitride grating coupler with distributed Bragg reflectors

In this paper we have designed, fabricated and characterized a high efficiency Silicon nitride grating coupler at 1490 nm. Distributed Bragg reflectors as bottom mirrors are employed to improve the coupling efficiency by reflecting the downward traveling light. The peak coupling efficiency obtained is about -2.5 dB and the 1-dB bandwidth is 53 nm. The fabrication process is CMOS-compatible and is ready to be integrated with photonic circuits.

Coupled spiral-shaped microdisk resonators with non-evanescent asymmetric inter-cavity coupling

We study coupled spiral-shaped microdisk resonators with non-evanescent asymmetric inter-cavity coupling via seamlessly jointed notches. Our finite-difference time-domain numerical simulations reveal that the throughput-port transmissions are reciprocal between counterclockwise (CCW) and clockwise (CW) traveling-wave modes, while the drop-port transmissions and modal field distributions are input-port dependent. By introducing a slight mismatch in radii between two coupled microdisks while preserving their seamlessly jointed notches, we are able to show selectively enhanced extinction ratio for one of the split modes while suppressing the other. Our experiments using coupled spiral-shaped microdisk resonators in silicon nitride-on-silica suggest split resonances with an extinction ratio of ~20 dB using identical coupled microdisks, and an enhanced resonance extinction ratio of ~24 dB using slightly mismatched coupled microdisks. The non-evanescent coupling preserves high-Q resonances.