Andrew Wing On Poon

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.

Multimode resonances in square-shaped optical microcavities.

Square-shaped two-dimensional optical microcavities (micro-cavities) were investigated for possible applications as filters for dense wavelength-division multiplexing. Multimode cavity resonances were observed in the elastic scattering of approximately 200-microm square-shaped micro-cavities in fused silica. Based on a two-dimensional k-space representation, we accounted for the multimode spectrum by different normal modes with rays confined by total internal reflection. The cavity-mode trajectories need not be closed after each round trip. Single-mode spectra are expected from smaller square-shaped micro-cavities.

Low-Loss Multimode-Interference-Based Crossings for Silicon Wire Waveguides

We report multimode-interference (MMI)-based crossings in high-index-contrast silicon wire waveguides. Our experiments show an MMI crossing of ~0.4-dB insertion loss, which suggests ~0.9-dB improvement from a control plain crossing of the same size and refractive index contrast. We also experimentally observe significant crosstalk suppression in our MMI crossing. The MMI crossing transmission exhibits wavelength dependence of less than 0.2 dB within the 1520-1580-nm wavelength range

Silicon electro-optic modulators using p-i-n diodes embedded 10-micron-diameter microdisk resonators

We demonstrate a silicon electro-optic modulator using a 10-micron-diameter microdisk resonator with a laterally integrated p-i-n diode surrounding essentially the entire microdisk. Our experiments reveal a modulation bandwidth of 510 MHz using a Q ~ 16,900 resonance mode under a square-wave drive voltage of ~0.9 V forward bias and ~-6 V reverse bias.

Silicon microring carrier-injection-based modulators/switches with tunable extinction ratios and OR-logic switching by using waveguide cross-coupling.

We demonstrate silicon microring carrier-injection-based modulators/switches with waveguide cross-coupling. We tune the modulator extinction ratio by forward-biasing either the microring or cross-coupled waveguide p-i-n diode, while modulating the other. We also demonstrate OR-logic switching functionality by simultaneously applying two different electrical data streams across the microring and cross-coupled waveguide diodes. For both the modulator and the switch, we observe NRZ pattern-dependent optical waveform distortions.

Electrically reconfigurable silicon microring resonator-based filter with waveguide-coupled feedback

We demonstrate an electrically reconfigurable silicon microring resonator-based filter with waveguide-coupled feedback. Our experiments and scattering-matrix-based modeling show that the resonance wavelengths, extinction ratios, and line shapes depend on the feedback coupling and can be controllably tuned by means of carrier injection to the feedback- waveguide. We also demonstrate nearly uniform resonance line shapes over multiple free-spectral ranges by nearly phase-matching the feedback and the microring.

Fano resonance-based electrically reconfigurable add-drop filters in silicon microring resonator-coupled Mach-Zehnder interferometers

We report a Fano resonance-based electrically reconfigurable add-drop filter using a microring resonator-coupled Mach-Zehnder interferometer (MZI) on a silicon substrate. Our experiments reveal a pair of complementary Fano resonance line shapes that can be electrically tuned and output coupled from the MZI output ports. A near symmetrical resonance peak can be flipped to a near symmetrical resonance dip by applying a forward-bias voltage of less than 1V across a laterally integrated p-i-n diode in the MZI non-resonator-coupled arm. Our scattering-matrix-based modeling shows good agreement with the experiments and indicates ways to enhance the resonance routing functionality. Our work demonstrates the feasibility of an integrated reconfigurable add-drop filter with actively interchangeable throughput and drop ports.

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.

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 cross-connect filters using microring resonator coupled multimode-interference-based waveguide crossings.

We report silicon cross-connect filters using microring resonator coupled multimode-interference (MMI) based waveguide crossings. Our experiments reveal that the MMI-based cross-connect filters impose lower crosstalk at the crossing than the conventional cross-connect filters using plain crossings, while offering a nearly symmetric resonance line shape in the drop-port transmission. As a proof-of-concept for cross-connection applications, we demonstrate on a silicon-on-insulator substrate (i) a 4-channel 1 x 4 linear-cascaded MMI-based cross-connect filter, and (ii) a 2-channel 2 x 2 array-cascaded MMI-based cross-connect filter.