Qianfan Xu

Micrometre-scale silicon electro-optic modulator

Metal interconnections are expected to become the limiting factor for the performance of electronic systems as transistors continue to shrink in size. Replacing them by optical interconnections, at different levels ranging from rack-to-rack down to chip-to-chip and intra-chip interconnections, could provide the low power dissipation, low latencies and high bandwidths that are needed. The implementation of optical interconnections relies on the development of micro-optical devices that are integrated with the microelectronics on chips. Recent demonstrations of silicon low-loss waveguides, light emitters, amplifiers and lasers approach this goal, but a small silicon electro-optic modulator with a size small enough for chip-scale integration has not yet been demonstrated. Here we experimentally demonstrate a high-speed electro-optical modulator in compact silicon structures. The modulator is based on a resonant light-confining structure that enhances the sensitivity of light to small changes in refractive index of the silicon and also enables high-speed operation. The modulator is 12 micrometres in diameter, three orders of magnitude smaller than previously demonstrated. Electro-optic modulators are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures.

Guiding and confining light in void nanostructure

We present a novel waveguide geometry for enhancing and confining light in a nanometer-wide low-index material. Light enhancement and confinement is caused by large discontinuity of the electric field at high-index-contrast interfaces. We show that by use of such a structure the field can be confined in a 50-nm-wide low-index region with a normalized intensity of 20 microm(-2). This intensity is approximately 20 times higher than what can be achieved in SiO2 with conventional rectangular waveguides.

12.5 Gbit/s carrier-injection-based silicon micro-ring silicon modulators

We show a scheme for achieving high-speed operation for carrier-injection based silicon electro-optical modulator, which is optimized for small size and high modulation depth. The performance of the device is analyzed theoretically and a 12.5-Gbit/s modulation with high extinction ratio >9dB is demonstrated experimentally using a silicon micro-ring modulator.

Experimental demonstration of guiding and confining light in nanometer-size low-refractive-index material

We experimentally demonstrate a novel silicon waveguide structure for guiding and confining light in nanometer-wide low-refractive-index material. The optical field in the low-index material is enhanced because of the discontinuity of the electric field at high-index-contrast interfaces. We measure a 30% reduction of the effective index of light propagating in the novel structure due to the presence of the nanometer-wide low-index region, evidencing the guiding and confinement of light in the low-index material. We fabricate ring resonators based on the structure and show that the structure can be implemented in highly integrated photonics.

Silicon microring resonators with 1.5-µm radius

We demonstrate a junction between a silicon strip waveguide and an ultra-compact silicon microring resonator that minimizes spurious light scattering and increases the critical dimensions of the geometry. We show cascaded silicon microring resonators with radii around 1.5 µm and effective mode volumes around 1.0 µm3 that are critically coupled to a waveguide with coupled Q’s up to 9,000. The radius of 1.5 µm is smaller than the operational wavelength, and is close to the theoretical size limit of the silicon microring ring resonator for the same Q. The device is fabricated with a widely-available SEM-based lithography system using a stitch-free design based on a U-shaped waveguide.

All-optical logic based on silicon micro-ring resonators

We demonstrate all-optical logic in a micron-size silicon ring resonator based on the free-carrier dispersion effect in silicon. We show AND and NAND operation at 310 Mbit/s with ~10-dB extinction ratio. The free-carrier-lifetime-limited bit-rate can be significantly improved by active carrier extraction.

Cascaded silicon micro-ring modulators for WDM optical interconnection

We experimentally demonstrate cascaded silicon micro-ring modulators as the key component of a WDM interconnection system. We show clean eye-diagrams when each of the four micro-ring modulators is modulated at 4 Gbit/s. We show that optical inter-channel crosstalk is negligible with 1.3-nm channel spacing.

Carrier-induced optical bistability in silicon ring resonators.

We demonstrate optical bistability in a micrometer-sized silicon ring resonator based on the free-carrier dispersion effect in silicon. We measure the transfer function of the resonator showing a hysteresis loop with an input optical power of less than 10 mW. The influence of the thermal optical effect, which is minimized in the experiment by use of nanosecond pulses, is evaluated theoretically. Applications include sequential logic operations for all-optical routing.

All-optical slow-light on a photonic chip

We demonstrate optically tunable delays in a silicon-on-insulator planar waveguide based on slow light induced by stimulated Raman scattering (SRS). Inside an 8-mm-long nanoscale waveguide, we produce a group-index change of 0.15 and generate controllable delays as large as 4 ps for signal pulses as short as 3 ps. The scheme can be implemented at bandwidths exceeding 100 GHz for wavelengths spanning the entire low-loss fiber-optics communications window and thus represents an important step in the development of chip-scale photonics devices that process light with light.

Time-resolved study of Raman gain in highly confined silicon-on-insulator waveguides

We show time-resolved measurement of Raman gain in Silicon submicron-size planar waveguide using picosecond pump and probe pulses. A net nonlinear gain of 6 dB is obtained in a 7-mm long waveguide with 20.7-W peak pump power. We demonstrate an ultrafast all-optical switch based on the free-carrier dispersion effect in the silicon waveguide, whose transmission is enhanced by more than 13 dB due to the Raman effect.