Ciyuan Qiu

Wavelength tracking with thermally controlled silicon resonators

We experimentally demonstrate feedback controlling of the resonant wavelength of a silicon dual-ring resonator. The feedback signal is the difference in optical scattering from the two coupled microring resonators, and the control mechanism is based on thermo-optic tuning with micro-heaters. This control scheme keeps the central wavelength of the resonator aligning with the input wavelength, which can be used to compensate fabrication variations, environmental temperature shift and the drift of laser wavelength. This feedback control scheme allows microring-based electro-optic modulators to be used in a dynamic environment.

Efficient Modulation of 1.55 μm Radiation with Gated Graphene on a Silicon Microring Resonator

The gate-controllability of the Fermi-edge onset of interband absorption in graphene can be utilized to modulate near-infrared radiation in the telecommunication band. However, a high modulation efficiency has not been demonstrated to date, because of the small amount of light absorption in graphene. Here, we demonstrate a ∼ 40% amplitude modulation of 1.55 μm radiation with gated single-layer graphene that is coupled with a silicon microring resonator. Both the quality factor and resonance wavelength of the silicon microring resonator were strongly modulated through gate tuning of the Fermi level in graphene. These results promise an efficient electro-optic modulator, ideal for applications in large-scale on-chip optical interconnects that are compatible with complementary metal-oxide-semiconductor technology.

High-contrast terahertz modulator based on extraordinary transmission through a ring aperture.

We demonstrated extraordinary THz transmission through ring apertures on a metal film. Transmission of 60% was obtained with an aperture-to-area ratio of only 1.4%. We show that the high transmission can be suppressed by over 18 dB with a thin layer of free carriers in the silicon substrate underneath the metal film. This result suggests that CMOS-compatible terahertz modulators can be built by controlling the carrier density near the aperture.

Demonstration of reconfigurable electro-optical logic with silicon photonic integrated circuits

We demonstrate a scalable and reconfigurable optical directed-logic architecture consisting of a regular array of integrated optical switches based on microring resonators. The switches are controlled by electrical input logic signals through embedded p-i-n junctions. The circuit can be reconfigured to perform any combinational logic operation by thermally tuning the operation modes of the switches. Here we show experimentally a directed logic circuit based on a 2×2 array of switches. The circuit is reconfigured to perform arbitrary two-input logic functions.

A Novel Composite Method for Ultra-Wideband Doublet Pulses Generation

A novel composite approach to generate ultra-wideband (UWB) doublet pulses in optical domain through an electrical Gaussian pulse is proposed and demonstrated in this letter. Two pulses with reversed polarity generated by polarization modulation are fed into a differential group delay device and a semiconductor optical amplifier (SOA) wherein the optical pulse profile is modified. Doublet pulses are obtained after optical-electrical detection due to gain saturation and recovery mechanism in the SOA. The fractional bandwidth of doublet pulses exceeds 180%, fulfilling the UWB requirements.

Slow-light delay enhancement in small-core pure silica photonic crystal fiber based on Brillouin scattering

We demonstrate that we realize large-delay slow light based on stimulated Brillouin scattering in a short length of our fabricated small-core photonic crystal fiber (PCF). The cavity effect from the partially reflective splices in the end of the PCF enhances slow-light delay significantly. Our experiments show that large slow-light delay can be easily realized in a very short length of the PCF with a moderate pump power. Up to a one-half pulse-width delay is achieved in only 50 m of PCF in a single pump segment.

Active dielectric antenna on chip for spatial light modulation

Integrated photonic resonators are widely used to manipulate light propagation in an evanescently-coupled waveguide. While the evanescent coupling scheme works well for planar optical systems that are naturally waveguide based, many optical applications are free-space based, such as imaging, display, holographics, metrology and remote sensing. Here we demonstrate an active dielectric antenna as the interface device that allows the large-scale integration capability of silicon photonics to serve the free-space applications. We show a novel perturbation-base diffractive coupling scheme that allows a high-Q planer resonator to directly interact with and manipulate free-space waves. Using a silicon-based photonic crystal cavity whose resonance can be rapidly tuned with a p-i-n junction, a compact spatial light modulator with an extinction ratio of 9.5 dB and a modulation speed of 150 MHz is demonstrated. Method to improve the modulation speed is discussed.

Ultraprecise measurement of resonance shift for sensing applications.

Resonator-based optical sensors detect the change of refractive index in the environment by measuring the resonance shift. The sensitivity of such sensors is determined by how precise one can locate the resonant wavelength, which is thought to be limited by the bandwidth and the quality factor of the resonator. Here we show that, with a tunable resonator, one can determine the resonant wavelength with ultrahigh precision. Using a silicon microring resonator with an embedded p-i-n junction for electro-optic tuning, whose quality factor is only 14,000, we measured the resonant wavelength with a resolution of 0.06 pm, which corresponds to an index sensitivity of ~10(-7). This resonance measurement for sensing purposes can be done using a fixed-wavelength laser.

Compact tunable silicon photonic differential-equation solver for general linear time-invariant systems

We propose and experimentally demonstrate an all-optical temporal differential-equation solver that can be used to solve ordinary differential equations (ODEs) characterizing general linear time-invariant (LTI) systems. The photonic device implemented by an add-drop microring resonator (MRR) with two tunable interferometric couplers is monolithically integrated on a silicon-on-insulator (SOI) wafer with a compact footprint of ~60 μm × 120 μm. By thermally tuning the phase shifts along the bus arms of the two interferometric couplers, the proposed device is capable of solving first-order ODEs with two variable coefficients. The operation principle is theoretically analyzed, and system testing of solving ODE with tunable coefficients is carried out for 10-Gb/s optical Gaussian-like pulses. The experimental results verify the effectiveness of the fabricated device as a tunable photonic ODE solver.

Efficient coupler between chip-level and board-level optical waveguides.

We propose an efficient optical coupler between a submicrometer-sized silicon waveguide on a silicon photonic chip and a multi-micrometer wide polymer waveguide on an optical printed circuit board for interchip optical networks. We show low coupling loss <0.4 dB with high lateral and angular tolerance to misalignment so that coupling can be done by automatic pick-and-place equipments with high throughput and low cost. The coupler has a wide optical bandwidth from 1470 to 1650 nm.