Charlie Lin

Silicon photonic grating-assisted, contra-directional couplers.

We demonstrate, in both theory and experiment, 4-port, electrically tunable photonic filters using silicon contra-directional couplers (contra-DCs) with uniform and phase-shifted waveguide Bragg gratings. Numerical analysis, including both intra- and inter-waveguide couplings, is performed using coupled-mode theory and the transfer-matrix method. The contra-DC devices were fabricated by a CMOS-photonics manufacturing foundry and are electrically tunable using free-carrier injection. A 4-port, grating-based photonic resonator has been obtained using the phase-shifted contra-DC, showing a resonant peak with a 3-dB bandwidth of 0.2 nm and an extinction ratio of 24 dB. These contra-DC devices enable on-chip integration of Bragg-grating-defined functions without using circulators and have great potential for applications such as wavelength-division multiplexing networks and optical signal processing.

Ultra-compact, flat-top demultiplexer using anti-reflection contra-directional couplers for CWDM networks on silicon

Wavelength-division-multiplexing (WDM) networks with wide channel grids and bandwidths are promising for low-cost, low-power optical interconnects. Wide-bandwidth, single-band (i.e., no free-spectral range) add-drop filters have been developed on silicon using anti-reflection contra-directional couplers with out-of-phase Bragg gratings. Using such filter components, we demonstrate a 4-channel, coarse-WDM demultiplexer with flat passbands of up to 13 nm and an ultra-compact size of 1.2 × 10(-3) mm(2).

Universal grating coupler design

A universal design methodology for grating couplers based on the silicon-on-insultator platform is presented in this paper. Our design methodology accomodates various etch depths, silicon thickness (e.g., 220 nm, 300 nm), incident angles, and cladding materials (e.g., silicon oxide or air), and has been verified by simulations and measurement results. Further more, the design methodology presented can be applied to a wide range, from 1260 nm to 1675 nm, of wavelengths.

Grating-coupled silicon microring resonators

We propose to integrate grating-assisted asymmetric couplers with microring resonators for selective resonance excitation and control of coupling efficiency. A grating-coupled microring add-drop filter is experimentally demonstrated using a silicon-on-insulator technology compatible with complementary metal-oxide-semiconductor processing. A dominant longitudinal mode with a side-mode suppression ratio of 8 dB has been obtained across a broad spectral range of over 130 nm. The selected mode shows an out-of-band-rejection ratio of 19 dB and a quality factor (Q) of 25 000. The concept of integrating grating-assisted couplers for mode selectivity can be applied to other microring-based devices for high-Q, single-mode operations.

Coupler-apodized Bragg-grating add–drop filter

We demonstrate an apodization technique using tapered coupler gaps in grating-assisted asymmetric couplers, which circumvents the need for tapered waveguide perturbations and enables a higher tolerance to fabrication errors. A high sidelobe suppression ratio of 30 dB has been obtained on the submicrometer silicon-on-insulator platform using a CMOS-compatible photonics process with 193 nm lithography.

Ultra-compact, high-Q silicon microdisk reflectors

We demonstrate wavelength-selective reflectors based on silicon microdisk resonators integrated with compact Y-branch splitters, using a CMOS-photonics technology. A high quality factor (Q) of ~ 88,000 was measured in the reflection spectrum for a 2.5-μm-radius device with a small footprint of 6 × 17 μm(2) and a wide free-spectral range (FSR) of over 41 nm. As the radius is reduced to 1.5 μm, corresponding to a device footprint of 4 × 15 μm(2), the spectrum shows an ultra-wide FSR of over 71 nm with the compromise of having a reduced Q of ~ 4000. The coupling between a microdisk and a waveguide is numerically investigated. We further propose a multichannel sensing system using cascaded microdisk reflectors.

Electrically tunable resonant filters in phase-shifted contra-directional couplers

We demonstrate an electrically tunable resonant filter with FSR-free operation using a silicon photonic quarter-wavelength phase-shifted, grating-assisted contra-directional coupler. The spectrum shows a stop-band of 7nm, an extinction of 24dB, and a Q of 7,000.

Differential measurement of transmission losses of integrated optical components using waveguide ring resonators

Development of large-scale photonic integrated circuits requires an accurate, simple, and space-efficient method for characterizing the optical losses of integrated optical components. Here we present a ring-resonator-based technique for transmission-loss measurement of integrated optical components. Y-branch splitters are used to demonstrate the concept. This measurement techique is based on characterizing the spectral response of a waveguide ring resonator with a number of Y-branches inserted inside the cavity. The measurement accuracy is intrinsically limited by the optical loss of the ring waveguide and is independent of fiber-to-waveguide coupling losses. The devices were fabricated using a CMOS-compatible silicon-on-insulator technology. Our results show that the proposed technique is promising for high-accuracy, high-efficiency characterization of optical losses. Limitations of and potential improvements to the technique are also discussed.

Single-band add-drop filters using anti-reflection, contra-directional couplers

We demonstrate a wide-bandwidth add-drop filter using silicon contra-directional couplers with a novel anti-reflection design. Single-band operation, with a bandwidth of 6.5nm and an extinction ratio of over 20dB, has been experimentally obtained.

Photonic device design flow : from mask layout to device measurement

Mask layout design is an important part in silicon photonic device design flow; the space used and the quality of the mask directly affect the cost of fabrication and quality of the outcome. To effectively minimize time spent on drawing masks, fixing design violations, and reducing unused spacings between each structure, we use effective approaches in the mask design process to ensure the listed criteria are met. Using the PCell and the hierarchy drawing methods, GDS files that contain different device parameters can be generated efficiently. As a result, direct GDS modeling efficiency is improved. An experimental setup that is capable of obtaining high quality measurement data is critical to device measurement. The concept of an automated measurement station can effectively reduce work needed from the experimenter while providing quality results. With the implemented fiber-to-fiber and fiber array automated measurement station, multi-device measurement can be set up to run automatically in minutes whereas traditional manual measurement stations require one’s presence and constant attention. In this thesis, we have illustrated several mask drawing approaches and showed the drawing steps of two masks in detail. We have described two automated experimental setups, fiber-to-fiber and fiber array, in detail and included various measurement results to show the capabilities of these two stations.