Kapil Debnath

Cascaded modulator architecture for WDM applications

Integration density, channel scalability, low switching energy and low insertion loss are the major prerequisites for on-chip WDM systems. A number of device geometries have already been demonstrated that fulfill these criteria, at least in part, but combining all of the requirements is still a difficult challenge. Here, we propose and demonstrate a novel architecture consisting of an array of photonic crystal modulators connected by a dielectric bus waveguide. The device architecture features very high scalability and the modulators operate with an AC energy consumption of less than 1fJ/bit. Furthermore, we demonstrate cascadeability and multichannel operation by using a comb laser as the source that simultaneously drives 5 channels.

Dielectric waveguide vertically coupled to all-silicon photodiodes operating at telecommunication wavelengths

We present a method for detecting light in the 1550 nm wavelength window based on a silicon nitride waveguide that is vertically coupled to a silicon photonic crystal cavity. The absorption in silicon arises from deep-levels created by ion implantation, thereby providing excellent CMOS compatibility. We demonstrate a responsivity of 0.108 A/W at −10 V reverse bias with a dark current of 9.4 nA. Our work demonstrates one of the smallest wavelength selective photodectors realised to date. By cascading such detectors we also demonstrate a two-channel demultiplexer.

Lithographic wavelength control of an external cavity laser with a silicon photonic crystal cavity-based resonant reflector

We report the experimental demonstration of a new design for external cavity hybrid lasers consisting of a III-V semiconductor optical amplifier (SOA) with fiber reflector and a photonic crystal (PhC)-based resonant reflector on SOI. The silicon reflector is composed of an SU8 polymer bus waveguide vertically coupled to a PhC cavity and provides a wavelength-selective optical feedback to the laser cavity. This device exhibits milliwatt-level output power and side-mode suppression ratios of more than 25 dB.

Low-Loss Silicon Waveguides and Grating Couplers Fabricated Using Anisotropic Wet Etching Technique

We report low-loss silicon waveguides and efficient grating coupler to couple light into them. By using anisotropic wet etching technique, we reduced the side wall roughness down to 1.2nm. The waveguides were patterned along the [112] direction on a [110] SOI substrate. The waveguide boundaries are decided by the [111] planes which are normal to the [110] surface. Fabricated waveguides show minimum propagation loss of 0.85 dB/cm for TE polarization and 1.08dB/cm for TM polarization. The fabricated grating couplers show coupling efficiency of -4.16dB at 1570nm with 3dB bandwidth of 46nm.

Low-Loss Slot Waveguides with Silicon (111) Surfaces Realized Using Anisotropic Wet Etching

We demonstrate low-loss slot waveguides on silicon-on-insulator (SOI) platform. Waveguides oriented along the (11-2) direction on the Si (110) plane were first fabricated by a standard e-beam lithography and dry etching process. A TMAH based anisotropic wet etching technique was then used to remove any residual side wall roughness. Using this fabrication technique propagation loss as low as 3.7dB/cm was realized in silicon slot waveguide for wavelengths near 1550nm. We also realized low propagation loss of 1dB/cm for silicon strip waveguides.

Photonic crystal waveguides on silicon rich nitride platform

We demonstrate design, fabrication, and characterization of two-dimensional photonic crystal (PhC) waveguides on a suspended silicon rich nitride (SRN) platform for applications at telecom wavelengths. Simulation results suggest that a 210 nm photonic band gap can be achieved in such PhC structures. We also developed a fabrication process to realize suspended PhC waveguides with a transmission bandwidth of 20 nm for a W1 PhC waveguide and over 70 nm for a W0.7 PhC waveguide. Using the Fabry-Pérot oscillations of the transmission spectrum we estimated a group index of over 110 for W1 PhC waveguides. For a W1 waveguide we estimated a propagation loss of 53 dB/cm for a group index of 37 and for a W0.7 waveguide the lowest propagation was 4.6 dB/cm.

Toolkit for Photonic Integrated Circuits Based on Inverted Rib Waveguides

We have performed an exploration of inverted rib waveguide platform for use in optical backplanes. This entailed the design, optimization, and characterization of a variety of passive optical components that may serve as a basis for the functions required of an on-chip optical networks. The presented design introduces an inverted-rib template, which consists of a polymer waveguide layer. We have successfully fabricated and demonstrated low-loss waveguides, and also functional passive devices such as directional couplers, multimode interferometers, waveguide bends and crossings, and distributed Bragg reflectors. We also demonstrate a way of coupling active components (e.g., in silicon) to such a photonic integrated circuit.

Low insertion loss modulator based on a vertically coupled photonic crystal resonator

We experimentally demonstrate a simple but more efficient technique to modulate and multiplex multiple WDM channels. Our design is based on a bus waveguide vertically coupled to multiple Photonic Crystal (PhC) resonator, each of which modulates an individual channel in place. The Photonic crystal resonator modulator provide very low switching energies (~fJ) while the bus waveguide can be made from a material with a low refractive index thereby allowing very efficient coupling with an optical fiber.

Slowlight enhanced photonic crystal modulators

We use the slow light effect to enhance modulation in a carrier injection Mach Zehnder interferometer and enhance the modulation efficiency by a factor of 8 relative to a rib waveguide device.

Fabrication of Arbitrarily Narrow Vertical Dielectric Slots in Silicon Waveguides

Slot waveguides are used for many photonic applications. However, existing fabrication techniques impose restrictions on the width of the slot region. Here, we propose and experimentally demonstrate a fabrication process to realize an arbitrarily narrow vertical dielectric slot in a silicon waveguide. Using this fabrication method, we have realized silicon slot waveguides with a 10-nm oxide slot region. The propagation loss of the fabricated slot waveguide was 1.36 ± 0.3 dB/mm.