Thach G. Nguyen

Rigorous Modeling of Lateral Leakage Loss in SOI Thin-Ridge Waveguides and Couplers

The transverse-magnetic to transverse-electric (TM-TE) mode coupling properties and the lateral leakage loss behavior of the propagating TM-like mode in silicon-on-insulator thin-ridge waveguides and directional couplers are investigated. Accurate calculation of the lateral leakage is performed by a mode matching technique in which the calculation window is left fully open in the lateral direction.

High Q factor chalcogenide ring resonators for cavity-enhanced MIR spectroscopic sensing

We report the characteristics of high Q factor chalcogenide ring resonators designed for sensing in the mid-infrared (MIR). The resonators consisted of an exposed Ge11.5As24Se64.5 core on a Ge11.5As24S64.5 bottom cladding and were fabricated in the racetrack coupled ring structure. Loaded Q factors at 5.2μm up to 58,000were obtained, corresponding to an intrinsic Q of 145,000 and a waveguide propagation loss of 0.84dB/cm.

Elastomer-Based Pneumatic Switch for Radio Frequency Microdevices

This paper reports the realization and characterization of a pneumatic microswitch integrated with a high-frequency radio frequency (RF) transmission line on an elastomer substrate. A process for the fabrication of low-loss RF coplanar transmission lines on flexible elastomeric polydimethylsiloxane (PDMS) substrates was developed, and devices realized using this process were used to determine the characteristics of PDMS as an RF substrate with uniform low loss and low dielectric constant being measured. To demonstrate the capabilities of this elastomer-based RF platform, a micromechanical switch exploiting a pneumatic membrane valve was integrated with the PDMS RF transmission line. Repeatable switching was observed with greater than 20-dB suppression in the “off” state and minimal degradation of the transmission line characteristics in the “on” state being achieved over a multioctave 2-20-GHz bandwidth. These valve-integrated transmission lines had an insertion loss of 0.16 dB·mm-1 at 20 GHz. This proof-of-concept device represents a novel combination of the areas of micropneumatics, flexible electronics, and broadband microwave devices with excellent RF properties, low interference, bias-free pneumatic switching, and relatively simple fabrication.

Integrated frequency comb source based Hilbert transformer for wideband microwave photonic phase analysis

We demonstrate a photonic RF Hilbert transformer for broadband microwave in-phase and quadrature-phase generation based on an integrated frequency optical comb, generated using a nonlinear microring resonator based on a CMOS compatible, high-index contrast, doped-silica glass platform. The high quality and large frequency spacing of the comb enables filters with up to 20 taps, allowing us to demonstrate a quadrature filter with more than a 5-octave (3 dB) bandwidth and an almost uniform phase response.We demonstrate a photonic RF Hilbert transformer for broadband microwave in-phase and quadrature-phase generation based on an integrated frequency optical comb, generated using a nonlinear microring resonator based on a CMOS compatible, high-index contrast, doped-silica glass platform. The high quality and large frequency spacing of the comb enables filters with up to 20 taps, allowing us to demonstrate a quadrature filter with more than a 5-octave (3 dB) bandwidth and an almost uniform phase response.

Lateral leakage of TM-like mode in thin-ridge Silicon-on-Insulator bent waveguides and ring resonators

We present the first prediction of lateral leakage behavior of the TM-like mode in thin-ridge SOI curved waveguides and ring resonators. A simple phenomenological model is first presented which predicts that the lateral leakage in these structures is significantly impacted by both the ring radius and waveguide width. This prediction is verified using full vectorial mode matching and finite element methods. We show that specific combinations of waveguide width and ring radius can lead to very low-loss propagation in the TM-like mode. This finding is critical for the design of high-Q resonators on such waveguide platforms and will have major impact on the field of silicon lasers and sensing applications.

Compact Brillouin devices through hybrid integration on silicon

A range of unique capabilities in optical and microwave signal processing and generation have been demonstrated using stimulated Brillouin scattering (SBS). The need to harness SBS in mass-manufacturable integrated circuits has led to a focus on silicon-based material platforms. Remarkable progress in silicon-based Brillouin waveguides has been made, but results have been hindered by nonlinear losses present at telecommunications wavelengths. Here, we report on a new approach to surpass this issue through the integration of a high Brillouin gain material, As2S3, onto a silicon-based chip. We fabricated a compact spiral device within a silicon circuit, achieving an order-of-magnitude improvement in Brillouin amplification. To establish the flexibility of this approach, we fabricated a ring resonator with free spectral range precisely matched to the Brillouin shift, enabling the first demonstration, to our knowledge, of Brillouin lasing in a planar integrated circuit. Combining active photonic components with the SBS devices shown here will enable the creation of compact, mass-manufacturable optical circuits with enhanced functionalities.

Thin-ridge Silicon-on-Insulator waveguides with directional control of lateral leakage radiation

In this paper, we propose a Silicon-On-Insulator waveguide structure which when excited with TM guided light emits controlled TE polarized radiation from one side of the structure only. The validity of the proposed structure is analyzed using eigenmode expansion and supermode techniques. It is shown that care must be taken to select the gap between the radiating elements such that both the phase and the amplitude of the radiating modes are maintained along the propagation direction to achieve the desired directional control of radiation. Steps toward practical demonstration of the proposed structure are identified.

Long-range coupling of silicon photonic waveguides using lateral leakage and adiabatic passage

We present a new approach to long range coupling based on a combination of adiabatic passage and lateral leakage in thin shallow ridge waveguides on a silicon photonic platform. The approach enables transport of light between two isolated waveguides through a mode of the silicon slab that acts as an optical bus. Due to the nature of the adiabatic protocol, the bus mode has minimal population and the transport is highly robust. We prove the concept and examine the robustness of this approach using rigorous modelling. We further demonstrate the utility of the approach by coupling power between two waveguides whilst bypassing an intermediate waveguide. This concept could form the basis of a new interconnect technology for silicon integrated photonic chips.

Thin Shallow-Ridge Silicon-on-Insulator Waveguide Transitions and Tapers

We analyze transitions from low-loss “magic” width to strongly radiating “anti-magic” width thin, shallow-ridge silicon-on-insulator waveguides operating in the transverse magnetic mode, using a vector eigenmode expansion method. It is shown that the transition produces a beam of transverse electric (TE) radiation with a pattern, which is strongly dependent on the geometry of the transition. It is shown that controlled, highly coherent, and low-divergence TE beam can be emitted from a relatively compact linear taper. Methods for side lobe suppression are also analyzed and avenues for more sophisticated beam shaping are identified drawing inspiration from leaky wave antennas.

Adiabatic two-photon quantum gate operations using a long-range photonic bus

Adiabatic techniques have much potential to realize practical and robust optical waveguide devices. Traditionally, photonic elements are limited to coupling schemes that rely on proximity to nearest neighbour elements. We combine adiabatic passage with a continuum based long-range optical bus to break free from such topological restraints and thereby outline a new approach to photonic quantum gate design. We explicitly show designs for adiabatic quantum gates that produce a Hadamard, 50:50 and 1/3:2/3 beam splitter, and non-deterministic controlled NOT gate based on planar thin, shallow ridge waveguides. Our calculations are performed under conditions of one and two-photon inputs.