William R. Headley

Polarization-independent optical racetrack resonators using rib waveguides on silicon-on-insulator

In an effort to find low-cost alternatives for components currently used in dense wavelength division multiplexing, optical ring resonators fabricated on silicon on insulator are currently being investigated. Their performance can be further enhanced if they are polarization independent. Herein we use rib waveguides to control the polarization properties of the devices and hence produce polarization-independent racetrack ring resonators. Transverse electric and transverse magnetic resonant peaks are measured to within 2 pm of one another over four cycles of the free spectral range. The racetrack resonators also exhibit measured Q factors of approximately 90 000 and finesse values of 12.

A high efficiency input/output coupler for small silicon photonic devices

Coupling light from an optical fibre to small optical waveguides is particularly problematic in semiconductors, since the refractive index of the silica fibre is very different from that of a semiconductor waveguide. There have been several published methods of achieving such coupling, but none are sufficiently efficient whilst being robust enough for commercial applications. In this paper experimental results of our approach called a Dual-Grating Assisted Directional Coupler, are presented. The principle of coupling by this novel method has been successfully demonstrated, and a coupling efficiency of 55% measured.

Freestanding waveguides in silicon

Using a direct-write process for the production of three dimensional microstructures on a semiconductor, freestanding waveguides have been realized in silicon. The waveguides are produced by a focused beam of high energy protons that is scanned over a silicon substrate. The latent image of the scan is subsequently developed by electrochemical etching. Herein the authors report on the fabrication method as well as determining the propagation loss of these structures. Propagation loss values of 13.4 and 14.6 dB/cm were obtained for these preliminary structures for transverse electric and transverse magnetic polarizations, respectively.

Fabrication of low-loss silicon-on-oxidized-porous-silicon strip waveguide using focused proton-beam irradiation

We have successfully fabricated low-loss silicon-on-oxidized-porous-silicon (SOPS) strip waveguides with high-index contrast using focused proton-beam irradiation and electrochemical etching. Smooth surface quality with rms roughness of 3.1 nm is achieved for a fluence of 1x10(15)/cm(2) after postoxidation treatment. Optical characterization at a wavelength of 1550 nm shows a loss of 1.1+/-0.4 dB/cm and 1.2+/-0.4 dB/cm in TE and TM polarization respectively, which we believe is the lowest reported loss for SOPS waveguides. This opens up new opportunities for all-silicon-based optoelectronics applications.

Free carrier lifetime modification for silicon waveguide based devices

We investigate the effect of silicon ion irradiation on free carrier lifetime in silicon waveguides, and thus its ability to reduce the density of two-photon-absorption (TPA) generated free carriers. Our experimental results show that free carrier lifetime can be reduced significantly by silicon ion implantation. Associated excess optical absorption from the implanted ions can be reduced to an acceptable level if irradiation energy and dose are correctly chosen. Simulations of Raman scattering suggest that net gain can be achieved in certain cases without the need for an integrated diode in reverse bias to remove the photo-generated free carriers.

Three-dimensional control of optical waveguide fabrication in silicon

In this paper, we report a direct-write technique for three-dimensional control of waveguide fabrication in silicon. Here, a focused beam of 250 keV protons is used to selectively slow down the rate of porous silicon formation during subsequent anodization, producing a silicon core surrounded by porous silicon cladding. The etch rate is found to depend on the irradiated dose, increasing the size of the core from 2.5 microm to 3.5 microm in width, and from 1.5 microm to 2.6 microm in height by increasing the dose by an order of magnitude. This ability to accurately control the waveguide profile with the ion dose at high spatial resolution provides a means of producing three-dimensional silicon waveguide tapers. Propagation losses of 6.7 dB/cm for TE and 6.8 dB/cm for TM polarization were measured in linear waveguides at the wavelength of 1550 nm.

Issues Associated With Polarization Independence in Silicon Photonics

Interest in silicon photonics is experiencing a dramatic increase due to emerging applications areas and several high profile successes in device and technology development. Despite early work dating back to the mid-1980s, dramatic progress has been made only in the recent years. While many approaches to research have been developed, the striking difference between the work of the early to mid-1990s, and more recent work, is that the latter has been associated with a trend to reduce the cross sectional dimensions of the waveguides that form the devices. The question arises therefore, as to whether one should move to very small strip waveguides (silicon wires) of the order of 250 nm in height and a few hundred nanometres in width for improved device performance but with little hope of polarization independence, or to utilize slightly larger rib waveguides that offer more opportunity to control the polarization dependence of the devices. In this paper, we discuss the devices suitable for one approach or the other, and present the designs associated both with strip and rib waveguides. In particular, we present the designs of polarization-independent ring resonators with free spectral ranges up to 12 nm, we propose modulators for bandwidths in the tens of gigahertz regime, and present grating-based couplers for rib and strip waveguides, and/or for wafer scale testing, as well as a novel means of developing Bragg gratings via ion implantation

Multi-stage racetrack resonator filters in silicon-on-insulator

In an effort to find low-cost alternatives for components currently used in dense wavelength division multiplexing (DWDM), various devices fabricated on silicon-on-insulator (SOI) have been investigated. Many include modulators, filters, and switches that can be realized with a ring or racetrack resonator. For such devices to be commercially viable, they need to be insensitive to the polarization state of the input signal. Herein we discuss the design of polarization-independent multi-stage racetrack filters in SOI and compare this design with a single-stage configuration.

Silicon photonics: the early years

The purpose of this paper is to set the scene for what promises to be an outstanding conference. To this end the paper will survey the early work in silicon photonics from the late 1980s to the mid 1990s. This was when the more fundamental studies of basic building blocks were carried out, such as study of the silicon optical waveguide itself, the contributions to loss and improvement of waveguiding devices. Issues such as how to achieve modulation, and how to implement a modulator, the criteria for single mode propagation will also be covered, as well as work on the beginnings of optical circuits in silicon and SOI. The focus will be upon pure silicon, usually, but not exclusively in the form of Silicon on Insulator (SOI), as opposed to work on compounds such as SiGe or SiC. Much of this work still resonates with work being carried out today, because the move to smaller and more efficient devices means that some of these issues must be revisited in order to achieve optimal device performance. Hence the paper will provide a summary of the early work on silicon photonics, and attempt to relate it to some of the issues being studied today.

High frame-rate, 3-D photorefractive holography through turbid media with arbitrary sources, and photorefractive structured illumination

In this paper, we briefly review our work on low-coherence photorefractive holography and report on the current state of the art. We present what is, to the best of our knowledge, the fastest-ever three-dimensional (3-D) imaging system and present results obtained with imaging at 470 frames/s (fps). We demonstrate the versatility of photorefractive holography using various sources, including LEDs, high-power diode arrays, and a novel, all solid-state broad-band laser. We present preliminary results obtained by combining the technique of structured illumination with photorefractive holography for the first time. We demonstrate that this novel holographic optical sectioning technique may be implemented for both reflection and fluorescence imaging.