Branislav Timotijevic

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.

Vibration and shock reliability of MEMS: modeling and experimental validation

A methodology to predict shock and vibration levels that could lead to the failure of MEMS devices is reported as a function of vibration frequency and shock pulse duration. A combined experimental–analytical approach is developed, maintaining the simplicity and insightfulness of analytical methods without compromising on the accuracy characteristic of experimental methods. The minimum frequency-dependent acceleration that will lead to surfaces coming into contact, for vibration or shock inputs, is determined based on measured mode shapes, damping, resonant frequencies, and an analysis of failure modes, thus defining a safe operating region, without requiring shock or vibration testing. This critical acceleration for failure is a strong function of the drive voltage, and the safe operating region is predicted for transport (unbiased) and operation (biased condition). The model was experimentally validated for over-damped and under-damped modes of a comb-drive-driven silicon-on-insulator-based tunable grating. In-plane and out-of-plane vibration (up to 65 g) and shock (up to 6000 g) tests were performed for biased and unbiased conditions, and very good agreement was found between predicted and observed critical accelerations.

Silicon photonic waveguides for different wavelength regions

In this paper, we present our work on three silicon waveguide structures that are suitable for three different wavelength regions: near-, mid- and far-infrared. Design rules for standard rib SOI waveguides are given. Both single mode and polarization independence in these waveguides are discussed. A hollow-core waveguide suitable for gas-sensing applications in the mid-infrared wavelength region is also analysed. Finally, fabrication and experimental results for free standing waveguides, which may find application in the mid- and perhaps far-infrared wavelength regions, are presented.

Design Rules for Single-Mode and Polarization-Independent Silicon-on-Insulator Rib Waveguides Using Stress Engineering

There is a trend towards miniaturization of silicon photonic circuits due to superior performance and small cost. Design rules that must be imposed on the geometry of optical waveguides to make them behave as polarization-independent and single-mode devices are well known for waveguides with relatively large cross sections and for some small cross-sectional rib waveguides with vertical sidewalls and an air top cladding. The influence of the top oxide cover on waveguide birefringence was analyzed recently, but only for relatively large cross-sectional waveguides. This paper reports simulations for both single-mode and polarization-independent behavior for small cross-sectional waveguides with variable rib width, etch depth, top oxide cover thickness, and side-wall angle. The results show that the stress-induced effects must be taken into account to satisfy both requirements. Design rules to maintain birefringence-free operation and to satisfy single-mode behavior for small rib silicon-on-insulator (SOI) waveguides are presented.

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.

Tailoring the spectral response of add/drop single and multiple resonators in silicon-on-insulator

Channel dropping waveguide filters based on single and multiple resonators in silicon-on-insulator (SOI) technology are of great interest due to their compactness and high wavelength selectivity, which is a desirable feature for photonic modulators, detectors, and other optically integrated components in telecommunication systems, in particular for wavelength division multiplexing (WDM) systems. Particular advantage of these filters is that they are capable of producing relatively large free spectral range (FSR) as well as narrow 3-dB bandwidth of the filter resonances. Herein we report experimental results and discuss the possibility of designing mono-mode and (nearly) polarization independent SOI ring and racetrack resonators with the FSR in excess of 30 nm.

Polarization-insensitive directional couplers based on SOI wire waveguides

Optical directional couplers based on SOI-wire waveguides have been modelled by a semi-analytical approach based on the Coupled Mode Theory and Finite Element Method. The modelling is used to obtain analytically optical power at the parallel and cross ports by utilizing numerically calculated coupling coefficients. Geometrical dimensions of the couplers have been optimized to obtain a polarization-independent behaviour. The influence of non-vertical sidewalls on the coupler performance has also been addressed.

Silicon Photonics: Are Smaller Devices Always Better?

There is a worldwide trend towards miniaturising silicon photonic waveguides for both performance and cost reasons. It is clear than in many circumstances the shrinking of the device dimensions provides advantages in terms of cost and packing density, modulation bandwidth, improved performance in resonant structures, and an increase in optical power density within the devices. However, the size reduction comes at some costs in increased difficulty in maintaining single mode operation of the waveguides whilst controlling the polarisation properties of the device. Furthermore, the difficulty of coupling into and out of these devices is increased due to the mismatch in size and refractive index with an optical fibre.

Characteristics of rib waveguide racetrack resonators in SOI

Optical ring/racetrack resonators have the sufficient flexibility to realise many functions in a single device, from filters/multiplexers, to modulators, to switches. The use of Silicon-On-Insulator (SOI) material, coupled with Ultra Large Scale Integration (ULSI) processing techniques, may allow the cost of these devices to become economically advantageous over current components. This paper describes our recent work in developing polarisation independent ring resonators, and subsequent, work on increasing the limited free spectral range and full width half maximum of the resonance. There are two key components that comprise a polarisation-independent racetrack resonator: a polarisation-independent rib waveguide and a polarisation-independent directional coupler. Polarisation independence is achieved in the waveguides when the geometrical design ensures that both polarisation modes propagate with the same effective index. We report on such devices together with polarisation independent couplers, which are achieved by allowing different inter multiples of the coupling length for the TE and the TM modes. By combining these components, the resulting device is a polarisation independent ring resonators. These devices have been thermally modulated by means of a modulated visible laser and alternatively via small heaters fabricated on the waveguides. We have also modelled ring resonator modulators via carrier injection and depletion. Subsequently we have improved the device characteristics by employing smaller bend radii to increase the free spectral range by a factor of 5, and by cascading racetracks to improve the full width half maxima of the resonance by almost 40%. Experimental results are reported for most of the above characteristics. We will further investigate the opportunities for increasing the FSR whilst retaining polarisation independence, the possibility of retaining polarisation independence whilst utilising the properties of the ring resonator to form improved modulators.