Goran Z. Masanovic

Dual grating-assisted directional coupling between fibers and thin semiconductor waveguides

A novel dual grating-assisted directional coupler (DGADC) for coupling an optical fiber with a thin semiconductor waveguide is proposed. As an example, a DGADC in silicon-on-insulator is discussed. Maximum coupling efficiency in excess of 90% can be obtained using this coupler, while the highest coupling efficiency previously reported was only 40%.

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.

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.

Coupling optical fibers to thin semiconductor waveguides

We present a novel type of grating-assisted coupler for coupling light from an optical fibre to a thin semiconductor waveguide. The Dual Grating-Assisted Directional Coupler (DGADC) consists of two gratings, one thick waveguide with a refractive index similar to the refractive index of the fibre, a thin semiconductor waveguide and a layer with refractive index which is between that of the thick and thin waveguides. Specifically, a coupler in silicon on insulator (SO!) has been analysed. The maximum coupling efficiency for this coupler can approach 1 00%, while the minimum coupling length can be around 2mm.

Preliminary experimental results of a dual grating-assisted directional coupler on SOI

The recent interest in silicon based photonics, and the trend to reduced device dimensions in photonic circuits generally, has led to the need for mode converters to couple from optical fibres to such small devices. A range of structures have been proposed and in some cases demonstrated, including three dimensional tapers, inverted tapers and micromachined prisms. We have previously reported theoretical analyses of a Dual Grating Assisted Directional Coupler (DGADC), which promises high efficiency coupling over modest spectral linewidths. In this paper we report preliminary experimental results on the fabrication of such devices, together with an evaluation of the coupling efficiency. The approach has been to fabricate a demonstrator device for a particular arrangement of waveguide coupling parameters, i.e. we have fabricated a device that couples easily from fibre, because the input waveguide is approximately 5μm in cross sectional dimensions. The mode converter then couples to a 0.25μm silicon waveguide, primarily because comparisons exist in the literature. These results are compared with the predicted efficiency, and the results are discussed both in terms of the constituent parts of the DGADC, as well as the fabrication limitations. Whilst our device is not optimised we demonstrate that it has promise for very high efficiency coupling.

Characterization of integrated Bragg gratings on silicon-on-insulator rib waveguides

Waveguide based Bragg grating devices have the potential of integration with passive or active optical components. A narrow bandwidth Bragg reflection filter or Fabry-Perot resonant structures can be realised using the approach of periodic refractive index modulation in waveguide gratings to form reflective structures. Most authors have considered 1st order Bragg gratings with periods of the order of 228nm operating at 1550nm but at the expense of complexity and high cost of fabrication. This paper describes the design of Silicon-On-Insulator (SOI) rib waveguides operating in the single mode regime that exhibit low polarisation dependence. A rigorous leaky mode propagation method (LMP) has been used to investigate the influence of etch depth in 3rd order Bragg gratings on the reflectance and bandwidth in the waveguides.

Coupling to 1-μm silicon modulators

Optical modulation in silicon photonics is performed either via the plasma dispersion effect, or by thermal means. Both are relatively slow processes when utilized in large (multi micron) waveguide structures. However, modulators based on the plasma dispersion effect become much faster in smaller waveguides, but coupling to such waveguides is then inefficient. In this paper we discuss both the operation of small waveguide modulators and a potentially more efficient means of coupling to such waveguides. A number of design parameters of the modulators are discussed including the optimum configuration of a three terminal p-i-n diode around a rib waveguide, and the optimization of both the power efficiency and the operating speed of the device. Operating speed is theoretically increased by as much as 7400% with respect to devices in the literature. The problem of coupling to these waveguides is addressed via gratings. All work is theoretical, but sets the groundwork for subsequent fabrication.

Experimental evidence of modal properties using directional couplers in silicon-on-insulator

The single-mode optical rib waveguide is a fundamental building block for many, more complex optical circuits. Recent modelling has been provided in the literature that has investigated polarisation and modal properties of small, deeplyetched rib waveguides in SOI. In this paper we present work that has utilised a total of 160 directional couplers fabricated from rib waveguides of various waveguides dimensions, to investigate the validity of the published modelling. In particular 5 waveguide designs have been used to fabricate directional couplers of differing lengths, to map out the variation in coupling of power within the directional couplers. For a singlemode device, a characteristic sinusoidal variation is expected, but the sinusoid will be corrupted in the presence of higher order modes, each of which will have a different coupling length as compared to the fundamental mode. We have observed experimental results that are consistent with the modelling for each of the 5 waveguide designs, and hence we present experimental evidence of higher-order mode behaviour that is consistent with modelling.

Small devices in SOI: fabrication and design issues

There is a trend in photonic circuits to move to smaller device dimensions for improved cost efficiency and device performance. However, the trend also comes at some cost to performance, notably in the polarisation dependence of the circuits, the difficulty in coupling to the circuits, and in some cases, in increased device complexity. This paper discusses a range of Silicon-on-Insulator (SOI) based optical devices, and the advantages and disadvantages in moving to smaller waveguide dimensions. In particular optical phase modulators based upon the plasma dispersion effect and ring resonators are considered, together with a device for coupling to small waveguides, the so-called Dual Grating Assisted Directional Coupler (DGADC). The advantages of moving to small dimensions are considered, and some preliminary experimental results are given. In particular, progress of the DGADC is evaluated in the light of promising experimental results.

A grating based coupler for fibre to silicon waveguide excitation

Recently there has been a strong trend to fabricate smaller photonic devices. In the literature, the problem of coupling optical fibres with thin semiconductor waveguides has not been solved sufficiently well to obtain both high coupling efficiency and good fabrication tolerances. This paper discusses a new approach, the Dual Grating-Assisted Directional Coupling (DGADC), which can result in a robust and very efficient device, with relaxed fabrication tolerances. Theoretical investigation of the coupler is presented. Coupling efficiency and device length are determined as functions of layer thicknesses and refractive indices, grating periods, depths and duty ratios, and finally wavelength. Fabrication of the coupler is also given, as well as preliminary experimental results.