Seong Phun Chan

Single-mode and polarization-independent silicon-on-insulator waveguides with small cross section

The fabrication restrictions that must be imposed on the geometry of optical waveguides to make them behave as single-mode devices are well known for relatively large waveguides, with shallow etch depth. However, the restrictions for small waveguides (/spl sim/1 /spl mu/m or less in cross section) are not well understood. Furthermore, it is usually a requirement that these waveguides are polarization independent, which further complicates the issues. This paper reports on the simulations of the conditions for both single-mode behavior and polarization independence, for small and deeply etched silicon-on-insulator (SOI) waveguides. The aim is to satisfy both conditions simultaneously. The results show that at larger waveguide widths, waveguide etch depth has little effect on the mode birefringence because the transverse-electric (TE) mode (horizontal-polarized mode) is well confined under the rib region. However, at smaller rib widths, the etch depth has a large influence on birefringence. An approximate equation relating the rib-waveguide width and etch depth to obtain polarization-independent operation is derived. It is possible to achieve single-mode operation at both polarizations while maintaining polarization independence for each of the waveguide heights used in this paper but may be difficult for other dimensions. For example, a 1-/spl mu/m SOI rib waveguide with an etch depth of 0.64 /spl mu/m and rib width of 0.52 /spl mu/m is predicted to exhibit such characteristics.

Optical phase modulators for MHz and GHz modulation in silicon-on-insulator (SOI)

This paper reports the simulation of the direct current (dc), transient, and optical characteristics of low-loss single-mode optical phase modulators based on silicon-on-insulator (SOI) material. The devices operate by injecting free carriers to change the refractive index in the guiding region and have been modeled using the two-dimensional (2-D) device simulation package SILVACO and the optical simulator BeamPROP to determine their electrical and optical performance, respectively. These simulators have been employed to optimize the overlap between the injected free carriers in the intrinsic region and the propagating optical mode. Attention has been paid to both the steady state and transient properties of the device. In order to produce quantitative results, a particular p-i-n device geometry has been employed in the study, but the trends in the results are sufficiently general to be of help in the design of many modulator geometries. The specific example devices used are designed to support a single optical guided mode and are of approximately 1 mm in cross-sectional dimensions. The modeling results predict that the transient performance of the device is affected significantly by the contact width and the rib doping depth. Results presented encompass Gaussian and constant doping profiles in the n/sup +/ regions. The doping profile of the contacts has a tremendous effect on both the dc and transient performances. Phase modulators with drive currents as low as 0.5 mA and transient rise times of 0.3 ns and fall times of 0.12 ns are predicted. Following from these results, a realistic doping profile is proposed that surpasses the electrical results of the Gaussian and most of the constant doping profiles. The improvements in electrical device characteristics are at the expense of a slightly increased optical absorption loss. An alternative switching technique is also presented that could further improve the device speed.

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

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.

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.

Flat spectral response silicon arrayed waveguide gratings via ion implantation

A flat spectral response has long been a requirement in photonic networking. In order to find a low cost alternative compared to some other technologies, a novel method is demonstrated to achieve such a response in silicon-on-insulator arrayed waveguide gratings (AWG) through free carrier absorption, implemented by ion implantation of dopant species. The AWG is designed using 1.5mum Si-overlayer on an SOI wafer utilising rib waveguides with a width of 1.1mum and an etch-depth of 0.88mum to facilitate the single-mode, birefringence-free operation. It is also essential to achieve a uniform dopant concentration throughout the guiding region to avoid any phase errors resulting from the free carriers. This can be achieved using multiple ion implantation steps. Both n and p type dopants are investigated and results showed significant reduction of doping length is achieved by using n-type dopant as compared to a p-type dopants. The broadened passband is measured to be 0.5nm, a 5 times broadening from the Gaussian peak.

Single mode, polarisation independent waveguides in silicon-on-insulator

The conditions for single mode and zero birefringence for small and deeply etched SOI waveguides are presented and discussed. It is difficult to satisfy both conditions simultaneously.

Polarisation independent devices in small SOI waveguides

In silicon photonics, there is a trend to smaller device dimensions. This means that polarisation independent devices become more difficult to design and fabricate whilst maintaining monomode behaviour. By way of example we discuss a polarisation independent ring resonator.

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.