The mid-infrared spectral region is interesting for bio-chemical sensing, environmental monitoring,
free space communications, or military applications. Silicon is relatively low-loss from 1.2 to 8 μm and from
24 to 100 μm, and therefore silicon photonic circuits can be used in mid- and far- infrared wavelength ranges.
In this paper we investigate several silicon based waveguide structures for mid-infrared wavelength region.
Silicon Photonics is a field that has seen rapid growth and dramatic changes in the past 5 years. According to the MIT
Communications Technology Roadmap , which aims to establish a common architecture platform across market
sectors with a potential $20B in annual revenue, silicon photonics is among the top ten emerging technologies. This has
in part been a consequence of the recent involvement of large semiconductor companies around the world, particularly in
the USA. Significant investment in the technology has also followed in Japan, Korea, and in the European Union. Low
cost is a key driver, so it is imperative to pursue technologies that are mass-producible.
Therefore, Silicon Photonics continues to progress at a rapid rate. This paper will describe some of the work of the
Silicon Photonics Group at the University of Surrey in the UK. The work is concerned with the sequential development
of a series of components for silicon photonic optical circuits, and some of the components are discussed here. In
particular the paper will present work on optical waveguides, optical filters, modulators, and lifetime modification of
carriers generated by two photon absorption, to improve the performance of Raman amplifiers in silicon.
As Silicon Photonics is developing further towards integration on a single platform, the need for precise fabrication is
paramount and no matter how developed a technology is, there is always potential for error at the wafer and chip level. In
combination with Focused Ion Beam (FIB) technology, we present direct write methods to fabricate and tailor Silicon
Photonic devices to offer the potential of prototyping, testing and correction in a post-processing environment. However,
inherent in most FIB processing is the introduction of large optical loss due to damage and implantation of Gallium,
because Gallium is typically the gas species used in FIBs. Therefore, methods of processing to minimise potential loss
and changes to the original device design will be presented alongside results and a discussion offering a comparison with
other potential methods.
We investigate the effects of silicon ion irradiation on free carrier lifetime and propagation loss in silicon rib
waveguides, and thus its ability to reduce the density of two-photon-absorption (TPA) generated free carriers, an
undesired effect of the Raman process in crystalline silicon. Our experimental results show that free carrier lifetime can
be reduced significantly by silicon ion implantation. Associated excess optical absorption from the implanted silicon ions
can be kept low if irradiation energy and dose are correctly chosen. Simulations of Raman amplification in silicon rib
waveguides suggest that net gain can be achieved in certain cases without the need for an integrated diode in reverse bias
to sweep out the photo-generated free carriers.
We report a novel technique for the fabrication of an all-silicon channel waveguide using direct proton beam writing
and subsequent electrochemical etching. A focused beam of high energy protons is used to selectively inhibit porous
silicon formation in the irradiated regions. By over-etching beyond the ion range, the irradiated region becomes
surrounded by porous silicon cladding. Waveguide characterization carried out at 1550 nm on the proton irradiated
waveguide shows that the propagation losses improve significantly from 20±2 dB/cm to 9±2 dB/cm after vacuum
annealing at 800°C for 1 hour.
The field of Silicon Photonics has gained a significant amount of momentum in recent years. Announcements of high
speed modulators and cost-efficient light sources in the Silicon-on-insulator material system have helped to make Silicon
Photonics a viable contender as a low-cost active photonic platform. As a pioneer in the field, the University of Surrey
continues to investigate the prospects of silicon photonics. Herein we present a summary of our work on several key
areas such as ion implanted grating devices, high-speed modulators, switches and ring resonators. We conclude with a
discussion on an advanced fabrication technique, proton beam writing.
In this paper we report two novel fabrication techniques for silicon photonic circuits and devices. The techniques are
sufficiently flexible to enable waveguides and devices to be developed for telecommunications wavelengths or indeed
other wavelength ranges due to the inherent high resolution of the fabrication tools. Therefore the techniques are
suitable for a wide range of applications. In the paper we discuss the outline fabrication processes, and discuss how they
compare to conventional processing. We compare ease of fabrication, as well as the quality of the devices produced in
preliminary experimental fabrication results. We also discuss preliminary optical results from fabricated waveguide
devices, as measured by conventional means. In these preliminary results we discuss fundamental properties of the
waveguides such as loss and spectral characteristics, as it is these fundamental characteristics that will determine the
viability of the techniques. Issues such as the origins of the loss are discussed in general terms, as resulting fabrication
characteristics such as waveguide surface roughness (and hence loss), or waveguide profile and dimensions may be
traded off against cost of production for some applications. We also propose further work that will help to establish the
potential of the technique for future applications.
Silicon Photonics is experiencing a significant increase in interest due to emerging applications and several high profile
successes in device and technology development. One of the most prominent trends in silicon photonics recently has
been a trend towards miniaturising waveguides. 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. In this paper we analyse several silicon photonics devices based on both small
rib and strip waveguides. We have previously reported on issues related to single mode propagation and polarisation
independence of silicon waveguides, and produced design rules for such small waveguides that are reviewed here. We
have previously reported a modulator based on a small rib waveguide with the height of < 500nm for high speed
operation. However, in this paper we consider slightly larger designs to accommodate polarisation independence.
Finally we discuss the characteristics of ring and racetrack resonators based on both rib and strip waveguides and
methods of improving free spectral range whilst considering polarization effects, and the difficulty in coupling to such
strip waveguide based devices. Both theoretical and experimental results are presented. The maximum free spectral
range that we have demonstrated experimentally is approximately 43nm.
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.
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.
Recently, we have realised a polarisation independent optical racetrack resonator whose resonant dips for TE and TM align to better than 1pm. The devices had a Free Spectral Range (FSR) of only several hundred picometres. This in large part was to the relatively large bend radius (~ 400μm) designed and fabricated with initial focus on producing low bend loss devices. Modelling of the bend loss of the same dimension devices shows that the bend radius can be reduced significantly (down to ~25μm) to produce race track ring resonator with an FSR that is approximately 400% larger than that of those previously fabricated, whilst retaining polarisation independence. This paper will focus on the proposed enhancement of these devices as well as the impetus for their investigation.
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.
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.
Because of their compact size, ring resonators can be a cost effective solution for many Dense Wavelength Division Multiplexing (DWDM) components, as well as many low cost applications such as part of optical sensor circuits, or low cost optical signal processing. Modulators, filters, add-drop multiplexers, and switches are all components that can be realised with a ring resonator. Their potentially large Free Spectral Range (FSR), finesse, and quality factor, together with the potential for low cost fabrication, make them a viable alternative to many current DWDM devices. However, for such devices to be commercially viable, they need to be insensitive to the polarisation state of the input signal. The results obtained herein show that a single input/output optical racetrack resonator has been fabricated so that the minima in the resonance spectra align to better than 1pm. The rings also exhibit relatively low loss with measured Q-factors of approximately 90,000 and finesse values of 12.
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.
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.
In an effort to determine low-cost alternatives for components currently used in DWDM, optical ring resonators are currently being investigated. The well-known microfabrication techniques of silicon, coupled with the low propagation loss of single crystal silicon, make SOI an attractive material. Laterally coupled racetrack resonators utilising rib waveguides have been fabricated and preliminary results are discussed. An extinction ratio of 15.9 dB and a finesse of 11 have been measured.
In silicon based photonic circuits, optical modulation is usually performed via the plasma dispersion effect or via the thermo-optic effect, both of which are relatively slow processes. Until relatively recently, the majority of the work in Silicon-on-Insulator (SOI) was based upon waveguides with cross sectional dimensions of several microns. This limits the speed of devices based on the plasma dispersion effect due to the finite transit time of charge carriers, and on the thermo-optic effect due to the volume of the silicon device. Consequently moving to smaller dimensions will increase device speed, as well as providing other advantages of closer packing density, smaller bend radius, and cost effective fabrication. As a result, the trend in recent years has been a move to smaller waveguides, of the order of 1 micron in cross sectional dimensions. In this paper we discuss both the design of small waveguide modulators (of the order of ~1 micron) together with a presentation of preliminary experimental results. In particular two approaches to modulation are discussed, based on injection of free carriers via a p-i-n device, and via thermal modulation of a ring resonator.
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 3<sup>rd</sup> order Bragg gratings on the reflectance and bandwidth in the waveguides.
We report the first results of optical coherence imaging (OCI) of rat tumor spheroids. OCI is a full-frame variant of optical coherence tomography (OCT). The coherent image spatially modulates a high-sensitivity dynamic holographic film composed of a photorefractive quantum well (PRQW). Full-frame readout out of the hologram is observed in real time on a video camera. This system may be considered generally as a video camera with a coherence filter on the lens. Tumor spheroids are small (100-1000 m) balls of tumor cells that are cultured in vitro. Larger spheroids have increasingly complex inner structure. Necrosis and calcification form and expand, reminiscent of structure in malignant cysts in human tumors. In addition, rafts of tumor cells become separated by fluid-filled voids. These features are within the resolution limit of the OCI system, and produce highly structured coherent images.
Low coherence photorefractive holography is a wide-field technique for 3-D imaging that offers a unique mechanism to discriminate against a background of diffuse light. This provides a wide-field method to image through scattering materials that we have demonstrated may be implemented at frame rates as high as ~ 470/second. We present our recently developed low coherence photorefractive microscope and demonstrate how it may be realized using a spatially coherent broadband c.w. diode-pumped solid-state laser. This can provide real-time sectioned images of moving 3-D objects using only a simple uncooled 8-bit CCD camera. We also demonstrate a photorefractive 3-D imaging technique that exploits structured illumination and photorefractive holography to achieve a real-time wide-field sectioning microscope that may be applied to fluorescence, as well as reflected light. We also discuss issues for improving the sensitivity and spectral coverage of photorefractive holography using semi-insulating MQW devices.
Optical coherence imaging (OCI) is an autocorrelation imaging technique that uses short-coherence light and holographic recording and reconstruction to perform laser-ranging into translucent media. OCI is a full-frame variant of OCT and shares excellent discrimination against scattered light from heterogeneous media. We present the first use of OCI to image into a heterogeneous translucent media: sandstone. There are two motivations for studying sandstone. First, it is an excellent example of a heterogeneous translucent medium on which to study the effects of holographic reconstruction in the presence of static scattered speckle. Second, it is of intrinsic interest for energy production as an excellent example of an oil or gas reservoir rock. Using Optical Coherence Imaging (OCI) we have imaged several layers of grains in a sandstone sample. Information on grain geometry was obtained as deep as 400 microns into the sample.
We report high speed (~ 470 frames/s) 3-D imaging using photorefractive holography with sources of diverse temporal and spatial coherence and discuss design considerations for real-world high bit-rate imaging systems. We also propose a new real-time optical sectioning technique based on structured illumination with photorefractive holography to detect fluorescence.