Programmable Micro Diffraction Gratings (PMDG) are one dimensional arrays of individual mirrors, the height of which can be adjusted to produce a desired spectrum at a given angle. They can function both as spatial light modulators and as reconfigurable generators of high-resolution spectra. Fig. 1a shows the way the PMDG is actuated, while Fig. 1b presents an example of applications for PMDGs, the generation of synthetic spectra . MEMS PMDGs have been used in microspectrometers, compact projection displays, optical communication systems and miniaturized external cavity lasers [1-4] because of their optical properties and compactness.
In future space missions for Universe and Earth Observation, scientific return could be optimized using MOEMS devices. Large micromirror arrays (MMA) are used for designing new generation of instruments. In Universe Observation, multi-object spectrographs (MOS) are powerful tools for space and ground-based telescopes for the study of the formation and evolution of galaxies. This technique requires a programmable slit mask for astronomical object selection; 2D micromirror arrays are perfectly suited for this task. In Earth Observation, removing dynamically the straylight at the entrance of spectrographs could be obtained by using a Smart Slit, composed of a 1D micro-mirror array as a gating device. We are currently engaged in a European development of micro-mirror arrays, called MIRA, exhibiting remarkable performances in terms of surface quality as well as ability to work at cryogenic temperatures. MMA with 100 × 200 μm2 single-crystal silicon micromirrors were successfully designed, fabricated and tested down to 162 K. In order to fill large focal planes (mosaicing of several chips), we are currently developing large micromirror arrays to be integrated with their electronics. 1D and 2D arrays are built on wafer with Through Wafer Vias in order to allow routing of the device on wafer backside, foreseeing integration with dedicated ASICs. The yield of these devices as well as contrast enhancement have been successfully implemented.
We have designed, fabricated and tested narrow-band Fabry-Perot filters in the infrared using gold porous mirrors and a silicon spacer layer. The filter peaks at 10 μm and 15 μm have approximately 10% transmission and a 1.5% linewidth. A Fabry-Perot structure with plane metal layers having a similar linewidth would have a transmission of only 0.2%. Thus, for the same linewidth we have improved the transmission by a factor of 50. Apart from the optical enhancements, these filters also have the advantage that they can be made inexpensively in a standard silicon MEMS technology and that their resonances can be finely tuned through post processing.
Continuous and accurate monitoring of acceleration and temperature inside large turbo- and hydro-generators is of crucial importance to prevent extremely expensive system damages and false positives. Development of optical, metalfree sensors for such systems has gained a lot of attention due to the fact that they are resistant to typically very strong electromagnetic fields and that they are non-conductive. We present miniature temperature and accelerometer optical sensors using a common silicon MEMS platform. A linear response with a deviation as small as 1% between set and measured accelerations has been obtained in an acceleration range 0-40g. Preliminary tests for temperature sensors indicate a linear response with sensitivity better than 1°C in a range of 20°C to 150°C.
In this paper we show that using optical photolithography it’s possible to obtain submicron features for periodic structures using the Talbot effect. To use the Talbot effect without the need of an absolute distance measurement between the mask and the wafer we integrate over several exposures for varying wafer mask distances. Here we discuss the salient features of ‘integrated Talbot lithography’. Particularly, we show that to obtain good contrasts an excellent control of the illumination light is essential; for this we use the MO Exposure Optics (MOEO) developed by SUSS MicroOptics (SMO). Finally we show that 1μm and 0.55μm diameter holes can be made using this technique.
Programmable MEMS diffraction gratings are used for spectroscopic applications because of their potential in tailoring visible and infrared spectra. A fully programmable MEMS diffraction grating (FPMDG), where every micro-mirror can move independently in a range 0 - λ/2, where λ is the wavelength of light, leads to a better control of the intensity for each wavelength in the synthetized spectrum – the intensity can take any value from 0 (micro-mirror λ/4-condition) to the maximum (no micro-mirror displacement). The FPMDG chip contains 64 micro-mirrors which are actuated electrostatically. Rigid Si micro-mirrors are connected to side electrodes via linkage arms, permitting the micro-mirror to follow a pure vertical displacement, reducing the micro-mirror bending throughout actuation. Microfabrication is based on a 4 mask photolithography process, using SOI and Pyrex wafers. Each micro-mirror of the FPMDG chip can move by 1.25μm at voltages below 100 V. Two families of micro-mirrors, 50μm or 80μm wide, show negligible cross-talk during actuation. The micro-mirror bowing is as small as 0.14 μm over 700 μm and remains unchanged throughout actuation. Extinction ratios of up to 100 have been achieved by actuating only 3 adjacent micro-mirrors. The measurements have shown high stability and good reproducibility over time. Finally, FPMDGs are used to demonstrate shaping of the input spectrum: the intensity in a particular wavelength region is controlled through independent actuation of a set of adjacent micro-mirrors. The result is attenuation or cancellation of the corresponding wavelengths.
A review of three different systems based on the MEMS tunable blazed grating technology is presented. A MEMS tunable blazed grating is a versatile optical element providing a compact tunable mechanism for optical systems. The MEMS chip measures 5x5 mm2, including a position encoder, and is shock resistant up to 3000 g. The grating can operate in different spectral regions (Visible to Mid-IR) and high optical throughput is guaranteed at all wavelengths by operating it in Littrow condition. The first system shown uses the MEMS grating in a compact wavemeter. It is tested as Fiber Bragg Grating interrogating system. At 1.5 μm wavelength, it detects lines as narrow as 0.2 nm, resolves lines 2 nm apart and retrieves the central wavelength with accuracy better than 20 pm. By using the position encoder the expected accuracy can be on the order of 1pm. The second system shown demonstrates a compact (<10 cm3) tunable external cavity Quantum Cascade Laser using the MEMS grating. The resulting laser operates at a center wavelength of 9.5 μm and is tunable over a range of 150 nm. Finally a double stage monochromator is presented. Two MEMS chips with different grating periods are cascaded, in order to cancel out undesired grating orders, and to improve the filter linewidth (~1nm) and the extinction ratio (26 dB). The cascaded filter can be combined with a broadband source to select an arbitrary wavelength in the 400-800 nm range or the 800-1600 nm range.
In this paper we show that it is possible using optical photolithography to obtain micron and submicron features for
periodic structures in non-contact using the Talbot effect. In order for this effect to work it is important to have good
control of the illumination light and here we show that the MO Exposure Optics (MOEO) developed by SUSS
MicroOptics provides uniform and well collimated illumination light suitable for Talbot lithography. The MOEO can
easily be incorporated into a standard mask aligner. Here we show 1μm and 0.65μm diameter holes in a hexagonal array
in photoresist made in large-gap proximity printing.
This work describes a method for tracking the dynamics of electrostatic discharge (ESD) sensitive MEMS structures
during ESD events, as well as a model for determining the reduced combdrive snap-in voltage under vibration and shock.
We describe our ESD test setup, based on the human body model, and optimized for high impedance devices. A brief
description of the MEMS tunable grating, the test structure used here, and its operation is followed by results of the
measured complex device dynamics during ESD events. The device fails at a voltage up to four times higher than that
required to bring the parts into contact. We then present a model for the snap-in of combfingers under shock and
vibration. We combine the results of the analytical model for combdrive snap-in developed here with a shock response
model to compute the critical shock acceleration conditions that can result in combdrive snap-in as a function of the
operating voltage. We discuss the validity regimes for the combdrive snap-in model and show how restricting the
operation voltage below the snap-in voltage is not a sufficient criterion to ensure reliable operation especially in
environments with large disturbances.
The project is a consortium based activity involving researchers from the UK institutions of the
Universities of Surrey, St. Andrews, Leeds, Warwick, and Southampton, as well as the commercial
research institution QinetiQ. The aims of the project are to progress the state of the art in Silicon
Photonics, in the areas of waveguides, modulators, couplers, detectors, Raman processes, and integration
with electronics. Thus the field is vast, and impossible to cover comprehensively in one project, nor
indeed in one paper. The programme is run on a truly collaborative basis, with members from each
institution running one or more work packages within the project, each co-ordinating work from their
own plus other institutions. To date, the most well developed work has emerged from the activity on basic
waveguides and their characteristics, the modulator activity, optical filters, and work on Raman
Amplifiers. This work will be the main focus of this paper, but an attempt will be made to update the
audience on the remaining activities within the project. By the nature of the project, much of the work is
medium term, and hence some activities are not expected to yield viable results until at least next year,
hence the concentration on some activities rather than all activities at this stage.
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.
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.
Silicon-on-Insulator (SOI) has emerged as promising material choice for various integrated optoelectronic devices. Two
issues make SOI attractive for complex optical systems: the cost reduction due to compatibility with CMOS technology
and high refractive index contrast between core and cladding, which is an important property for good confinement of
light and efficient guiding and coupling in sub-micron waveguides. However, for those devices that are intended to be
part of broadband optical networks, for example multiplexers and de-multiplexers, it is desirable to demonstrate a high
selectivity and a tunable response. Thus, it is necessary to provide wavelength selective elements with the ability to filter
input data streams producing a large Free Spectral Range (FSR), a small Full Width at Half Maximum (FWHM), and a
high quality factor (Q), all conditions set by communication standards. Owing to the generic and adaptable operation,
ring-resonator-types of filters in SOI are often considered as candidates to meet these demands. Herein two different
designs are investigated from both experimental and modelling standpoints in order to tailor the filter transfer function.
These are mutually coupled (Vernier) resonators and cascaded resonators based on small SOI photonic wires. Fabricated
filters designed to provide a large FSR and a polarisation independent (PI) response are analysed and improvements
proposed. Issues associated with temperature control of the transfer function have also been addressed.
The requirement of a precise and controllable reflection interface in total internal reflection type optical switches is
widely acknowledged. When these switches are based upon carrier injection such as those fabricated in silicon-oninsulator
the ability to set up a precise reflection interface becomes difficult due to the diffusion of carriers. This
diffusion of carriers across the reflection interface creates a refractive index gradient which is likely to cause the input
light to be imperfectly reflected into the output port, which is obviously less efficient than reflection from a precise
interface in terms of loss due to the absorption by the free carriers and the directivity of the reflected wave. In our work
we propose the use of a barrier positioned along the reflection interface, and around a completely enclosed injection
region to prevent diffusion of carriers, and therefore set up a precise reflection interface. The barrier will also improve
the injection efficiency since the carriers are being injected into a much smaller volume. This will, in turn, lead to a
reduced switching current and faster switching speeds. This paper reports the modeling of the device and predicts the
bandwidth performance for one specific switch design.
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.
The transfer function of a photonic filter is significantly influenced by the profile of the waveguides forming the device. In this work we discuss requirements for devices based on two geometries, rib and wire shaped waveguides in Silicon-on-Insulator, from both the modal and polarisation standpoints. General guidelines and recommendations for the design of single-mode and polarisation-independent ring resonator filters with large Free Spectral Range (>30nm) are given, together with supportive experimental results.
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 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.