K. K. M. B. Dilusha Silva

Bifocal optical coherenc refractometry of turbid media

We propose and demonstrate a novel technique, which we term bifocal optical coherence refractometry, for the rapid determination of the refractive index of a turbid medium. The technique is based on the simultaneous creation of two closely spaced confocal gates in a sample. The optical path-length difference between the gates is measured by means of low-coherence interferometry and used to determine the refractive index. We present experimental results for the refractive indices of milk solutions and of human skin in vivo. As the axial scan rate determines the acquisition time, which is potentially of the order of tens of milliseconds, the technique has potential for in vivo refractive-index measurements of turbid biological media under dynamic conditions.

Scintillation index of the free space optical channel: Phase screen modelling and experimental results

Scintillation index (SI) is a key metric for free space optical communications (FSOC), and measures the normalised intensity variance caused by atmospheric turbulence. It is a function of the refractive index structure parameter Cn2, range, and receiver aperture. There is a need for an atmospheric simulation model of the effects of scintillation because testing of FSOC performance in the environment is difficult and time consuming. In this paper we compare experimental results with numerical simulations using phase screens for channels involving three receivers of different size apertures. There is good agreement in the results of experiment and model.

MEMS-Based Tunable Fabry–Perot Filters for Adaptive Multispectral Thermal Imaging

This paper reports on a proof-of-concept microelectromechanical system-based Fabry-Perot filter that is capable of electrically tuning within the long-wave infrared thermal imaging band of 8-12 μm. The device employs a single-layer quarter-wavelength thick tensile germanium membrane for the suspended top mirror in order to achieve nanometer-scale as-released mirror flatness across an area of several hundred square micrometers without any extraneous stress management techniques. Mechanical and optical characterization of the tunable filters of various sizes are presented and compared. A 200-μm dimension square filter is demonstrated with <;100-nm top mirror bowing and near-theoretical spectral characteristics across the entire tuning range of 8.5-11.5 μm, namely, peak transmission above 80%, full-width at half-maximum of spectral passband of approximately 500 nm, and out-of-band rejection greater than 40:1. Optical modeling shows that this filter can achieve a pixel-to-pixel transmission peak wavelength variation of less than 1.2% across the entire 200 μm × 200-μm optical imaging area. These results exceed the optical performance requirements for passive multispectral thermal imaging applications based on large-area focal plane arrays. In comparison, the 500 and 1000-μm dimension filters are shown to exhibit significant mirror bowing with actuation and, thus, for a pixel-to-pixel transmission peak wavelength non-uniformity of <; 4%, demonstrate narrower usable spectral tuning ranges of 9.3-11.4 and 10.3-11.3 μm, respectively.

A versatile instrumentation system for MEMS-based device optical characterization

Future improvements in spectral imaging systems can be attained through the integration of MEMS-based optical transmission devices matched with pixelated arrays. Such integrated module designs will require a detailed knowledge of the MEMS device optical properties at high spatial resolution and over a wide range of operating conditions. A substantially automated low-cost optical characterization system has been developed, which enables the optical transmission of the MEMS device be measured with high spatial and spectral precision. This Optical Metrology System (OMS) can focus light on the device under test (DUT) to a spot diameter of less than 30 μm, and characterize devices at near infrared for wavelengths within the spectral band from 1.4 μm to 2.6 μm. A future upgrade to the OMS will enable measurements to be carried out across a wide range of DUT temperatures and with a spectral range from visible to long wave infrared wavelengths.

Long-term stability of ICPCVD a-Si under prolonged heat treatment

Inductively coupled plasma enhanced chemical vapor deposited (ICPCVD) a-Si is used as a structural material in many microelectromechanical systems (MEMS). For a-Si to function as a sound structural component, the material must display long term mechanical stability. This paper evaluates the Youngs modulus, hardness, and residual stress of a-Si under prolonged heat treatment. It is found that Youngs modulus and hardness are not impacted by heat treatment, while the residual stress becomes more tensile with increased annealing time. Increased tensile stress is a result of hydrogen offgassing which can lead to improved film stability [1].

Large-Area MEMS-Based Distributed Bragg Reflectors for Short-Wave and Mid-Wave Infrared Hyperspectral Imaging Applications

We present the design, fabrication, and optical characterization of silicon-air-silicon-based distributed Bragg reflectors, or quarter wavelength mirrors, in sizes ranging from 200 μm × 200 μm to 5 mm × 5 mm. Such mirrors can be used in conjunction with either single-element photodetectors or large-area focal plane arrays to realize tunable multispectral sensors or adaptive focal plane arrays from the short-wave infrared wavelength ranges (1500-3000 nm) to mid-wave infrared wavelength (3000-6000 nm) ranges. Surface optical profile measurements indicate a flatness of the order of 20-30 nm in the fabricated structures across several millimetres. Single point spectral measurements on devices show excellent agreement with simulated optical models. The fabricated distributed Bragg reflectors show ~94% reflectivity, which is in close agreement with theoretical reflectivity. The demonstrated high reflectivity across a wide wavelength range renders them suitable as broadband reflectors. Finally, we present optical transmittance modeling results for Fabry-Pérot filters based on these distributed Bragg reflectors.

Suspended Large-Area MEMS-Based Optical Filters for Multispectral Shortwave Infrared Imaging Applications

We present the design, fabrication, and optical and mechanical characterization of silicon-/silicon-oxide-based optical filters and distributed Bragg reflectors in sizes ranging from 500 μm × 500 μm to 5 mm × 5 mm. They are designed to be used in conjunction with either single-element photodetectors or large-area focal plane arrays to realize tunable multispectral sensors or adaptive focal plane arrays in the shortwave infrared wavelength range. Surface optical profile measurements indicate a flatness of the order of 30 nm in the fabricated structures across several millimeters. Single-point spectral measurements on devices show an excellent agreement with simulated optical models, and demonstrate Si-SiOx-Si fixed optical filters with a 94% transmission at 1940 nm with a full-width at half-maximum of 250 nm. Distributed Bragg reflectors demonstrate 90% reflectance across the 1560-2050-nm wavelength range, making them suitable as broadband reflectors. The optical spatial uniformity across a 3-mm × 3-mm device shows only a 3% variation across the entire optically active area. Finally, the mechanical resonance characteristic of a 1-mm x 1-mm fabricated device shows the lowest resonant frequency of the suspended structure to be 39 kHz, indicating excellent immunity to extraneous low-frequency vibrations.

Ge/ZnS-Based Micromachined Fabry–Perot Filters for Optical MEMS in the Longwave Infrared

This paper reports on the successful demonstration of Ge/ZnS-based Fabry-Perot filters operating in the longwave infrared (LWIR). The suitability of thermally deposited Ge and ZnS as thin-film mirror materials for micromachined LWIR Fabry-Perot filters has been fully investigated, and it is shown that a film growth temperature higher than 150 °C is key to depositing durable ZnS films. The optical constants of Ge and ZnS films in the LWIR band reveal that the material pair possesses high refractive index contrast and excellent LWIR transparency. Fixed-cavity LWIR Fabry-Perot filters with a 150-μm circular single-layer Ge top mirror and a four-layer Ge/ZnS/Ge/ZnS bottom mirror were fabricated. Curvature in the suspended top mirror was corrected using a thin SiNx stress-compensation layer. After curvature correction, a mirror flatness of 550 nm was achieved, and the filter demonstrated a 60% peak transmission with a full-width at half-maximum of 700 nm as well as a out-of-band rejection of 24:1.

Silicon-Air-Silicon Distributed Bragg Reflectors for Visible and Near Infrared Optical MEMS

This letter presents the design, fabrication, and optical characterization of silicon-air-silicon-based surface micro-machined distributed Bragg reflectors (DBRs) for the visible to near infrared wavelength range (540-960 nm). The DBR (mirror) consisted of two quarter wave thick silicon films separated by a quarter-wave air gap. A mirror array was successfully fabricated, consisting of mirrors ranging in diameter between 270 and 420 μm. Calibrated optical measurements indicate that a peak reflectivity close to 92% has been achieved for visible wavelengths, despite the fact that silicon has strong absorbtion in the visible wavelength range. The mirrors are shown to be broadband reflectors, having 85% or more reflectivity over a 160-nm wavelength range. A spatially resolved optical transmission mapping and optical transmission profile of the mirrors demonstrates high uniformity across the fabricated array of DBRs.

Optimization of ICPCVD Amorphous Silicon for Optical MEMS Applications

In this paper, we present the optimization of optical and mechanical properties of inductively coupled plasma chemical vapor deposited (ICPCVD) amorphous silicon thin films for fabrication of high-quality optical microelectromechanical systems-based devices operating from visible to short-wave infrared wavelengths (450-3000 nm). Our results indicate that, at relatively high deposition temperatures for plasma CVD, a decrease in the ICP power results in films with lower tensile stress, higher refractive index, and lower extinction coefficient. We show that hydrogen concentration alone is not a sufficient parameter for controlling optical and mechanical quality of the films. In particular, both the hydrogen concentration and the hydrogen-silicon bonding nature together play a vital role in determining the optical and the mechanical quality of the silicon thin films. As a demonstration vehicle, three layer silicon-silicon oxide-silicon-based distributed Bragg reflectors were fabricated for the visible (500-700 nm), near infrared (700-1000 nm), and short-wave infrared (2000-3000 nm) wavelength ranges using an optimized silicon fabrication recipe. The measured optical transmission spectra show close to 90% peak reflectivity. Finally, stress optimization was evaluated by fabricating 270-μm diameter circular suspended silicon membranes, which demonstrate a flatness variation on the order of <;6 nm across the entire lateral dimension.