Adaptive Focal Plane Arrays Research Articles

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 $\mu \text{m}\times 200~\mu \text{m}$ to 5 mm $\times$ 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 $\sim 94$ % 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-Perot filters based on these distributed Bragg reflectors. [2015-0161]

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-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> -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.

Mid-Infrared Tunable Resonant Cavity Enhanced Detectors.

Mid-infrared detectors that are sensitive only in a tunable narrow spectral band are presented. They are based on the Resonant Cavity Enhanced Detector (RCED) principle and employing a thin active region using IV-VI narrow gap semiconductor layers. A Fabry-Pérot cavity is formed by two mirrors. The active layer is grown onto one mirror, while the second mirror can be displaced. This changes the cavity length thus shifting the resonances where the detector is sensitive. Using electrostatically actuated MEMS micromirrors, a very compact tunable detector system has been fabricated. Mirror movements of more than 3 μm at 30V are obtained. With these mirrors, detectors with a wavelength tuning range of about 0.7 μm have been realized. Single detectors can be used in mid-infrared micro spectrometers, while a detector arrangement in an array makes it possible to realize Adaptive Focal Plane Arrays (AFPA).

Wireless focal planes 'On the road to amacronic sensors'

Progress in microoptic technology and concepts that blend epitaxial layers of analog circuitry, microoptics, microlaser arrays, and detector arrays with neural retinal designs are discussed. The sensor developments that can lead to tightly coupled and adaptive focal plane arrays are reviewed. Characteristic amacrine functions, such as motion detection, edge enhancement, and space-variant dynamic range compression, are outlined. It is shown that, with optically cross-linked detector arrays, competitive nonlinear center-surround designs that form the basis of biological amacrine functions can bridge the image processing chasm that previous technologies have failed to bridge.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>