Reconfigurable mask for adaptive coded aperture imaging (ACAI) based on an addressable MOEMS microshutter array

Abstract

Coded aperture imaging has been used for astronomical applications for several years. Typical implementations use a fixed mask pattern and are designed to operate in the X-Ray or gamma ray bands. More recent applications have emerged in the visible and infra red bands for low cost lens-less imaging systems. System studies have shown that considerable advantages in image resolution may accrue from the use of multiple different images of the same scenerequiring a reconfigurable mask.We report on work to develop a novel, reconfigurable mask based on micro-opto-electro-mechanical systems (MOEMS) technology employing interference effects to modulate incident light in the mid-IR band (3-5µm). This is achieved by tuning a large array of asymmetric Fabry-Perot cavities by applying an electrostatic force to adjust the gap between a moveable upper polysilicon mirror plate supported on suspensions and underlying fixed (electrode) layers on a silicon substrate.A key advantage of the modulator technology developed is that it is transmissive and high speed (e.g. 100kHz) -allowing simpler imaging system configurations. It is also realised using a modified standard polysilicon surface micromachining process (i.e. MUMPS-like) that is widely available and hence should have a low production cost in volume. We have developed designs capable of operating across the entire mid-IR band with peak transmissions approaching 100% and high contrast. By using a pixelated array of small mirrors, a large area device comprising individually addressable elements may be realised that allows reconfiguring of the whole mask at speeds in excess of video frame rates.

Challenges in the evolution of advanced imaging systems

“…A key enabler is the technology required for creating the adaptive mask and the degree of agility provided by that technology, not to say the increase in computational work-load required for the resulting imagery to be provided on a quasi-real time basis. In another paper at this conference, the group at QinetiQ [25] has explored the use of a transmissive MEMS-based shutter array, which has been configured for use in the midinfrared. For the visible, it would be possible to consider reflective spatial light modulators based on liquid crystal on silicon technology or the Texas Instruments digital light projection engine.…”

An investigation of the potential for the use of a high resolution adaptive coded aperture system in the mid-wave infrared

“…The target solid angle depends on the target size and target distance and the background solid angle depends on the camera geometry (aperture and mask to detector distance, in this case giving a field of view of approximately +/-12 degrees). The mask transmission is less than 50% to take into account the likely transmission through the mask apertures and the existence of opaque gaps between the individual apertures [3]. For a typical scene, there will be many sources of light spread across the field of view, resulting in a relatively low-contrast intensity pattern being recorded by the detector array.…”

“…Practical adaptive coded aperture imaging (ACAI) is made possible by recent advances in both optical component technologies and digital signal processing (DSP): the coding mask is reconfigurable (for example it could be a spatial light modulator) and/or the decoding algorithms and parameters used to recover the image are varied. A companion paper [3] discusses a novel micro-optical electro mechanical systems (MOEMS) etalon SLM for use as an ACAI component in the infrared. Section 4 discusses some decoding techniques of relevance to ACAI.…”

MEMS-based programmable reflective slit mask for multi-object spectroscopy

“…We decided to use the same packaging that was successfully tested in cryogenic environment [10]. The array was glued into a ceramic pin grid array (PGA) and electrical connections between the PGA and the chip were performed by wire-bonding.…”