Protecting optical sensors and detectors against excessive input signals is not only important for dealing with unintentional overloads, but also central for protecting sensors against intentional high-intensity sources such as flares or laser beams directed as countermeasures against the sensor system. In prior work, a microelectromechanical system (MEMS)-based, all optically driven deformable mirror was designed and fabricated. In this work, an optical limiter using this optically addressed, deformable mirror is proposed and designed.

The proposed architecture for the MEMSBased Optical Limiter. A plane wave is incident on the membrane of a back-illuminated device, and the deflected membrane acts as a parabolic mirror and focuses the light up to its diffraction limit. The shortest focal length occurs at the saturated deflection. If the light at the saturation focal point is aperturized by a pinhole that has a diameter equivalent to the diffraction limit, then the light intensity that is transmitted through the pinhole saturates as a function of back-illumination intensity.
The architecture of the mirror device consists of a pixelated, metalized membrane mirror suspended over an optically addressed, photoconductive substrate such as GaAs or InP. A grid of insulating material, such as photoresist, is used to support the suspended membrane, while a transparent electrode (ZnO) is placed on the back side of the substrate. A bias is applied between the metalized membrane and the transparent electrode. Illuminating the device from the back side changes the photoconductivity across the substrate and, consequently, the voltage drop across the membrane and substrate. This leads to membrane deflection. The membrane deflects in a parabolic form with the deflection being proportional to the square of the applied field.

Several operating mechanisms of this device have been described before and include: DC bias, DC bias accompanied by AC light modulation, and a combination of AC and DC biases. In the DC bias operating mechanism, the device de flects in a binary fashion. In the AC light modulation with the DC bias mode, it is possible to control the impedance only in a non-steady-state situation. This suggests that this operating mechanism is suitable for moving targets or similar situations where only response to transient events is desired. In very-high-frequency AC light modulation, it is possible to control the impedance between the membrane and substrate under both transient and steady-state conditions, while a combination of AC and DC provides further control of the heterojunction capacitance between the transparent ZnO electrode and substrate. For the last three operating mechanisms it was found, both experimentally and theoretically, that the membrane deflection saturates as a function of the back illumination intensity. Both the saturable deflection of the membrane and the parabolic form of this deformation are used to create a MEMS optical limiter, focused in particular on using the very-high-frequency AC bias operating mode.

This work was done by Jed Khoury and Charles L. Woods of the Air Force Research Laboratory; Bahareh Haji-saeed and John Kierstead of Solid State Scientific Corp; and William D. Goodhue of the University of Massachusetts. AFRL-0128

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MEMS-Based Optical Limiter

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This article first appeared in the August, 2009 issue of Defense Tech Briefs Magazine.

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