Compact High-Speed Thin Membrane Deformable Mirror
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Solution Overview
Problem
Existing adaptive optics systems are hindered by large and heavy deformable mirrors due to large actuator spacing, which limits their compactness and efficiency in correcting wavefront aberrations, especially in airborne applications, and current membrane DMs suffer from high residual stress, limited actuator pitch, and sensitivity to environmental factors.
Innovation Solution
A compact, high-speed deformable mirror with a thin membrane and a proprietary transparent conductor design that reduces residual stress and actuator spacing, allowing for low-voltage operation and high spatial frequency response, while being hermetically sealed to minimize environmental interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional bulk micromachined membrane DMs are used with DC bias voltage to achieve static parabolic shape, then the membrane can be deformed to correct wavefront aberrations, but the device size and weight become large due to required actuator spacing of 2.0 mm or more
Solution Approach 1:
The patent changes the critical parameter of actuator spacing from conventional 2.0 mm or more to less than 1.0 mm (e.g., 0.5 mm). This parameter change enables achieving the same wavefront correction capability with significantly reduced device size and weight, as the optical bench size varies linearly with beam size which depends on actuator number and spacing
Solution Approach 2:
The patent uses a thin membrane (e.g., 2.0 microns thick silicon nitride) as the deformable element. This thin film structure allows for reduced actuator spacing while maintaining membrane integrity and deformability, enabling compact design with actuator spacing less than 1.0 mm compared to conventional bulk micromachined membranes
2Reliability
If conventional membrane DMs operate at control voltages of 200-300 V to achieve phase throw of ~4 microns, then wavefront correction is achieved, but the gap between membrane and metal conductors must be large (40-100 microns) which limits compactness
Solution Approach 1:
The patent changes the operating voltage parameter from conventional 200-300 V to lower voltages (e.g., 50 V or less for low spatial frequency operation). This voltage reduction allows for smaller gap distances between membrane and conductors, achieving compact design with gaps of 10-30 microns while maintaining required phase throw capability through increased actuator density
Solution Approach 2:
The patent applies a DC bias voltage to the membrane to pre-establish tensile stress that pulls the membrane into a flat shape. This preliminary action allows the membrane to start from a flat configuration and be deformed by smaller AC voltage signals applied to actuators, enabling reduced gap distances and lower operating voltages while maintaining correction capability
3Ease of operation
If conventional membrane DMs use silicon nitride membrane with DC bias, then the membrane can be deformed, but dielectric relaxation occurs resulting in short-term drift in deflection vs. voltage response
Solution Approach 1:
The patent changes the material composition parameter by using alternative membrane materials or composite structures that do not exhibit dielectric relaxation under DC bias. This material change maintains membrane deformability while eliminating the short-term drift in deflection vs. voltage response, improving long-term operational stability
Solution Approach 2:
The patent applies DC bias voltage to pre-stress the membrane into a flat configuration, establishing a stable baseline state. This preliminary action separates the DC bias function (shape control) from the AC actuation function (wavefront correction), allowing the membrane to respond linearly and stably to actuator voltages without dielectric relaxation drift
4Shape
If conventional membrane DMs are designed to allow parabolic shape formation with gap of 40-100 microns, then the membrane can achieve static shape, but the actuator spacing is limited to about 2 mm and coupling between actuators is significant
Solution Approach 1:
The patent changes the gap distance parameter from conventional 40-100 microns to smaller values (10-30 microns). This parameter change allows for reduced actuator spacing (less than 1.0 mm) while maintaining sufficient membrane deformability and reducing actuator coupling effects, as the membrane can achieve required shapes with smaller gaps and higher actuator density
Solution Approach 2:
The patent uses DC bias to pre-establish the membrane in a flat or near-flat configuration rather than requiring large gaps for parabolic shape formation. This preliminary shaping action allows the membrane to be deformed by smaller local actuator displacements, enabling compact design with reduced actuator spacing and minimal coupling
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution enables high optical phase throw at low voltage, reduced size and weight, and improved high-order Zernike profile accuracy, achieving kHz frequency operation and enhanced dynamic range for wavefront correction.
Implementation Method 1
Electrostatic devices exhibit a quadratic dependence of electrostatic pressure P on the voltage V and gap d, according to the equation: P=∈o(V/d)2
Implementation Method 2
a bias voltage is applied to the transparent conductor and actuator voltages are applied to the plurality of electrostatic actuators; the deformation of the membrane is induced by voltage differentials between the bias voltage applied to the transparent conductor and the actuator voltages applied to the plurality of electrostatic actuators
Data Source
AI summary
Provided is a compact, high-speed deformable mirror for use with an adaptive optic. The mirror or wavefront correction device corrects and/or compensates for wavefront aberrations present in a wavefront received by the optics. The mirror includes a deformable membrane which may be made of a semiconductive, metallic or insulating material. Positioned in close proximity to a front surface of the membrane is a transparent conductor, which may be covered by a window having an anti-reflective coating. A plurality of electrostatic actuators is located in close proximity to a back surface of the membrane, the conductor and actuators separated by a gap of approximately 10 μm. In operation, a bias voltage is applied to the transparent conductor and an actuator voltage is applied to the plurality of actuators. The resultant voltage differential across the membrane defines the amount of membrane deformation, which in turn compensates for distortions in a subsequently reflected wavefront.