Phillip A. Himmer

3-D MOEMS mirror for laser beam pointing and focus control

A biaxial torsion scan mirror with a deformable membrane surface is described. The 700-mm-diameter optical surface can be tilted /spl plusmn/4/spl deg/ about two orthogonal axes and can be deformed to a parabola with greater than 3-mm sag at the membrane center. The surface, therefore, acts as a lens with variable focal length ranging from /spl infin/ to 10 mm. Mirror architecture and applications in three-dimensional beam pointing and scanning are discussed.

Micromachined silicon nitride deformable mirrors for focus control

We have built a 1000mum-diameter silicon nitride deformable mirror for focus-control applications, using micro-optoelectromechanical systems technology. We achieved variable focal lengths from 36 to 360 mm while maintaining zero primary spherical aberration, using a maximum control voltage of 100 V. Active control of spherical aberration of approximately two waves at 660 nm was demonstrated.

Doppler optical coherence tomography with a micro-electro-mechanical membrane mirror for high-speed dynamic focus tracking

An elliptical microelectromechanical system (MEMS) membrane mirror is electrostatically actuated to dynamically adjust the optical beam focus and track the axial scanning of the coherence gate in a Doppler optical coherence tomography (DOCT) system at 8 kHz. The MEMS mirror is designed to maintain a constant numerical aperture of approximately 0.13 and a spot size of approximately 6.7 microm over an imaging depth of 1mm in water, which improves imaging performance in resolving microspheres in gel samples and Doppler shift estimation precision in a flow phantom. The mirrors small size (1.4 mm x 1 mm) will allow integration with endoscopic MEMS-DOCT for in vivo applications.

Silicon nitride biaxial pointing mirrors with stiffening ribs

Gold-coated silicon nitride mirrors designed for two orthogonal rotations were fabricated. The devices were patterned out of nitride using surface micromachining techniques, and then released by a sacrificial oxide etch and bulk etching the silicon substrate. Vertical nitride ribs were used to stiffen the members and reduce the curvature of the mirrored surfaces due to internal stress in the nitride and the metal layer. This was accomplished by initially etching the silicon substrate to form a mold that was filled with nitride to create a stiffening lattice-work to support the mirrored section. Mirror diameters ranging from 100 mm to 500 mm have been fabricated, with electrostatic actuation used to achieve over four degrees of tilt for each axis.

Off-axis variable focus and aberration control mirrors

Elliptical-boundary deformable mirrors have been developed for focus control of an optical beam incident at forty-five degrees with respect to the surface normal. The mirrors are silicon nitride membranes 1.4×1 mm in size, designed to accommodate a 1 mm diameter beam. Two electrostatic actuation zones provide control over spherical aberration. Focal lengths ranging from infinity to 36 mm have been achieved, and the mirror surface figure has been characterized to quantify aberration. Residual aberrations have been observed to be less than λ/5 (peak to peak) measured at λ = 660 nm.

Miniature high-resolution imaging system with 3D MOEMS beam scanning for Mars exploration

A compact confocal imaging instrument is described that makes use of a high-performance bi-axial Silicon torsion mirror, in concert with a reflective dynamic parabolic membrane mirror to provide 3D beam scanning. This beam scan engine is incorporated into a confocal imaging Raman spectrometer under development for exploration of Martian rocks and soil, designed to achieve optical resolution of 1 micrometers at (lambda) equals 850 nm, with a field of view of 300 micrometers and focus control of more than 200 micrometers . Fast x-y beam scanning is achieved with the bi-axial scanner, while the parabolic membrane provides both static and dynamic focus control for gross instrument focus as well as on-the fly field curvature correction or substrate contour tracing. In this paper we describe the MOEM elements as well as on-the-fly curvature correction or substrate contour tracing. In this paper we describe the MOEM elements as well as the overall instrument architecture. We also present initial imaging results using the torsion mirror scanner, and we describe the dynamic focus element fabrication, modeling and preliminary experimental characterization.

Spherical aberration correction using a silicon nitride deformable membrane mirror

A gold-coated silicon nitride membrane mirror 1.25 mm in diameter corrects up to 3 waves of spherical aberration at 633 nm. A 2times improvement in edge response is observed in an N.A. 0.55 optical system

Feedback-stabilized deformable membrane mirrors for focus control

This paper describes a method to extend the range of motion of a deformable, continuous membrane mirror beyond the limit of open-loop electrostatic instability through feedback control. The feedback scheme employs capacitive sensing directly at the mirror actuation electrodes and is based on frequency modulation of a coupled ring oscillator using a differential measurement technique. Analysis of the system shows that the range of stable deflection depends on the relative dynamics of the device and the feedback control circuitry. Experimental results demonstrate stable closed-loop deflection of our silicon nitride membrane test device to 69% of the air gap and confirm the dependence of the maximum stable displacement on overall loop dynamics.

Focus Tracking in Time Domain Optical Coherence Tomography using Membrane Mirrors Operated Near Snap-Down

We describe MEMS membrane mirrors for dynamic focus tracking in time-domain OCT. We demonstrate stable, distortion-compensated nearly sinusoidal motion at 10 kHz at an amplitude of 50% of the air gap

Pixel-by-pixel aberration correction for scanned-beam micro-optical instruments

Variable aberration compensation elements designed to correct the primary aberrations, and capable of sufficient speed for on-the-fly correction, can significantly extend the diffraction-limited field of view of scanned-beam instruments using practical microlens systems. In this paper we review the relevant aberration theory and discuss the requirements for compensation elements as well as appropriate architectures for correction of a scanned-beam instrument. We report correction of astigmatism and field curvature in an F/20 optical system using deformable polysilicon reflective membranes. Devices were successfully demonstrated that compensated more than 1.5 waves of defocus and more than 1 wave of astigmatism with less than 1/10 wave of spherical aberration, and with a bandwidth in excess of 20 kHz, which is suitable for high speed beam scanning applications such as video-rate imaging.