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.
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 the use of a feedback control scheme. The feedback scheme is based
on capacitive sensing of the mirror. The sensing is achieved by coupling the actuation electrodes to a ring oscillator.
We use a differential technique, where the frequency of the coupled oscillator is compared to a reference ring oscillator.
Analysis of the system using a simplified parallel-plate model shows that the range of stable deflection depends on the
dynamics of the device and control circuitry, and suggests that stable full-gap displacement can be achieved under
certain conditions. Experimental results are provided, showing stable closed-loop deflection of our silicon nitride test
device to 61% of the air gap, consistent with the predictions of our model.
This paper presents experimental data illustrating the dynamic behavior of micromachined deformable mirrors operated as cyclical focus control elements. The mirrors used for this study are circular silicon nitride membranes 1 mm in diameter with a fundamental resonance at 144 kHz and a quality factor in air of 0.86. Electrostatic actuation of the mirrors is provided by two concentric electrodes to control focus and spherical aberration. Mirror displacement has been characterized in terms of frequency response and dynamic surface figure. Surface shape was examined for variation with frequency over a range from 1 kHz to 180 kHz for small mirror deflection. Intra-cycle variation of both quartic and quadratic surface curvature terms was also measured at 60 kHz. Surface figure data are presented showing less than λ/10 spherical aberration for a mirror operating at 10 kHz with a focal length varying cyclically from ∞ to 32 mm.
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.
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.
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.
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.
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