Aligning of multiple micro-optical components is required for many systems composed of arrays of multiple lens elements, apertures, and filters. Methods of aligning two such wafers using mechanical features are discussed here. Alignment features include binary holes and posts, or grooves and ridges. With the circular holes or rectangular grooves etched into the two wafers, the mating pins or ridges are formed on both sides of a separate element to set both the lateral and vertical positioning. Grayscale technology allows for the printing of V-grooves and V-cones onto any substrate material over a wide range of aspect ratios. When integrated with cylindrical (fiber) or spherical (ball lens) mechanical features, this allows for accurate positioning. Some techniques allow for repositioning as well as disassembly and reassembly. The designs are kinematic or nearly kinematic. The paper discusses tolerances on mating components, and the associated precision of the overall alignment.
A process to diffractively structure GaAs for enhanced optical performance is described. The benefits of diffractively structuring an EOIR window material include improved FOR/FOV, consistent broadband performance, the ability to design and implement hyper-spectral characteristics directly into the substrate without incorporating a complex anti-reflective coating. Progress to date will be discussed including design evolution, process implementation, and optical characterization using the Automated Rasterable Integrated Spectrometer and TIS Measurement System (ARISTMS). Results will be presented on 100mm diameter samples.
In this paper, we propose a novel miniature MEMS based thermoacoustic cryo-cooler for thermal management of cryogenic electronic devices. The basic idea is to exploit a new way to realize a highly-reliable miniature cryo-cooler, which would allow integration of a cryogenic cooling system directly into a cryogenic electronic device. A vertical comb-drive is proposed as the means to provide an acoustic source through a driving plate to a resonant tube. By exciting a standing wave within the resonant tube, a temperature difference develops across the stack in the tube, thereby enabling heat exchange between two heat exchangers. The use of gray scale technology to fabricate tapered resonant tube provides a way to improve the efficiency of the cooling system, compared with a simple cylinder configuration. Furthermore, a tapered tube leads to extremely strong standing waves with relatively pure waveforms and reduces possible harmonics. The working principle of this device is described here. The fabrication of this device is considered, which is compatible with current MEMS fabrication technology. Finally, the theoretical analysis of key components of this cryo-cooler is presented.
A scanning two-axis tilt mirror has been modeled, fabricated and tested. The tilt mirror device is fabricated from single crystal silicon using bulk micromachining technology. The mirror is octagonal and is suspended by outer torsion hinges, a gimbal, and inner torsion hinges. Response to a driving voltage is investigated, along with frequency response. Finite element modeling was performed and the results compared with experimental data, with good agreement. Using automated and semi-automated placement equipment, linear arrays of the tilt mirrors have been produced.
Adaptive optics systems are used to maintain an optical system at its optimum performance through real time corrections of a wavefront. Deformable mirrors have traditionally been relatively large, expensive devices, suitable for systems such as large telescopes. The objective of the present work is to expand the range of systems that can employ adaptive optics by developing a small, low-cost MEMS deformable mirror. This deformable mirror uses a continuous membrane and has 61 actuators arranged in to approximate a circular pattern. Each actuator has an associated spring suspension, allowing it to push as well as pull on the membrane, producing locally convex or concave curvature. The folded springs are positioned so as to maximize the lateral stability. Maximum actuator displacement is six microns at less than 200 volts. The actuator resonant frequency, is greater than 10 kHz, allowing high-frequency updates of the mirror shape. To operate at high speed, the device must be sealed in a low-pressure environment. Each microactuator uses a vertical comb drive to achieve large travel at a reasonable voltage. The continuous membranes are made of silicon or silicon nitride. Both the actuator and membrane are fabricated with bulk micromachine process technologies. The design targets laser based communication specifications and medical imaging applications.
The performance of different MEMS mirrors from Boston University, MEMS Optical LLC, University of Colorado and OKO Technologies was studied in respect to an application in a model-free adaptive optics system. The frequency response characteristic was determined in a simple laser beam focusing set-up. Closed-loop adaptation experiments were performed using a VLSI controller system implementing a stochastic parallel gradient descent optimization algorithm. The system behavior using different MEMS mirror types, esp. adaptation speed, was compared.
Using two micro lens arrays and a MEMS micro shutter array, an intensity modulating Spatial Light Modulator is being developed at MEMS Optical, Inc. (patent pending) for high speed printing applications. The micro lens arrays are used to focus incident light to a point and then expand it back to its original size. At the focus point, a Foucault micro shutter array is used to modulate the amount of light allowed to pass through the aperture. The purpose for this device is for exposure control for high-speed electronic printing applications. The drive mechanism is based on an electrostatic lateral comb interdigitated drive. Design analysis shows a rise time of 1 - 2 microseconds for high voltage systems. This array of shutters is being implemented in a CMOS compatible process, and is capable of being integrated with on chip circuitry for opening and closing the shutters. The apertures are made using deep RIE etching, and the shutters are released using plasma etching. The result is an electronically controlled method of exposing a photosensitive surface at high speeds for the printing industry, with or without lasers.
An adaptive optics system is being developed, which uses integrated circuit technology along with diffractive optics to crete a very compact system. A lenslet array focuses incoming light onto individual actuators. Phase modulation is applied with electrostatic attraction. Gratings on the mirrors split off a part of the light for wavefront sampling. Optics on the back side of the lenslet array combine neighboring beams and focus onto detector elements. This creates a shearing measurement in two orthogonal directions. A resistive grid network reconstructs the wavefront from the individual measurements, and a feedback system nulls the outgoing wave. This paper contains simulations and analysis of the system. A 1D array was simulated, including the wavefront measurement and correction. A sine wave was input to the system, and the resulting phase and point spread function were calculated. System analysis of the wavefront reconstruction and feedback are discussed. Test results for a non-shearing interferometer are presented. Some test results from a test chip are also provided.
This paper discusses the application of MOEM technology to adaptive optics. An experiment is described in which a micromachined mirror array is used in a closed loop adaptive optic demonstration. An interferometer wavefront sensor is used for wavefront sensing. Parallel analog electronics are used for the wavefront reconstruction. Parallel operational amplifiers are used to drive the micromirrors. The actuators utilize a novel silicon design developed by SY Technology, Inc. The actuators have a measured frequency response of 15kHz, and a maximum usable stroke of 4 microns. The entire adaptive optic demonstration has a bandwidth exceeding 10kHz. Measured performance is described. The experiments conducted are designed to explore the feasibility of creating a single chip adaptive optic system, also described in this paper. This chip would combine all on a single VLSI chip aspects of a complete adaptive optics system, wavefront sensing, wavefront reconstruction, and wavefront correction. The wavefront sensing would be accomplished with a novel compact shearing interferometer design. The analog refractive and diffractive micro optics will be fabricated using a new single step analog mask technology. The reconstruction circuit would use an analog resistive grid solver. The resistive grid would be fabricated in polysilicon. The drive circuits and micromirror actuators would use standard CMOS silicon fabrication methods.
A high-frequency crossed-beam correlation (CBC) experiment was performed to determine the mean-squared fluctuating density, convection speed, and characteristic turbulent coherence length of a supersonic turbulent mixing layer. Aero-optical conditions were representative of actual flight. Orthogonal helium-neon laser beams intersected to interrogate a 100 micrometer -- diameter volume. Beam motion was sensed by two quadrant detectors, whose output signals were recorded after being digitally sampled at a 5 MHz rate. Cross-correlation of angular beam deviations was computed, and from this, the mean squared fluctuating density was determined. By offsetting the beams in the streamwise direction, convection speeds were determined, enabling turbulent cell sizes to be estimated. RMS densities reached approximately 15% of the local mean density in the mixing layer, and correlation length estimates ranged from 1.5 to 2 mm. Fluctuating densities were lower, and correlation lengths were higher than predicted by a simple model. This paper summarizes experimental design and procedures, and provides a theoretical treatment of the results.