Rodney L. Clark

Wavelength dependence of blur-circle size through turbulent flow

Aerodynamic flow surrounding a missile or aircraft in flight can degrade the performance of on-board optical sensor systems. The minimum resolvable spot or blur circle is a measure of optical system performance. The blur circle size may change by orders of magnitude throughout the course of the vehicle flight trajectory due to aerodynamic perturbations. This paper examines the wavelength dependence of blur circle size. It is shown that in many cases an optimum wavelength exists at which the blur circle size is minimized. Expressions are given for depicting the Point Spread Function shape and wavelength at which blur is minimized. The optimization expressions presented are suitable for use on a desk top computer or calculator.

Micro-opto-electro-mechanical (MOEM) adaptive optic system

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.

Aero-optic performance of supersonic mixing layers

Advances made in the field of aero-optical system modeling are applied in the technology of super- or hypersonic vehicles carrying optical seekers. The fundamentals discussed form a basis for performance predictions of these airborne optical systems. The focus is the inherent image degradation due to aerodynamic mixing layers. This paper examines the aero-optic properties of supersonic mixing layers. Recent experimental results on the aerophysics of supersonic mixing layers is combined with statistical aero-optics theory to create empirical equations governing the image degradation resulting from light propagating through a classical mixing layer. This model results in simple expressions for blur circle size, Strehl loss, jitter and boresight error. Expressions for the turbulent Modulation Transfer Function (MTF) and the Point Spread Function (PSF), assuming a diffraction-limited optical system, can also be determined.

Wave-optic model to determine image quality through supersonic boundary and mixing layers

Technological advancements in the field of mixing layer theory have allowed the design and subsequent construction of a Table Top Simulator of Aero-Optic Effects. This experimental facility simulates the supersonic boundary and mixing layers formed by the window coolant gas of an optically guided hypersonic vehicle. This paper discusses the foundations of wave-optic theory applied to model the propagation of optical radiation through such flow. The focus of the calculations is to determine performance quality parameters such as Strehl ratio, jitter, 50 percent contained energy diameter and boresight error. These quality measures will drive the performance requirements of the optical system and focal plane array of the seeker. Comparisons are made between wave-optic model results and actual aero-optic data collected from the Table Top experiment.

MEMS microshutter SLM for intensity modulation

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.

High-speed crossed-beam correlation with a Mach 1.5 turbulent flowfield

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.

Simulation of optical diagnostics for crystal growth: models and results

A computer simulation of a two-color holographic interferometric (TCHI) optical system was performed using a physical (wave) optics model. This model accurately simulates propagation through time-varying, 2-D or 3-D concentration and temperature fields as a wave phenomenon. The model calculates wavefront deformations that can be used to generate fringe patterns. This simulation modeled a proposed TriGlycine sulphate TGS flight experiment by propagating through the simplified onion-like refractive index distribution of the growing crystal and calculating the recorded wavefront deformation. The phase of this wavefront was used to generate sample interferograms that map index of refraction variation. Two such fringe patterns, generated at different wavelengths, were used to extract the original temperature and concentration field characteristics within the growth chamber. This proves feasibility for this TCHI crystal growth diagnostic technique. This simulation provides feedback to the experimental design process.

Evanescent coupling alternatives for YAG lasers

Diffraction gratings are proposed as an alternative technique to couple a laser diode pump beam into the YAG crystal of a solid-state laser. These binary diffraction gratings are on the long axis of the crystal and are etched in high refractive index coating material. The paper reveals the set of grating parameters and tolerances for transforming vertically incident light into horizontally propagating light inside the crystal with theoretical efficiencies of more than 90%. Under optimal conditions the diffraction grating behaves like an excellent leaky waveguide structure. Theoretical comparisons are made between the efficiencies of gratings directly etched into the laser crystal and gratings etched in to a high index coating material. The resulting zig-zag pumped laser cavity is uniformly excited in order to minimize thermal loads and lensing effects. The maskless binary sub-micron pattern transfer is realized by combining interferometry and lithography.

Three applications of pulse-coupled neural networks and an optoelectronic hardware implementation

Pulse Coupled Neural Networks have been extended and modified to suit image segmentation applications. Previous research demonstrated the ability of a PCNN to ignore noisy variations in intensity and small spatial discontinuities in images that prove beneficial to image segmentation and image smoothing. This paper describes four research and development projects that relate to PCNN segmentation - three different signal processing applications and a CMOS integrated circuit implementation. The software for the diagnosis of Pulmonary Embolism from VQ lung scans uses PCNN in single burst mode for segmenting perfusion and ventilation images. The second project is attempting to detect ischemia by comparing 3D SPECT images of the heart obtained during stress and rest conditions, respectively. The third application is a space science project which deals with the study of global aurora images obtained from UV Imager. The paper also describes the hardware implementation of PCNN algorithm as an electro-optical chip.

Three applications of pulse-coupled neural networks

Image segmentation is one of the major application areas for Pulsed Coupled Neural Networks (PCNN). Previous research has shown that the ability of PCNN to ignore minor variations in intensity and small spatial discontinuities in images is beneficial to image segmentation as well as image smoothing. This paper describes research and development projects in progress in which PCNN is used for the segmentation of three different types of digital images. The software for the diagnosis of Pulmonary Embolism from VQ lung scans uses PCNN in single burst mode for segmenting perfusion and ventilation images. The second project is attempting to detect ischemia by comparing 3D SPECT (Single Photon Emission Computed Tomography) images of heart obtained during stress and rest conditions, respectively. The third application is a space science project which deals with the study of global auroral images obtained from Ultraviolet Imager. The paper also describes an hardware implementation of PCNN as an electro-optical chip.