Michael K. Giles

Tracking speckle patterns with optical correlation

It has been shown that tracking small particle motion can be accomplished by tracking the speckle pattern is produces. This paper describes various methods of real-time tracking of speckle patterns obtained from ultrasonic flow imaging of blood and tissue motion using optical correlation. Results obtained from a gray scale joint transform correlator utilizing a twisted nematic liquid crystal spatial light modulator and from Sandia Labs acousto-optical correlator are presented. The experimental results demonstrate the feasibility of real-time tracking with accuracy comparable to that of template matching algorithms currently being implemented in digital hardware.

Closed-loop adaptive-optics system with a liquid-crystal television as a phase retarder.

We present a closed-loop adaptive-optics system that uses a liquid-crystal television (LCTV) as a phase retarder. The system consists of a LCTV inserted into one leg of a Mach-Zehnder interferometer so that one measures the wave-front function by analyzing the interferogram using a video CCD camera and a computer and corrects the wave-front distortion by placing the conjugate function on the LCTV. Experimental results are presented.

Implementation of ternary phase amplitude filters using a magnetooptic spatial light modulator

Ternary modulation can be achieved with a single SLM and improvedc orrelationd iscriminationc an bee xperimentally realized.

Reconfigurable Shack-Hartmann wavefront sensor

The conventional Shack-Hartmann wavefront sensor (SH-WFS) has a fixed subaperture area that is determined by consideration of several parameters such as the average atmospheric coherence diameter r 0 at the telescope site. Its SNR can be severely degraded due to low-light conditions caused by increasing turbulence, strong atmospheric scintillation, or simply viewing a faint object. Typically, the integration time of the sensor is increased to improve the SNR. Unfortunately, a decrease in bandwidth causes an increase in residual wavefront error that reduces image quality. We show that an increase in subaperture area produces a smaller residual wavefront error than an equivalent increase in integration time. Furthermore, we show that the ability to reconfigure the subaperture area, in combination with control bandwidth adjustment, provides superior performance over a system with fixed subaperture area when r 0 is different than the design point. We present a reconfigurable Shack-Hartmann wavefront sensor (RSH-WFS) with adjustable subaperture area implemented using a phase-modulated liquid crystal device (LCD). Experimental results demonstrate that the RSH-WFS increases system dynamic range by increasing the subaperture diameter whenever the Hartmann spot irradiance falls below the threshold of operation of the conventional SH-WFS.

Design considerations for miniature optical correlation systems that use pixelated input and filter transducers

Computer-controlled miniature optical correlators employing pixelated input and filter transducers can serve as either powerful plug-in modules for larger target-recognition systems, or as specialized, stand-alone portable correlators for specific image-processing and recognition applications. Attention is presently given to such design considerations for these devices as filter design, overall correlator length minimization, and the effect of aberrations on system performance. Two prototype miniature binary phase-only filter correlators employing magnetooptic spatial light modulators at the output and filter planes are presented.

Simple technique for measuring the phase property of a twisted nematic liquid crystal television

A simple technique to measure the phase retardation property as a function of the driving voltage of a liquid crystal television (LCTV) accurately using Young’s double-slit interference principle is presented. The phase property of an Epson LCTV is characterized by measuring the irradiance change at a fixed position at the center of the Young’s interference pattern. The phase retardation is also characterized by measuring the fringe shift as previous authors have done, and the measured results obtained with the two methods are consistent.

Implementation of an adaptive Shack-Hartmann sensor using a phase-modulated liquid crystal spatial light modulator

In this paper we demonstrate a closed-loop adaptive-optics system that uses a twisted nematic liquid-crystal television (LCTV) as an adaptive Shack-Hartmann wave front sensor (SHWS). This system writes a dynamic lenslet array onto the LCTV so that each subaperture generates a focal spot at the focal length of the lenslet array. The focal spots of the lenslet array are detected by a video CCD camera. The focal spots move around if turbulence exists in the system, therefore the locations of the distorted focal spots are computed using a centroid algorithm and used to correct the local tilted wave fronts. Using the centroid shift data from all of the subapertures of the lenslet array, the incident wave front can be calculated and the phase can be reconstructed. In this experiment we assume that the slope of the disturbed wave front in a subaperture is the spatially averaged wave front tilt, and a correcting LCTV uses a simplifying linear approximation to generate a compensating tilt at each subaperture.

Iterative technique for high-resolution phase distortion compensation in adaptive interferometers

High-resolution adaptive phase distortion suppression is ex- perimentally demonstrated for laser interferometers using a liquid crystal television as a phase modulator and an optoelectronic feedback loop. The experiments are carried out for both amplitude division (Mach- Zehnder) and rotational shear types of interferometers. The suggested iterative control algorithm is based solely on interference pattern infor- mation. The results of numerical simulations of a high-resolution adap- tive system based on rotational shear type interferometers show the sys- tems potential for atmospheric turbulence phase distortion suppression.

Setting up a liquid crystal phase screen to simulate atmospheric turbulence

Phase screens are often used to simulate atmospheric turbulence in systems designed to test adaptive optics techniques. This paper presents the design and implementation of a dynamic phase screen using a simple and inexpensive twisted nematic liquid crystal display taken from a video projector and placed in a pupil plane. The details of the optical system layout, the system alignment procedure, and the operating parameters of the liquid crystal display are discussed. Examples of turbulence (having strength and statistics similar to measured values of atmospheric turbulence in a variety of scenarios) are written to the phase screen, and the effects of the turbulence on image quality are measured and presented.

Adaptive optics system with micromachined mirror array and stochastic gradient descent controller

An adaptive laser beam focusing system comprising a 4 X 4 segment (5 X 5 actuator) MEMS deformable mirror developed at Boston University is presented. Mirror actuators were controlled by a circuit using VLSI chips implementing a stochastic parallel gradient descent algorithm. The system allowed for an enhancement of the iteration rate up to 5900 s-1, limited only by the used computer equipment. Results of experiments for characterization of the MEMS mirror response and the adaptation speed of the system are reported. A further improvement of adaptation speed was achieved by modification of the control algorithm implementing a self- optimization of one parameter.