Publications

We have developed a two-dimensional Shack-Hartman wavefront sensor that uses binary optic lenslet arrays to directly measure the wavefront slope (phase gradient) and amplitude of the laser beam. This sensor uses an array of lenslets that dissects the beam into a number of samples. The focal spot location of each of these lenslets (measured by a CCD camera) is related to the incoming wavefront slope over the lenslet. By integrating these measurements over the laser aperture, the wavefront or phase distribution can be determined. Since the power focused by each lenslet is also easily determined, this allows a complete measurement of the intensity and phase distribution of the laser beam. Furthermore, all the information is obtained in a single measurement. Knowing the complete scalar field of the beam allows the detailed prediction of the actual beam`s characteristics along its propagation path. In particular, the space- beamwidth product M{sup 2}, can be obtained in a single measurement. The intensity and phase information can be used in concert with information about other elements in the optical train to predict the beam size, shape, phase and other characteristics anywhere in the optical train. We present preliminary measurements of an Ar{sup +} laser beam and associated M{sup 2} calculations.

Optical diagnostics are extremely useful in fluid mechanics because they generally have high inherent bandwidth, and are non-intrusive. However, since optical probe measurements inherently integrate all information along the optical path, it is often difficult to isolate out-of-plane components in 3-dimensional flow events. It is also hard to make independent measurements of internal flow structure. Using an arrangement of one-dimensional wavefront sensors, we have developed a system that uses tomographic reconstruction to make two-dimensional measurements in an arbitrary flow. These measurements provide complete information in a plane normal to the flow. We have applied this system to the subsonic free jet because of the wide range of flow scales available. These measurements rely on the development of a series of one-dimensional wavefront sensors that are used to measure line-integral density variations in the flow of interest. These sensors have been constructed using linear CCD cameras and binary optics lenslet arrays. In designing these arrays, we have considered the coherent coupling between adjacent lenses and have made comparisons between theory and experimental noise measurements. The paper will present examples of the wavefront sensor development, line-integral measurements as a function of various experimental parameters, and sample tomographic reconstructions.

The advent of micro- or binary optics technology has made possible the fabrication of a variety of new optical devices. Optical fabrication is no longer limited by surfaces that can be made by grinding and polishing, or even diamond turning. In fact, optics with no symmetry, no smooth surfaces, and that perform multiple functions can be readily fabricated. While these optics have a large number of applications, they are extremely useful for systems that require arrays of small optics or aperture multiplexing, since these are fabricated using computer controlled photo-lithography and etching processes. We have applied binary optics technology to construct various wavefront sensing using four mask processes to create 16 level optics. They are binary in the sense that they use discrete phase levels, not in the sense of using only two levels (they might more properly be called digital optics). We have found that 16 levels is adequate for most systems, giving greater than 99% of efficiency.

We have measured the atmospheric isoplanatic angle in the near infrared by correlating widely separated time-resolved stellar images.