Harmonic diffractive lenses.

The harmonic diffractive lens is a diffractive imaging lens for which the optical path-length transition between adjacent facets is an integer multiple m of the design wavelength λ(o). The total lens thickness in air is mλ(o)/(n - 1), which is m times thicker than the so-called modulo 2π diffractive lens. Lenses constructed in this way have hybrid properties of both refractive and diffractive lenses. Such a lens will have a diffraction-limited, common focus for a number of discrete wavelengths across the visible spectrum. A 34.75-diopter, 6-mm-diameter lens is diamond turned in aluminum and replicated in optical materials. The sag of the lens is 23 µm. Modulation transfer function measurements in both monochromatic and white light verify the performance of the lens. The lens approaches the diffraction limit for 10 discrete wavelengths across the visible spectrum.

Optical flow computation in combustion image sequences.

A time-resolved sequence of laser shadowgraph images is computer processed to produce 2-D optical flow maps showing the motion of the combustion interface between hot products and cold reactants. Background-subtracted shadowgraph images are reduced to skeletons, followed by local correlation between adjacent frames for computing optical flow. The resulting flow maps represent an initial step toward the goal of computer-assisted flowfield analysis and interpretation.

Iterative technique for the synthesis of distortion-invariant optical correlation filters.

A procedure is presented for designing distortion-invariant correlation filters. Optical correlation filters designed using this technique retain full position invariance. The filter design begins by finding the distortion-invariant modes (eigenfunctions) for a particular image. The input image, filter, and correlation response are all spectrally expanded in terms of these orthogonal eigenfunctions. An iterative technique between the spatial domain and the spectral domain is used to rephase the invariant modes so that a filter composed of a linear combination of modes has the proper overall invariance. The iterative technique also controls the information content of the filter by maintaining specified amplitudes for each invariant mode in the filter. Targets are detected by spanning the filter to determine points of constant amplitude.

Optical median filtering using threshold decomposition.

A hybrid optical/electronic system performs median filtering and related ranked-order operations using threshold decomposition to encode the image. Threshold decomposition transforms the nonlinear neighborhood ranking operation into a linear space-invariant filtering step followed by a point-to-point threshold comparison step. Spatial multiplexing allows parallel processing of all the threshold components as well as recombination by a second linear space-invariant filtering step. An incoherent optical correlation system performs the linear filtering, using a magnetooptic spatial light modulator as the input device and a computergenerated hologram in the filter plane. Thresholding is done electronically. By adjusting the value of the threshold, the same architecture is used to perform median, minimum, and maximum filtering of images.

Synthesis of digital holograms by direct binary search.

A new approach to the design of computer-generated holograms makes optimal use of the available device resolution. An iterative search algorithm minimizes an error criterion by directly manipulating the binary hologram and observing the effect on the desired reconstruction. Several measures of error and efficiency useful in assessing the optimality of digital holograms are defined. Methods for designing digital holograms that are based on projections and error diffusion are presented as established techniques for comparison to direct binary search.

Experimental use of iteratively designed rotation invariant correlation filters.

Iteratively designed filters are incorporated into an optical correlator for position, rotation, and intensity invariant recognition of target images. The filters exhibit excellent discrimination because they are designed to contain full information about the target image. Numerical simulations and experiments demonstrate detection of targets that are corrupted with random noise (SNR approximately 05) and also partially obscured by other objects. The complex valued filters are encoded in a computer generated hologram and fabricated directly using an electron-beam system. Experimental results using a liquid crystal spatial light modulator for realtime input show excellent agreement with analytical and numerical computations.

Single-pulse, laser-saturated fluorescence measurements of OH in turbulent nonpremixed flames.

A single-pulse, laser-saturated fluorescence technique has been developed for absolute OH concentration measurements with a temporal resolution of 2 nsec, a spatial resolution of <0.1 mm(3), and an estimated accuracy of +/-30%. It has been applied in laminar, transitional, and turbulent hydrogen-air diffusion flames, providing the first reported quantitative measurements of average values, rms fluctuations, and probability density functions of OH radical concentration in nonpremixed flames.

Two-photon-excited fluorescence measurement of hydrogen atoms in flames.

We report the first two-photon-excited hydrogen-atom fluorescence measurements in flames made to our knowledge. The n = 3 level of the H atom was excited by 205.1-nm radiation generated by Raman shifting a 224-nm beam produced by frequency mixing. Fluorescence was observed at 656.3 nm as a result of radiative decay from n = 3 to n = 2, the Balmer-alpha transition. A novel technique, photoionization-controlled loss spectroscopy, is proposed to eliminate the quenching dependence of the fluorescence signal.

Temperature measurement by two-line laser-saturated OH fluorescence in flames.

A technique is proposed and demonstrated for measuring combustion temperatures using two-line laser-saturated fluorescence. The rotational temperature of OH is determined by saturating two different rotational transitions in the (0,0) band of the A(2)Sigma(+)-X(2)II electronic system and detecting fluorescence emission which originates from the laser-pumped upper rotational levels. Temperature is calculated from the ratio of the fluorescence intensities for the two different excitation-emission pairs. The method is demonstrated by measuring temperature profiles in subatmospheric H(2)/O(2)/Ar flat flames. Temperatures measured by two-line saturated fluorescence are compared with temperatures measured by coated thermocouples and OH absorption and with predictions from an elementary chemical kinetics code. The temperatures measured by the two-line fluorescence technique are accurate to 3-5% and exhibit low random error.

GABA levels in CSF of patients with psychiatric disorders.

The authors measured gamma-aminobutyric acid (GABA) levels in the lumbar CSF of patients with depression, with psychosis, or undergoing evaluation for a neurologic disorder. GABA levels in the CSF from depressed patients were significantly decreased compared with neurologic control patients. CSF GABA levels in psychotic patients were not different from those in neurologic patients, although the data suggested a decrease in CSF GABA levels in patients with schizoaffective disorder.

Balanced cross-rate model for saturated molecular fluorescence in flames using a nanosecond pulse length laser.

The balanced cross-rate model is proposed to analyze laser-induced molecular fluorescence signals when the laser pulse length is of the order of nanoseconds. Nanosecond pulse length lasers, specifically Q-switched Nd:YAG-pumped dye lasers, are attractive for saturated molecular fluorescence spectroscopy because of their high peak power and because their short pulse length minimizes the risk of laser-induced chemistry. In the balanced cross-rate model, single upper and lower rotational levels are assumed to be directly coupled by the laser radiation. Because the laser-induced processes which couple these levels are so fast at saturation intensities, a steady state is established between the two levels within picoseconds. Provided that the total population of the two laser-coupled rotational levels is constant during the laser pulse, the total molecular population can be calculated from the observed upper rotational level population using a two-level saturation model and Boltzmann statistics. Numerical simulation of the laser excitation dynamics of OH in an atmospheric pressure H(2)/O(2)/N(2) flame indicates that the balanced cross-rate model will give accurate results provided that the rotational relaxation rates in the upper and lower sets of rotational levels are approximately equal.

Infrared holography using the thermochromic material Cu(2)Hgl(4).

Thermochromic materials can be used to record infrared holograms at 10.6 microm as has been demonstrated by others. The objective here was to determine quantitatively the capability of holographic recording techniques that utilize thermochromics. Particular emphasis is placed on cuprous mercuric iodide, but the procedure developed is valid for other thermochromic materials. The thermochromic Cu(2)HgI(4) mixed with a suitable binder and coated onto a temperature controlled substrate was used to record holograms at a wavelength of 10.6 microm. Thermal diffusivity and conductivity of the recording material (Cu(2)HgI(4) + binder) were measured and used to analyze the thermal behavior of the detector. Theory shows that a resolution of 50 l/mm can be attained. Experiments were conducted to demonstrate the theoretical predictions. Holograms were recorded and reconstructed at the wavelength of 10.6 microm.

Materials processing with CO(2)laser holographic scanner systems.

The complexity of laser material processing can be greatly reduced using computer-generated phase reflection holographic scanners. These scanners direct and focus the beam of a carbon dioxide laser into a spot on the workpiece and then translate this spot over some general 2-D pattern as the scanner undergoes a simple 1-D motion. Procedures for constructing these scanners are presented, and the first-order aberrations introduced by them are analyzed. The primary aberrations cause the diffracted beam to focus to an astigmatic spot on the work surface. The severity of the astigmatism is proportional to the scan rate, scan angle, and f/number. A technique is presented in which the design of the scanner is adjusted so that the astigmatic image is aligned with the scan direction. The resolution perpendicular to the scan direction is the same as that of a scanner without aberrations of the same f/number. Materials processed using these scanners are presented to show their capabilities for carbon dioxide laser material processing. Power densities on the order of 10(6)/cm(2) can be readily obtained using the proposed technique.

Infrared holography: an analysis of the thermal behavior of thin films.

This paper presents an analysis describing the thermal behavior of a thin film subjected to a spatially modulated irradiation field. The analysis shows that it is possible using a cw laser to establish a temperature field in such a film that maps the irradiation pattern with a resolution adequate for recording high quality holograms. Effects of the thermal properties of the film and substrate on such recording characteristics as linearity, sensitivity, and resolution are examined. A high diffraction efficiency recording technique based on the sublimation of a thin film situated on a constant temperature substrate is proposed as an example of the application of the analysis.

Reconstruction of three-dimensional refractive index fields from multidirectional interferometric data.

Numerical and analytical techniques are presented that allow three-dimensional, asymmetric, refractive index fields to be reconstructed from optical pathlength measurements, which can be obtained using multidirectional holographic interferometry. Analytical reconstruction techniques that have been used in radioaptronomy and electron microscopy for a number of years, and recently in interferometry, are presented in the context of interferometric applications in the refractionless limit. These techniques require that optical pathlength data be collected over a 180 degrees angle of view. The required pathlength sampling rate is discussed. An efficient numerical procedure is developed for direct inversion of the data. Several numerical techniques are developed that do not require that data be collected over a full 180 degrees angle of view. All such techniques require redundant data to achieve accurate reconstructions. The required degree of redundancy increases as the angle of view decreases. Numerical simulations using six different reconstruction techniques indicate that with a 180 degrees angle of view, all are capable of providing accurate reconstructions. Four of the techniques were used to analyze simulated interferometric data recorded over an angle of view of less than 180 degrees . Examples of reasonably accurate reconstructions using data with angles of view as low as 45 degrees are presented.

Measurement of vibrational amplitude by modulation of projected fringes.

A full-field, variable sensitivity technique for mapping the vibrational amplitude of a diffuse surface is demonstrated. The surface under study is placed in an optical field consisting of two mutually coherent plane waves which propagate in different angular directions. At any instant of time the interference of these waves causes a fringe pattern to be formed on the surface under study. The time average, over one or more vibrational periods, of the intensity pattern on the surface can be obtained in real time by utilizing visual persistance or by making a time-average recording of an image of the surface. The visibility of the fringe pattern thus obtained is found to be modulated by a function of the local amplitude of vibration. This modulation can be observed directly or a photograph of the pattern can be optically processed to yield fringes that are loci of constant vibrational amplitude.