Donald C. O'Shea

GRAY-SCALE MASKS FOR DIFFRACTIVE-OPTICS FABRICATION. I: COMMERCIAL SLIDE IMAGERS

Fabrication of diffractive optics with binary masks requires multiple photolithographic processes to produce high-efficiency elements. Alignment or etching errors at any stage of fabrication decrease the efficiency of the element. We developed an easily accessible procedure that reduces fabrication complexity and costs by using a single gray-scale mask. The gray-scale patterns are generated by commercial slide imagers and are then photoreduced onto low-contrast film plates. Multiple-level or continuous relief structures (kinoforms) may be constructed by use of the photoreduced gray-scale patterns as lithographic masks. Diffractive-optic lenses and blazed gratings were fabricated in photoresist with this procedure. First-order diffraction efficiencies as high as 85% were measured for the blazed gratings. The advantages and the limitations of this technique are discussed.

Gray-scale masks for diffractive-optics fabrication: II. Spatially filtered halftone screens.

Fabrication of diffractive optics with binary masks requires multiple photolithographic processes to produce efficient, continuous profile elements (kinoforms). Alignment or etching errors at any stage of fabrication decrease the efficiency of the element. We developed two accessible procedures that minimize fabrication complexity, component turnaround time, and cost. The first technique [Appl. Opt. 34, 7507-7517 (1995)] uses gray-scale masks produced by commercial slide-imager systems. Here, we report on an alternative technique for producing gray-scale masks by spatial filtering of halftone screens. Using the photoreduced gray-scale patterns as lithographic masks, we fabricated diffractiveoptic blazed gratings and lens arrays in both photoresist and quartz. First-order efficiencies as high as 70% are reported. Also, the strengths and limitations of this technique are compared with the previously reported slide-imager method as well as other fabrication methods.

Binary-mask generation for diffractive optical elements using microcomputers

A new technique for generation of binary masks for the fabrication of diffractive optical elements is investigated. This technique, which uses commercially available desktop-publishing hardware and software in conjunction with a standard photoreduction camera, is much faster and less expensive thanhe conventional methods. The short turnaround time and low cost should give researchers a much greater degree of flexibility in the field of binary optics and enable wider application of diffractive-optics technology. Techniques for generating optical elements by using standard software packages that produce PostScript output are described. An evaluation of the dimensional fidelity of the mask reproduction from design to its realization in photoresist is presented.

Raman scattering and fluorescence in calcium fluoride

Abstract Raman scattering from a high purity calcium fluoride crystal excited by a 6328 A He-Ne laser has been observed. Contrary to results reported by Russell, no Raman structure was observed except for the first order line at 322 cm -1 . The strong spectrum, reported by Russell as second order Raman scattering, is found to closely resemble the fluorescence spectrum of erbium in calcium fluoride.

Fidelity of POSTSCRIPT-generated masks for diffractive optics fabrication.

A page description language such as POSTSCRIPT provides inexpensive and rapid output to high-resolution graphics engines for generating binary masks. However, the limitations of this technology have not been considered beyond a statement of the resolution of the output device. Measurements of three high-resolution laser image setters to quantify the limitations inherent to this type of mask fabrication show that the spot size and spot placement affect the performance of the final element. The expression of the graphics figure within the illustration program used to print the masks must also be considered. Strategies for compensating for print errors are described.

Temperature dependent Raman spectrum of strontium titanate

Abstract The Raman spectrum of crystalline SrTiO 3 has been observed in the temperature range 27 to 563°K and is believed to be of first order. A mode has been found which follows the behaviour predicted by Cochran for ferroelectric crystals, but its frequency is higher by a factor of approximately three than the mode observed by Cowley and by Barker and Tinkham.

Reduction of the zero-order intensity in binary Dammann gratings

The source of the reduction in the zero-order intensity in binary Dammann gratings is described as an error in the areas in the phase areas within the unit cell of the grating. Equations for determining the amount of error required to produce a specific reduction ratio are given. A two-dimensional, N = 1, Dammann grating that creates a 3 × 3 beam fan-out with a 24% reduction of the zero order provides an example of such an effect. The calculation shows agreement with the measured error.

Four-plane space-variant Fresnel-transform optical processor with a random phase encoder

We introduce a four-plane Fresnel-transform space-variant optical processor consisting of an input plane and two filter planes. One filter mask is programmable with a spatial light modulator. The second filter mask is a fixed random binary phase array with a known pseudorandom distribution of pixels. The order of the masks can be interchanged, giving different output characteristics. In one case the Horner efficiency of the correlator increases dramatically. In the other case the edge enhancement of the output image is removed. We discuss the theory for this general processor and its implementation with phase-only masks. We present experimental results when a binary magneto-optic spatial light modulator was used.

Group velocity dispersion using commercial optical design programs

Three commercial optical design programs are used to model familiar geometries in ultrafast optics. A set of macros has been created to calculate the pulse delay, group velocity dispersion, and third-order dispersion caused by the components in an ultrafast optical system. The results correspond to published values. This opens the possibility of using the well-developed optimization routines to improve the performance of ultrafast systems.

Light modulation from crossed phase gratings

A new device, consisting of two phase gratings oriented with a small angle between their grating grooves, acts as a variable-profile diffraction grating that produces variations in the diffraction orders as the illumination region is moved across the device. Combinations of phase gratings that optimize the effect and act as passive light modulators are given. An experiment that demonstrates the principle is described.