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.

Crossed phase gratings with diffractive optical elements.

Orienting two identical or complementary diffractive gratings with a small angle between the grating grooves allows a new crossed-grating device to be constructed. This device has an effective profile that varies locally. For understanding the effects of this variation and the diffraction efficiency of the gratings, the local profiles were correlated with the moiré period of the crossed-grating system by use of various techniques. Asymmetric intensity behavior in the first order of the crossed gratings was seen. Effectively, the diffraction efficiency of the crossed gratings yielded a response equivalent to that of a grating with variable blaze that could be useful in optical computing as a passive optical switching device. One of several models is described that creates greater asymmetric behavior.

Photoablation and lens damage from fractional Talbot images of Dammann gratings.

UV photoablation of materials is recorded for both the near and far fields after transmission through a Dammann grating. The fused silica Fourier lens used for far-field imaging was damaged by a near-field intensity pattern with the same periodicity as the Dammann grating. The lens was located inadvertently at one eighth of the Talbot distance Z (T) behind the Dammann grating. Patterns recorded in copper film at the even-fractional Talbot planes compare qualitatively with calculated intensities. On the basis of these findings, a near-field intensity pattern was used to ablate vias in copper and polyimide films. The pattern at a distance of Z(T)/8 was used for via ablation because it is the pattern with the most fluence per spot.

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.

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.

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.

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.

Specifying dispersion in the design of diffractive optics.

The chromatic aberration of a diffractive optical surface can be specified within a lens design program using the Sweatt model' by choosing the fictitious high refractive index equalto its corresponding wavelength in an appropriate unit.

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.

Conformational features of distamycin-DNA and netropsin-DNA complexes by Raman spectroscopy.

The binding of distamycin and netropsin to duplex DNA has been studied by Raman spectroscopy. Several changes occur in the Raman spectra of these drugs upon binding DNA. These changes were analyzed by assigning specific motions to the observed Raman bands through the use of molecular subunits of the drugs and normal mode calculations. Our analysis indicates that pyrrole ring and peptide group vibrations are altered upon binding to DNA. The environments of the pyrrole ring methyl groups are not affected by the binding. These data provide physical evidence consistent with a binding model in which the methyl groups on the pyrroles project away from the DNA and the peptide N-H groups form hydrogen bonds with the DNA.

Lensless real-image camera.

By hacksawing a Polaroid Super Shooter instant camera in two, a device can be made that solves the problem of recording real images (images that can be shown on a screen) for rapid evaluation in a laboratory. The real-image camera described here provides students, whose laboratory time is at a premium, with instant records of their work in an optics lab. The camera should also be useful for recording real images for optical measurements (interferometry, shadowgraphy, Schlieren, and deflection mapping) in research laboratories.

NO(2) Concentration Measurements in an Urban Atmosphere Using Differential Absorption Techniques.

NO(2) absorption coefficients for the most prominent argon-ion laser lines have been measured. A technique for measurement of NO(2) concentration in an urban atmosphere in real time is described. Preliminary results using the differential absorption of argon-ion laser lines by NO(2) in an urban (Atlanta, Georgia) atmosphere are presented. A simplified detection system using the differential absorption is proposed.