High-resolution magnetic field measurement system for recording heads and disks.

A high-resolution system for measuring the magnetic fields near recording heads and disks has been built. The perpendicular field components close to the head and disk surfaces are measured with an InSb Hall probe which is moved point-by-point under computer control. The computer also records the magnetic field strength as a function of position in a plane parallel to the head or disk surface. The active area of the probe is 2x2 microm and a roughly 1-microm separation between the probe and the surfaces being measured can be obtained. Measurements of the fields near transitions on a particulate disk and near the gap in in a ferrite recording head are presented and compared with calculated field distributions.

Lensless holographic recognition of spatially incoherent patterns in real time.

Self-luminous patterns and patterns on diffusing backgrounds can be recognized in real time by means of multiple holographic filters in an approach that requires no imaging lenses. Use of filters matched to features rather than to entire characters has allowed reading of alphabetic characters printed on white paper at contrast ratios down to 1.7:1. The experimental geometry eliminates aberrations as a problem.

Measurements of the stability of bleached photographic phase holograms.

Although efficient high quality, bleached holograms have been made using photographic film, the tendency of the bleach products to darken upon exposure to light seriously limits their usefulness in many practical situations. Nevertheless, the great sensitivity of photographic emulsions makes their use in holography highly desirable. To shed light on the problem of stability, relative measurements of light induced darkening were made on heavily exposed plates bleached to yield one of the reaction products: silver chloride, silver bromide, silver iodide, and silver ferricyanide. We have found that the resistance of the emulsion to darkening increases significantly as the molecular weight of the halide ion is increased. Efficient, high quality, silver iodide holographic gratings usable over the wavelength range from 450 nm to 1200 nm, have been made which exhibit complete immunity to darkening when exposed to white light. Representative measurements of holograms made on Agfa 8E70 plates using the more stable emulsions are presented.

An ultrasonic shutter for noise reduction in real-time optical correlators.

A coherent optical correlator can operate as a generator of complex waveforms when it is driven by a short-duration carrier pulse. However, the poor signal-to-noise ratio which results usually precludes operation in this mode. The ultrasonic shutter described herein can improve the signal-to-noise ratio by a factor approaching the time-bandwidth product of the system. No moving mechanical parts are involved.

Optical correlation of fresnel images.

A real-time optical correlator is described that uses an ultrasonic light modulator (ULM) in conjunction with an amplitude mask or replica of the signal to achieve linear, phase-coherent detection, without need for high quality optics. When the mask is placed in a Fresnel image plane of the ULM, optimum correlation and collinear heterodyning are achieved. Correlator response has been calculated and measured as a function of mask-ULM separation for a pulse modulated cw signal and a square wave modulated fm waveform. For essentially band-limited signals, the output is a pair of symmetrically time-shifted, matched filter responses, possessing negligible quadratic phase distortion.