F. A. Horrigan

Signal-to-noise relationships for coaxial systems that heterodyne backscatter from the atmosphere

This paper analyzes the signal-to-noise ratio for a coaxial laser system that heterodynes the signal backscattered from the atmospheric aerosol. The laser radiation, which is assumed to have a wavefront with a gaussian amplitude distribution, is transmitted into the atmosphere through a telescope. Radiation is collected by the same telescope and directed onto a detector where it is mixed with a local oscillator beam originating from the same laser source. The signal-to-noise ratio at the output of the detector is calculated under shot noise limited conditions. The calculation is general and applies for both near and far fields and for focused and unfocused systems. Three specific cases are considered. These are a pulsed system, a cw system illuminating an infinite target, and a cw system illuminating a target of finite extent.

FRESNEL ZONE PLATE IMAGING OF GAMMA RAYS; THEORY.

The use of a Fresnel zone plate as a coded aperture for imaging incoherent radiation such as gamma rays has been previously reported. The coded image is in many respects similar to a hologram and can be decoded or reconstructed with a coherent optical system. In this paper, the general theory of coded-aperture imaging is presented, first for an arbitrary code and then for an on-axis zone plate, an off-axis zone plate, and a one-dimensional zone plate (or linear chirp). With the on-axis plate, a matched imaging condition is suggested as a guide to optimizing image contrast. With the off-axis zone plate and the linear chirp, it is necessary to use a half-tone screen to spatially heterodyne the object spectrum into the passband of the aperture. In all three cases, expressions for the resolution, depth of field, field of view, and relative efficiency are derived. A simplified noise analysis is presented, and some practical system constraints are discussed.

Transition Probabilities for some Ar II Laser Lines

In the Ar II system transition probabilities are being calculated between states arising from the 3p44p and 3p44s configurations and between states of the 3p44s and 3p5 configurations. We compare these calculations with previous theoretical and experimental work relating to the above‐mentioned transitions as well as to laser data. It is found that the various observed laser transition thresholds between the considered configurations are in reasonable agreement with the calculations. It appears that the various upper maser states are being pumped nonuniformly. It is also found that laser action should persist to rather high current densities. We estimate that, in 2‐mm‐diam tubes, output powers of at least 10 W/cm3 of gas should be obtainable.

cw OPERATION OF HIGH‐PRESSURE FLOWING CO2 LASERS

The performance of N2–CO2–He lasers operating at pressures in the 10–120‐Torr range and with nitrogen flow rates of 20–90 standard liters per minute is discussed. A 10‐cm‐long, 13.5‐mm‐i.d. discharge tube produced 140‐W output power. These lasers operate as in‐line mixing lasers, producing about one photon per excited nitrogen molecule. Cooling is provided by rapid removal of hot gas from the discharge region.

Residue-Based Optical Processor

In the 1950s, digital designers noted that residue number systems offer the attractive feature of carry-free addition, subtraction, and multiplication. This interest waned, in part because residue number systems possess a multi-state character; flip flops and other binary state devices lend themselves more naturally to binary arithmetic than to multi-state arithmetic. Optics enjoys the requisite multi-state capability: lenses can resolve many discrete positions, gratings can resolve many discrete frequencies, and so forth. This fact has motivated efforts to create a residue based optical processor, which would combine the parallel, speed of light throughput of optics with processing accuracies possible only to digital systems. Our program has included the identification of appropriate roles for residue processors, the investigation of I/O techniques, such as analog to residue and residue to radix conversion, and the study of optical implementations of residue arithmetic.

One-Dimensional To Two-Dimensional Transformations In Signal Correlation

We explore one-dimensional (1-D) to two-dimensional (2-D) transformations suitable for optical correlation of signals. An intuitive, geometrical development establishes and relates several 2-D formats including variations of the familiar falling raster format. Insight into the folded spectrum is gained by considering a matched spatial filter implementation of correlation between two signals in falling raster formats.