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Summary

A technology for transmitting high power by laser beam over large distances using a quasi-coherent array of lasers is described. This technology achieves quasi-coherency by turning OFF selected laser elements, rather than trying to correct their phase as present technologies do. Such an array puts less power into a specified area in the far field than a fully coherent array but gains two major advantages: (1) laser array elements are greatly simplified and less expensive and (2) array lifetime can be lengthened. The degree of coherency can be chosen. The greater the coherency, the greater is the lifetime of the array (for a specified power transmission requirement). More laser elements remain inactive. Also, by making array element selection a dynamic process, the technology can actively correct any phase changes induced in the array. The technology is analyzed, results of a computer experiment are presented, and a laboratory experiment for further development of the technology is proposed.

Introduction

For many years, NASA has studied the transmission of power by laser beam (refs. 1 and 2), with potential applications identified both in space and on Earth. Such applications require high power (up to megawatts) diffraction limited laser systems. One method to obtain such systems is to assemble arrays of less powerful lasers which can be scaled to the laser power required. Arrays also lend themselves to building the large apertures required for transmission over large distances. The larger the transmission aperture is, the smaller the received laser spot size for a given range. Arrays have usually been described in terms of their ultimate power transmission capability (i.e., the maximum power that a coherent array could transmit into a specified area in the far field). Coherent arrays transmit optical amplitudes that are in phase in space and time. Although coherent arrays are feasible, they are very difficult to obtain in practice. They require complex and expensive electro-optical components that are still challenged by the dynamics of a real system (i.e., heat distribution, alignment, mechanical stress). Thus, the questions arise: Is the power transmission loss due to incomplete coherence worth the cost of obtaining complete coherence?" and Are there other advantages of a partially coherent array?" These questions have led to an idea labeled dynamic selection of emitting laser array elements." This idea represents a new approach to obtaining concentrated laser power in the far field. In this approach, laser array elements are not forced" to be perfect (coherent) but are simply turned OFF

if they are sufficiently imperfect (incoherent). The advantages of this approach are discussed, some of the results of a computer experiment with this theory are presented, a practical laboratory experiment is proposed, and its purposes are enumerated.

Dynamic Selection of Emitting Laser

Array Elements

The dynamic selection of emitting laser array elements (ref. 3) is, essentially, a simple method for obtaining a concentrated far-field radiation pattern from an array of lasers that are already temporally coherent but have random phase errors due to optical path differences or effects that can be interpreted as such. The idea represents a compromise between the beam concentration obtained with fully coherent laser arrays and that obtained with incoherent laser arrays. The price paid in reduced array coherence buys an easily implemented, partially coherent array with greatly simplified laser elements and extended lifetime. (See ref. 4 for an overview of related technology in the PILOT program and descriptions of complex laser systems that attempt full correction of incoherence.) The degree of coherence is adjustable. If a large degree of coherence is chosen, not many array elements will be turned ON because only a small fraction will meet the optical path-length-difference requirements. The resultant output power can be increased by increasing the total number of laser array elements. Since a large degree of coherence activates a small fraction of array elements, the other elements can be activated at later times to provide extended lifetimes.

Elements of the array need only be aligned (emitted beams approximately parallel) and temporally coherent (same frequency). The phases of the elements (spatial coherence) are randomly distributed. Partial coherence is obtained by simply turning OFF those elements that are not within a specified phase error of a reference element (and turning ON those that are). (Semiconductor waveguide amplifiers or similar devices for EXACT phase control are not needed.) This is a dynamic process; that is, each element is reexamined at later times to determine whether they should remain OFF or ON. In this manner, the array is continually readjusted to compensate for any factors that tend to destroy spatial coherence.

If, for example, only those laser elements that are within ?90ffi of the reference element are turned ON, about one half of the total laser elements would be turned ON (assuming, for simplicity, that the maximum phase variation is a multiple of 2ss radians). The total number of laser elements would have to be