Three documents discuss a method of controlling the diameter of a laser beam projected from Earth to any altitude ranging from low orbit around the Earth to geosynchronous orbit. Such laser beams are under consideration as means of supplying power to orbiting spacecraft at levels of the order of tens of kilowatts apiece. Each such beam would be projected by use of a special purpose telescope having an aperture diameter of 15 m or more. Expanding the laser beam to such a large diameter at low altitude would prevent air breakdown and render the laser beam eye-safe. Typically, the telescope would include an adaptive-optics concave primary mirror and a convex secondary mirror. The laser beam transmitted out to the satellite would remain in the near field on the telescope side of the beam waist, so that the telescope focal point would remain effective in controlling the beam width. By use of positioning stages having submicron resolution and repeatability, the relative positions of the primary and secondary mirrors would be adjusted to change the nominal telescope object and image distances to obtain the desired beam diameter (typically about 6 m) at the altitude of the satellite. The limiting distance DL at which a constant beam diameter can be maintained is determined by the focal range of the telescope 4λƒ² where λ is the wavelength and ƒ the ƒ/number of the primary mirror. The shorter the wavelength and the faster the mirror, the longer DL becomes.
This work was done by Harold E. Bennett of Bennett Optical Research, Inc., for Marshall Space Flight Center. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Dr. Harold Bennett, President
Bennett Optical Research, Inc.
916 N. Randall Street
Ridgecrest, CA 93555
Refer to MFS-32039-1, volume and number of this NASA Tech Briefs issue, and the page number.