A provision for controlled variation of the divergence of a laser beam or of multiple parallel laser beams has been incorporated into the design of a conceptual free-space optical-communication station from which the transmitted laser beam(s) would be launched via a telescope. The original purpose to be served by this provision was to enable optimization, under various atmospheric optical conditions, of the divergence of a laser beam or beams transmitted from a ground station to a spacecraft. Beyond the original purpose, the underlying design concept could be beneficial for terrestrial free-space laser communication, ranging, and scientific instrumentation applications in which there are requirements to vary the divergences of laser beams.
In order to be able to provide for controlled variation of beam divergence, one must first gain detailed understanding of the optical train from each laser to the primary mirror of the telescope. Gaussian propagation of each laser beam through all the optical elements must be computed. If multiple parallel beams were to be transmitted, then by means of previously developed optics, they would be positioned symmetrically about the optical axis. It would be necessary to perform paraxial ray tracing to ensure that the beams emerging from the primary mirror into free space were parallel to each other and to the main optical axis of the telescope.
The design concept reflects a requirement in the original application that final divergence of the beam(s) propagating out from the primary mirror into free space be varied by moving only one lens or lens assembly in the optical train and that this motion not cause the outgoing beam(s) to deviate from parallelism with the optical axis. To satisfy this requirement, the telescope would incorporate an afocal optical subassembly, within which either a single on-axis lens in the case of a single laser beam or a ring assembly of lenses in the case of multiple laser beams (see figure) would be moved. The lens or lenses must be designed to mode-match the laser output through the afocal sub-assembly and telescope optics to produce the required beam divergence. By moving this lens (or moving the assembly of lenses as a single unit) along the optical axis, one would cause the divergence of the outgoing laser beam(s) to vary through the required range. Care must be taken to ensure that there is no apodization or vignetting through any limiting apertures in the overall optical system and that power density of any laser beam must not be so high as to result in dielectric breakdown of air or in damage to any optic along the optical path.