Two techniques have been proposed to increase the efficiency of coupling of light from lasers to Cassegrain telescopes and, in general, telescopes with secondary or tertiary mirror obscuration. The need to increase the efficiency of coupling arises in laser transmitters of lidar and free-space optical communication systems that utilize Cassegrain telescopes. The vignetting caused by the secondary reflector and baffle in such a telescope reduces the transmitted power by a large fraction because (1) the obscured area is central and is a significant fraction of the telescope aperture and (2) the cross-sectional intensity profile of a typical laser beam is Gaussian, so that intensity is greatest in the obscured central area.
In a technique proposed previously for increasing the efficiency of coupling, an optical assembly comprising an axicon device and a folding mirror that would render the solid laser beam annular - in effect, turning the laser beam inside out - so that the laser light would be concentrated into an annular cross section that would not be obscured by the secondary reflector and baffle. Hence, most or all of the light would be coupled into the output beam.
Another prior efficiency-enhancing technique is denoted subaperture illumination. In this technique, the laser beam is displaced laterally with respect to the optical axis of the secondary reflector, such that the beam impinges on an off-axis subaperture of the primary reflector that is not obscured by the secondary reflector and baffle.
The disadvantages of the axicon approach are that it is difficult to fabricate an axicon device and that a small misalignment can strongly degrade its functionality. The disadvantage of the subaperture-illumination approach is that the beam transmitted by the telescope diverges more than it would if the entire aperture were illuminated.
In the first of the techniques now proposed, one would use a folding mirror in combination with a prism beam slicer that would function partly similarly to an axicon. Like an axicon device, the prism slicer would be an afocal refractive and reflective optical element. Like an axicon, the prism beam slicer would utilize both transmitting and reflecting optical surfaces. In the meridional cross-sectional detail in Figure 1, the prism slicer would look exactly like the axicon device.
Unlike the axicon device, the prism beam slicer would not have any curved optical surfaces: this would make it easier to fabricate and would make its functionality less sensitive to misalignment. The prism beam slicer would slice the laser beam into two or more beams that would have sector-of-circle cross sections, and that would be arranged symmetrically about the optical axis in unobscured off-axis positions. The difference between the axicon device and the prism beam slicer is that instead of a conical rear surface, the prism beam slicer would have an even number of flat rear surfaces in a pyramidal configuration at the same angle as that of the conical surface in the axicon device. If the number of pyramidal surfaces were made infinite, the prism beam splitter would revert to the axicon device.
The second technique now proposed would be a combination of the subaperture-illumination technique with a beam-splitting/beam-combining technique. As shown in Figure 2, the laser beam would be split into four beams that would be made to impinge on four faces of a pyramidal combining mirror and then further reflected by a flat combining mirror to generate four beams that would be parallel to the optical axis and would strike the primary mirror at unobscured off-axis positions 90° apart.
This work was done by Hamid Hemmati and Norman Page of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.
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