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.

Figure 1. The Prism Beam Slicer and Folding Mirror in a laser transmitter would concentrate most of the laser light into off-axis sector-of-circle cross sections that would not be obscured by the secondary reflector and baffle.

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.

Figure 2. The Laser Beam Would Be Split in four and then recombined in such a manner as to illuminate the primary mirror in four unobscured subapertures.

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.

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

Intellectual Assets Office
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240
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Refer to NPO-30574.

Topics:
Assembling Communication systems Exteriors Fabrication Imaging and visualization Lasers Lidar Manufacturing processes Mirrors Optics Photonics Physical Sciences Telecommunications Telescopes
This Brief includes a Technical Support Package (TSP).
Document cover
Efficient Coupling of Lasers to Telescopes With Obscuration

(reference NPO-30574) is currently available for download from the TSP library.

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    Photonics Tech Briefs Magazine

    This article first appeared in the January, 2003 issue of Photonics Tech Briefs Magazine (Vol. 27 No. 1).

    Read more articles from the archives here.

    Overview

    The document is a NASA Technical Support Package detailing innovative methods for efficiently coupling laser beams to telescopes, particularly those with secondary mirror obscuration, such as Cassegrain telescopes. The work, conducted by inventors Hamid Hemmati and Norman A. Page at the Jet Propulsion Laboratory, addresses a significant challenge in optical systems where a substantial portion of the laser beam is lost due to the obscuration caused by secondary mirrors and baffles.

    Typically, when coupling lasers with Gaussian (TEM00) output to these telescopes, about 50% or more of the useful portion of the beam is lost. This loss occurs because the central peak of the Gaussian beam is clipped by the obscured area, which can block a significant fraction of the beam area. The document introduces two novel approaches to mitigate this issue: a prism beam-slicer and a beam-splitter/combiner scheme. These methods are designed to be simpler to implement than existing techniques and aim to eliminate the typical losses encountered in such systems.

    The first approach, the prism beam-slicer, allows for the effective redirection of the laser beam, while the second approach, the beam-splitter/combiner scheme, facilitates the combination of multiple beams to enhance overall transmission efficiency. Both methods are positioned as improvements over prior art, which includes the use of axicon optical elements and sub-aperture elimination methods, the latter of which can lead to higher beam divergence.

    In addition to the laser coupling techniques, the document also discusses the calibration of atmospheric pressure measurements, particularly in the context of Mars exploration. It highlights the potential for calculating atmospheric pressure using temperature, known CO2 abundance, and spectral absorption measurements, which can reduce the need for atmospheric-pressure gauges and thus lower the mass of the instruments used in space missions.

    Overall, this technical report emphasizes the importance of improving laser coupling efficiency for various applications, including lidar and free-space optical communications, while also contributing to advancements in atmospheric science related to planetary exploration. The work is a testament to ongoing efforts to enhance the capabilities of optical systems in challenging environments.