Holographic retroreflectors that function equivalently to Plossl eyepieces have been developed and used in free-space optical communication systems that utilize laser beams. Plossl eyepieces are well known among telescope designers. They have been adopted for use as retroreflectors and as focusing elements (for reception) and collimating elements (for transmission) in optical communication systems. A retroreflector that incorporates a Plossl eyepiece is termed a cat’s eye retroreflector (see figure).
Plossl eyepieces have external pupils and are telecentric. Their telecentricity is what makes them useful as retroreflectors. A Plossl eyepiece is necessarily somewhat complex and expensive because it contains lenses that must be optically corrected to enable operation over a large field of view and a range of visible wavelengths.
In a free-space optical communication system, there is no need for lenses that function over a range of wavelengths because only one wavelength — the laser wavelength — is used to transmit information. A holographic optical element can readily be designed to perform equivalently to a corrected lens assembly at a single wavelength. If the Plossl eyepiece in a cat’s-eye retroreflector were replaced with a holographic optical element, the resulting optical assembly would be simpler and considerably lighter in weight. In addition, in mass production, such holographic optical elements would cost much less than do the corresponding lenses.
This work was done by Eugene Waluschka of Goddard Space Flight Center. GSC-14472-1