Shared-aperture multiplexed holographic scanning telescopes have been proposed for use in lidar transceivers and other laser transmitters and receivers in remote sensing instruments. Examples of instruments that could incorporate the proposed telescopes include airborne terrain mappers, and lidar wind-shear-profiling systems to increase safety of airplane takeoffs and landings. Unlike prior scanning telescopes, the proposed telescopes would contain no moving parts; hence, relative to prior scanning telescopes, the proposed telescopes could be made lighter, more compact, and more reliable.
Instead of conventional reflective or refractive optics, shared-aperture multiplexed holographic scanning telescopes would utilize diffractive optics in the form of holographic optical elements (HOEs). A telescope of this type is said to be shared-aperture multiplexed (SAM) because a number of HOEs would be multiplexed into a single film, the area of which would define a single, shared aperture. Each HOE in the film would be optically addressable by virtue of its angular selectivity, which would define a field of view (FOV) centered on a line of sight different from the lines of sight of the other HOEs. Thus, by optically addressing the various HOEs in sequence, one could aim the telescope sequentially along different lines of sight.
In addition to separate FOVs, the HOEs in the film would have separate field stops, for example, located at various angles around a circle (see figure). Each HOE would be optically addressed by transmitting a laser beam through the HOE along the appropriate line of sight, which would appear to emanate from one of the field stops. This could be accomplished by use of a separate laser for each line of sight. Alternatively, one could steer a single laser beam sequentially through virtual field stops, either by diffractive beam steering mechanisms, or mini-mechanical scanner, or even microeletromechanical systems (MEMS) technology.
In an alternative optical configuration, the central portion of the SAM optic would be used for transmitting only. In this case, the focal spots from which the laser beam would appear to emanate would be offset from the receiver foci and the central transmitting portion of the SAM optic would no longer be available for receiving. Moreover, in this case, there would be an option to superimpose the receiving foci so that a single detector could be used for all FOVs, provided that the transmitted laser pulses were sufficiently separated in time that lidar return signals would not overlap.
It is worth emphasizing that the proposed telescopes would scan in a step-and-stare mode rather than in the continuous mode of mechanical scanning. Although this may seem at first glance to be disadvantageous, it may not be; indeed, it could even be advantageous. In particular, there is some agreement within the Doppler lidar community that continuous scanning is not needed, and that a step-and-stare approach to gathering data from different look angles may be preferred because it would eliminate the need for lag-angle compensation. (In a mechanically scanned lidar system, the lag angle is a consequence of rotation of a scanner during the time it takes a light pulse from the laser transmitter to travel to the target and back.)
This work was done by Geary Karl Schwemmer of Goddard Space Flight Center and Richard Rallison of Ralcon, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Goddard Space Flight Center; (301) 286-7351.
Refer to GSC-14240.