Wide-aperture receivers for free-space optical communication systems would utilize Fresnel lenses instead of conventional telescope lenses, according to a proposal. Fresnel lenses weigh and cost much less than conventional lenses having equal aperture widths. Plastic Fresnel lenses are commercially available in diameters up to 5 m - large enough to satisfy requirements for aperture widths of the order of meters for collecting sufficient light in typical long-distance free-space optical communication systems.
Fresnel lenses are not yet suitable for high-quality diffraction-limited imaging, especially in polychromatic light. However, optical communication systems utilize monochromatic light, and there is no requirement for high-quality imaging; instead, the basic requirement for an optical receiver is to collect the incoming monochromatic light over a wide aperture and concentrate the light onto a photodetector.
should be possible to attain blur-circle diameters of less than2 mm. Preferably, the blur-circle diameter should match the width of the photodetector. For most high-bandwidth communication applications, the required photodetector diameters would be about 1 mm. In a less-preferable case in which the blur circle was wider than a single photodetector, it would be possible to occupy the blur circle with an array of photodetectors.
As an alternative to using a single large Fresnel lens, one could use an array of somewhat smaller lenses to synthesize the equivalent aperture area. Such a configuration might be preferable in a case in which a single Fresnel lens of the requisite large size would be impractical to manufacture, and the blur circle could not be made small enough. For example one could construct a square array of four 5-m-diameter Fresnel lenses to obtain the same light-collecting area as that of a single 10-m-diameter lens. In that case (see figure), the light collected by each Fresnel lens could be collimated, the collimated beams from the four Fresnel lenses could be reflected onto a common off-axis paraboloidal reflector, and the paraboloidal reflector would focus the four beams onto a single photodetector. Alternatively, detected signal from each detector behind each lens would be digitized before summing the signals.
This work was done by Hamid Hemmati of Caltech for NASA's Jet Propulsion Laboratory. 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
Innovative Technology Assets Management JPL Mail Stop 202-233 4800 Oak Grove Drive Pasadena
CA 91109-8099 (818) 354-2240 E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
Refer to NPO-40436.
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Fresnel Lenses for Wide-Aperture Optical Receivers
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Overview
The document titled "Fresnel Lenses for Wide-Aperture Optical Receivers" from NASA's Jet Propulsion Laboratory discusses the potential of using large diameter Fresnel lenses as low-cost optical communication ground receiver apertures. It emphasizes the need for effective photon collection systems to facilitate higher data-rate downlinks for free-space optical communication with deep-space probes and near-earth spacecraft.
The document outlines various configurations for optical receivers, including single large monolithic primaries, segmented primaries forming large diameter telescopes, and arrays of smaller telescopes. However, these traditional methods are often costly and time-consuming to develop. In contrast, Fresnel lenses are highlighted for their advantages: they are inexpensive, lightweight, and commercially available in large diameters.
Fresnel lenses, made from transparent plastic materials like PMMA, acrylic, polycarbonate, and rigid vinyl, are typically unsuitable for astronomical imaging due to diffraction-induced aberrations when collecting a broad spectrum of light. However, in laser communication applications, where the receiver focuses on a single wavelength (e.g., 1064 or 1550 nm) and has a narrow field of view, Fresnel lenses can effectively focus light into a small blur circle, making them viable for optical communication.
The document also describes specific configurations for using Fresnel lenses, such as a 5-meter laser light collection aperture synthesized with 25 1-meter Fresnel lenses arranged in a 5 by 5 array. Each lens is paired with a small conventional lens to collimate the light, which is then directed to a small parabola that focuses the combined light onto a single detector. This setup allows for efficient photon collection while maintaining a compact design.
Overall, the document presents a compelling case for the use of large diameter Fresnel lenses in optical communication systems, particularly for deep-space applications where traditional methods may be impractical. By leveraging the unique properties of Fresnel lenses, NASA aims to enhance the capabilities of ground-based optical receivers, ultimately improving data transmission rates and communication efficiency in aerospace missions.
