An innovation reported in “Two-Camera Acquisition and Tracking of a Flying Target,” NASA Tech Briefs, Vol. 32, No. 8 (August 2008), p. 20, used a commercial fish-eye lens and an electronic imaging camera for initially locating objects with subsequent handover to an actuated narrow-field camera. But this operated against a dark-sky background. An improved solution involves an optical design based on custom optical components for the wide-field optical system that directly addresses the key limitations in acquiring a laser signal from a moving source such as an aircraft or a spacecraft.
The first challenge was to increase the light collection entrance aperture diameter, which was approximately 1 mm in the first prototype. The new design presented here increases this entrance aperture diameter to 4.2 mm, which is equivalent to a more than 16 times larger collection area. One of the trades made in realizing this improvement was to restrict the field-of-view to +80° elevation and 360° azimuth. This trade stems from practical considerations where laser beam propagation over the excessively high air mass, which is in the line of sight (LOS) at low elevation angles, results in vulnerability to severe atmospheric turbulence and attenuation. An additional benefit of the new design is that the large entrance aperture is maintained even at large off-axis angles when the optic is pointed at zenith.
The second critical limitation for implementing spectral filtering in the design was tackled by collimating the light prior to focusing it onto the focal plane. This allows the placement of the narrow spectral filter in the collimated portion of the beam. For the narrow band spectral filter to function properly, it is necessary to adequately control the range of incident angles at which received light intercepts the filter. When this angle is restricted via collimation, narrower spectral filtering can be implemented. The collimated beam (and the filter) must be relatively large to reduce the incident angle down to only a few degrees. In the presented embodiment, the filter diameter is more than ten times larger than the entrance aperture. Specifically, the filter has a clear aperture of about 51 mm.
The optical design is refractive, and is comprised of nine custom refractive elements and an interference filter. The restricted maximum angle through the narrow-band filter ensures the efficient use of a 2-nm noise equivalent bandwidth spectral width optical filter at low elevation angles (where the range is longest), at the expense of less efficiency for high elevations, which can be tolerated because the range at high elevation angles is shorter. The image circle is 12 mm in diameter, mapped to 80×360° of sky, centered on the zenith.
This work was done by Norman A. Page, Jeffrey R. Charles, and Abhijit Biswas of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO-46945
This Brief includes a Technical Support Package (TSP).
Wide-Field Optic for Autonomous Acquisition of Laser Link
(reference NPO-46945) is currently available for download from the TSP library.
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Overview
The document outlines a technical support package for a wide-field optical system designed for autonomous acquisition of laser links, specifically targeting applications in communication, ranging, and targeting over moderate distances (hundreds to thousands of kilometers). The system addresses the challenges of tracking moving objects within a wide field of view (FOV) while maintaining high optical performance and compatibility with narrow band filters.
Key features of the optical design include an entrance aperture diameter that increases with lower elevation angles, a unique characteristic for optics with a large FOV. This design allows for a significant increase in light collection area, with the new system achieving an entrance aperture of 4.2 mm, over 16 times larger than its predecessor. The system is tailored for monochromatic laser link applications, ensuring high efficiency and good optical performance across the entire FOV, which spans 80° in elevation and 360° in azimuth.
The document highlights the limitations of prior art, which struggled to accommodate narrow band filters in wide-field optical systems without adding numerous ancillary optical elements. The innovative design presented here overcomes these limitations, allowing for effective use of narrow band filters while maintaining a wide FOV. The system is optimized for specific monochromatic laser wavelengths, enhancing its effectiveness in free-space laser communication.
Additionally, the design incorporates a collimated space that can accommodate a filter wheel, allowing for the use of various filters matched to the system's specifications. This flexibility is crucial for optimizing performance across different wavelengths, although the use of color filters may deviate from the monochromatic optimization.
The document emphasizes the importance of this technology in improving signal-to-background noise ratios and enhancing tracking capabilities against challenging backgrounds, such as bright daytime skies. The advancements in this optical system represent a significant step forward in the field, combining a wide FOV with a large aperture and narrow band filtration, thus enabling more effective tracking and communication in aerospace applications.
Overall, the document serves as a comprehensive overview of the innovative optical design, its applications, and the technical challenges it addresses, showcasing the potential for broader technological and commercial applications stemming from this research.
