The figure schematically depicts the use of a special-purpose holographic plate to focus light from (1) circular interference fringes generated by a Fabry-Perot etalon to (2) a series of points, each illuminated in a different wavelength interval. This holographic circle-to-point converter was invented to enable efficient utilization of the output light of a Fabry-Perot etalon in an incoherent Doppler lidar system. (The role of the etalon in such a system is to resolve, within a fairly narrow frequency band, the spectrum of light back-scattered by atmospheric particles that have been illuminated by a laser in the system.) If photodetectors are placed at the illuminated points to measure the intensities of light in the various wavelength intervals, then the photodetector outputs can be used to characterize the spectrum of light coming through the Fabry-Perot etalon.
The role of the holographic plate is that of a field lens. As depicted in simplified form in the figure, the holographic plate superficially resembles a Fresnel zone lens, but it does not function like one. The annuli in the holographic plate are designed to be registered with the annuli in the interference pattern, wherein successive annuli contain light in successive wavelength intervals. Unlike a Fresnel lens, the hologram in the plate focuses the light from each annulus to a unique off-axis point instead of to the center of the interference pattern.
The hologram can be constructed so that all annuli share a common focal length; that is, all the focal points lie on one plane that is parallel to the plane of the holographic plate. The number and sizes of annuli and the size of the holographic plate can be chosen to satisfy the design requirements for a specific instrument. In the example of the figure, the annuli of the holographic plate have equal areas to take advantage of the fact that equal areas in the circular Fabry-Perot interference pattern correspond to equal wavelength intervals; however, one could just as well use annuli with differing areas if a design called for unequal wavelength intervals.
This work was done by Vibart Stan Scott, Matthew J. McGill, and Marzouk Marzouk of Goddard Space Flight Center.
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-13869.