Optics: measuring and testing – Velocity or velocity/height measuring – With light detector
Reexamination Certificate
1999-07-02
2001-11-06
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Velocity or velocity/height measuring
With light detector
C356S340000, C356S340000
Reexamination Certificate
active
06313908
ABSTRACT:
BACKGROUND
Field of the Invention
The field of the invention is the use of holographic optical elements to convert the information of a spectral distribution of light to another form. For convenience, the term “light” will be used to mean electromagnetic radiation of any and all wavelengths.
Many applications require discrimination or selection of wavelengths, but with different resolution requirements. For example a prism can be used as a low resolution spectrometer to separate visible light into its constituent colors. However other applications require isolation of narrow spectral lines to resolve a spectral shape.
One such application is a lidar system to measure wind velocities by aerosol and/or molecular backscatter. In a direct, detection Doppler lidar or incoherent lidar, the Doppler shift resulting in a pulse of narrowband laser light from scattering by aerosols or molecules is measured. A zero-wind reference spectrum of an outgoing laser beam is measured by the collection of light scattered from the outgoing optics. The reference spectrum and a backscattered laser light return signal pass through receiving optics. The reference spectrum is subtracted from the return signal to determine the Doppler shift. A high resolution spectroscopic device, typically a Fabry-Perot interferometer is used to detect the wavelength shifts.
The Fabry-Perot interferometer produces a circular interference spectrum or fringe pattern of equal area rings representing equal wavelength intervals, sharing a common axis, at the infinity focus of an objective lens system. Different types of image plane detectors have been created which attempt to match the circular pattern. Converting the pattern itself to fit linear detectors has also been accomplished.
One such image plane detector is described in “Image plane detector for the Dynamics Explorer Fabry-Perot interferometer,” Timothy L. Killeen, B. C. Kennedy, P. B. Hays, D. A. Symanow, and D. H. Ceckowski,
Applied Optics
, Vol. 22, No. 22, Nov. 15, 1983, pp. 3503-3513. This device consisted of an S-20 photocathode, three microchannel plate electron multiplication stages, and an equal-area concentric-ring segmented anode to match the interference ring pattern. Another type of image plane detector was a multi-element detector of concentric rings of PIN photodiode material. (See U.S. Pat. No. 5,239,352 “Multiple Scattering Technique (MUST) Lidar,” Luc R. Bissonnette, Issue Date Aug. 24, 1993 and “Multiple field of view lidar returns from atmospheric aerosols,” D. L. Hutt, L. R. Bissonnette, and L. Durand,
Applied Optics
, Vol. 33, No. 12, Apr. 20, 1994, 2338-2348. The image plane detectors typically suffer from blurring of spot sizes and low quantum efficiency.
A different approach for converting the information in a Fabry-Perot fringe pattern to a more easily detectable form is described in U.S. Pat. No. 4,893,003, “Circle-to-Line Interferometer Optical System,” Paul B. Hays, Issue Date: Jan. 9, 1990 and “Circle to line interferometer optical system,” Paul B. Hays,
Applied Optics
, Vol. 29, No. 10, Apr. 10, 1990, 1482-1489. A 45 degree half angle internally reflecting cone segment is used to convert the circular Fabry-Perot interferometer fringe pattern into a linear pattern.
A small, high efficiency, compact, low cost device for obtaining the most information in a Fabry-Perot pattern while being compatible with linear arrays of detectors is desired. Being able to couple the pattern to solid state photodetectors as opposed to charge coupled device (CCD) detectors has the benefit of no cooling requirements when measuring atmospheric wind profiles. Also, solid state photodetectors can resolve the wavelength phase shift in the microsecond time interval needed for range resolved lidar measurements. In addition, a lidar system with multiple fields of view or, in other words, a very wide field of view is desired to avoid or lessen movement of the optical system in order to change zenith angle.
SUMMARY
A small, compact holographic optical element (HOE) with high transmission efficiency has been developed which may be used in an apparatus for resolving a spectral distribution of light. The holographic optical element of the present invention converts an incident spectral distribution to image points. The element comprises areas, each comprising a recorded hologram of a point source. Each area acts as a separate lens to focus the distribution in its area to an image point. Each of the image points is angularly separated from the others.
The image points can form a line and lie in the same focal or image plane which is convenient for use with a linear array detector.
The areas can be made in various geometries. For example, one embodiment of the element has concentric equal areas sharing a common axis to match the equal wavelength intervals in the circular fringe pattern of a Fabry-Perot interferometer. The Fabry-Perot pattern is focussed onto the HOE which in turn focuses the light on each concentric area or annulus to an image point. Detectors, such as analog detectors, charge coupled devices (CCD) or solid state photodetectors can then be used to measure the intensities of the image points.
All the areas of the HOE focus about the same wavelength. Another version of the HOE has additional recorded holograms overlaid for additional wavelengths so that this version of the element can focus light of different wavelengths to separate series' of points. For example, each area may be overlaid with holograms for wavelengths in the red and green portions of the visible spectrum. The result would be a series of image points for the red wavelength and a series of points for the green wavelength.
The holographic optical element and at least one detector can be integrated as the detection system in a incoherent or direct detection Doppler lidar system. Furthermore, a multiple field of view lidar system for measuring multiple scattering of an outgoing laser beam may have receiving optics comprising an embodiment of the holographic optical element placed at the focal plane of a wide angle telescope.
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“Circle to line interferometer optical system,” Paul B. Hays,Applied Optics,vol. 29, No. 10, Apr. 1, 1990, pp. 1482-1489.
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Marzouk Marzouk
McGill Matthew J.
Scott Vibart S.
Buczinski Stephen C.
The United States of America as represented by the Administrator
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