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The United States of America As Represented by the Administrator of the

James B Breckinridge

Robert A Frosch

Rudolf A Schindler

Roland V Shack

Norman Alan Page

Robert R Shannon

Robert H Norton

Fred G O'Callaghan

State of the art Fourier transform spectrometers are useful scientific tools. But, they are very complex precision electro-optical-mechanical instruments. To simplify the instrument, the need for a mechanical slide mechanism to create a path difference has been eliminated by the use of retro-reflecting mirrors (16, 18) in a monolithic interferometer assembly (11) wherein the mirrors (16, 18) are not at 90 degrees to the propagation vector (29, 43) of the radiation (27), but rather at a small angle (49). The resulting interference fringes (51, 53) create a double-sided interferogram (33) of the source irradiance distribution which is detected by a charge-coupled device (CCD) array (23). The position of each CCD pixel (25) in the array is an indication of the path difference between the two retro-reflecting mirrors (16, 18) in the monolithic optical structure. The Fourier transform of the signals generated by the CCD array (23) provide the spectral irradiance distribution of the source. For imaging, the interferometer assembly (11) scans the source of irradiation by moving the entire instrument, such as would occur if it was fixedly mounted to a moving platform, e.g., a spacecraft. During scanning, the entrance slot (21) to the monolithic optical structure send different pixels to corresponding interferograms detected by adjacent columns of pixels at the CCD array (23).

Integrated optics in an electrically scanned imaging Fourier transform

Disclosed is an optical system used in a spacecraft to observe a remote surface and provide a spatial and spectral image of this surface. The optical system includes aspheric and spherical mirrors aligned to focus at a first focal plane an image of the surface, and a mirror at this first focal plane which reflects light back on to the spherical mirror. This spherical mirror collimates the light and directs it through a prism which disperses it. The dispersed light is then focused on an array of light responsive elements disposed at a second focal plane. The prism is designed such that it disperses light into components of different wavelengths, with the components of shorter wavelengths being dispersed more than the components of longer wavelengths to present at the second focal plane a distribution pattern in which preselected groupings of the components are dispersed over essentially equal spacing intervals.

Optical system

A high resolution interferometer is provided which is insensitive to slight misalignment of its elements, avoids channeling in the spectrum, generates a maximum equal path fringe contrast, produces an even two-sided interferogram without critical matching of the wedge angles of the beamsplitter and compensator wedges, and is optically phase tunable. The interferometer includes a mirror along the path of each beam component produced by the beamsplitter, for reflecting the beam component from the beamsplitter to a corresponding retroreflector and for reflecting the beam returned by the retroreflector back to the beamsplitter. The fact that the mirror 'covers' the retroreflector, so that the mirror reflects the beam component before and after it reaches the retroreflector, results in the system being insensitive to slight tilting of the mirror. A wedge located along each beam component path, is large enough to 'cover' the retroreflector, so that each beam component passes through the wedge during movement towards the retroreflector and away therefrom, to assure that all portions of a beam component pass through the same thickness of glass of the wedge.

Interferometer

An interferometer of relatively simple design which is tilt compensated, and which facilitates adjustment of the path lengths of split light beams. The interferometer includes a pair of plate-like elements with a dielectric coating and an oil film between them, that forms a beamsplitter interface, and with a pair of reflector surfaces at the ends of the plates. A pair of retroreflectors are positioned so that each split beam component is directed by a retroreflector onto one of the reflector surfaces and is then returned to the beamsplitter interface, so that the reflector surfaces tilt in a direction and amount that compensates for tilting of the beamsplitter interface. The oil film is less than about 20 microinches thick, to pass approximately equal amounts of all wavelengths longer than the film thickness, and yet to provide a sliding surface that enables one plate to shift relative to the other. The plate-like elements are slightly tapered to avoid channeling effects.

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La Canada, CA, United States of America

James B Breckinridge