Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
1978-01-12
2001-10-16
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C356S329000, C356S365000, C359S577000
Reexamination Certificate
active
06304325
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of interferometry and more particularly, to a variable shear A.C. interferometer based on a rotating grating.
The invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the Department of the Air Force.
2. Description of the Prior Art
Interferometers are commonly used to measure wavefronts, and, in particular, are employed in a wide variety of applications to measure the departure of a wavefront from a true sphere (or from a plane wave in the case of collimated light). Such departures of a wavefront from its true shape typically occur as a result of imperfections in the optical path along which the wavefront travels. For example, the imperfections could be caused by a lens or some other optical system through which the light travels, and, in this situation, the interferometer could be used as an instrument to test the lens or other optical system.
In another application, the imperfections could be as a result of the effects of a turbulent atmosphere through which the light travels, and the interferometer can be employed to measure this distorted wavefront in a system to compensate for the effects of the atmosphere to permit more accurate imaging of distant objects such as a star or the like.
In general, interferometers operate by converting phase differences in the wavefront into intensity differences that can be monitored and read with a detector, and a variety of interferometers are known in the prior art. In U.S. Pat. No. 3,829,219 of James C. Wyant entitled “Shearing Interferometer,” a prior art A.C. grating interferometer based on double frequency phase gratings is described. In this system, known as the Wyant Interferometer, a wavefront being observed is converged to a focal point, and two diffraction gratings having slightly different line spacings are placed near the focal point of the wavefront. The diffraction gratings produce two first diffraction orders at two slightly different angles which result in a shearing interferogram in the region of overlap. This interferogram yields wavefront information in one direction, and by shearing the wavefront in two orthogonal directions as by providing two additional diffraction gratings, orthogonally oriented relative to the first two diffraction gratings, complete wavefront information in two directions can be obtained.
The interferometer described in the above patent, as well as most other prior art shearing interferometers suffer from several inadequacies. For one thing, they are essentially designed for use with coherent light and do not operate effectively, if at all, with white light. Also, their optical efficiency is usually quite limited because in such systems the diffraction orders are spatially separated and each are sheared with the result that riot much light gets through to each of the various orders.
SUMMARY OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, a variable shear A.C. (optical heterodyne) interferometer is provided which not only overcomes many of the inadequacies of the prior art, but also provides several desirable features not generally found in the prior art systems. In particular, the present invention relates to a variable shear A.C. interferometer based on the principle of a rotating radial grating which has been designed to provide the following important features:
(1) It is inherently achromatized and thus can be effectively used with white light extended sources;
(2) It has a high optical efficiency which can be made to approach 100%;
(3) The amount of shear can be easily varied by translating the grating rotation axis relative to the optical axis;
(4) It is a common path interferometer and, thus, is relatively insensitive to vibration and optical misalignment; and
(5) It requires only a small number of optical components.
As will be explained in much greater detail hereinafter, light is passed through a rotating radial grating to produce multiple sidebands diffracted at various angles. These multiple sidebands produce overlapping (sheared) images of the input wavefront at a detector plane causing intensity variations proportional to the phase difference between the sheared wavefronts. An array of detectors is positioned to monitor these images, and the detector output signals are electronically processed to obtain the slope of the wavefront at every location on the wavefront where the detectors make measurements. By making two sets of phase difference (slope) measurements, X and Y, with the shear in orthogonal directions, complete wavefront information can readily be obtained.
Specific details and further features of the invention will be set out hereinafter in conjunction with the detailed description of the preferred embodiments.
REFERENCES:
patent: 3468609 (1969-09-01), Sterrett et al.
patent: 3829219 (1974-08-01), Wyant
patent: 3921080 (1975-11-01), Hardy
patent: 4053773 (1977-10-01), Deresh et al.
Feinleib, Julius, et al., “Wideband Adaptive Optics for Imaging” Proceedings of the Society of Photo Optical Instrumentation Engineers, vol. 75, 1976 pp. 103-108 (Copy Supplied by Applicants).
Hardy John W.
Koliopoulos Chris L.
Lefebvre Joseph E.
Buczinski Stephen C.
Lenzen, Jr. Glenn H.
Raytheon Company
Schubert William C.
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