Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1999-04-01
2001-05-08
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
Reexamination Certificate
active
06229616
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally related to optical heterodyne wavefront sensors, and more particularly to an optical heterodyne wavefront sensor that uses heterodyne interferometry to measure a two dimensional optical wavefront.
2. Description of the Prior Art
A conventional laser beam wavefront sensor responds to a two dimensional light beam and uses a camera to record an interferogram of the phase of the beam across its aperture. The phase information is used to correct aberrated beams, such as may occur when a light beam is propagated and steered through a long distance or through a turbulent atmosphere. The camera is usually read serially, and thus requires a relatively long period of time to produce a result. This is undesirable for high speed applications or for rapidly changing wavefronts.
An example of a heterodyne interferometer system is found in U.S. Pat. No. 4,688,940, “Heterodyne Interferometer System”, invented by Gary E. Sommargren and Moshe Schaham. As disclosed, the heterodyne interferometer system utilizes a single stabilized frequency linearly polarized laser input beam from a light source. This is provided to an acousto-optic device along with a frequency stabilized electrical reference signal for transforming the input beam into a pair of orthogonally polarized beams. The beams differ in frequency by the reference signal frequency prior to providing the beams to a polarization type interferometer. A mixing polarizer mixes the beams after they traverse the interferometer and provides the mixed beams to a photoelectric detector where they are processed into an electrical measurement signal. The electrical measurement signal is processed in a phase meter/accumulator along with the reference signal to produce an output signal which is the sum of the single resultant phase difference on a cycle-by-cycle basis between the measurement signal and the reference signal. The phase meter/accumulator includes an analog-to-digital converter and a memory register for the previous cycle, with the measurement resolution being determined by the number of bits of the analog-to-digital converter. The system, however, does not measure the phase of an entire field or serve to correct the individual phase differences of the array of subapertures that constitute the aberrated beam, such as one which has propagated through a turbulent environment where the phase of each subaperture across the beam varies.
What is needed, therefore, is an optical heterodyne wavefront sensor that is capable of simultaneously measuring the phase of each subaperture across an optical wavefront, that allows two dimensional measurements of the state of phase of the wavefront in parallel, and that is capable of measuring the optical phases of more than one wavelength.
SUMMARY OF THE INVENTION
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides generally an optical heterodyne wavefront sensor.
Briefly the optical heterodyne wavefront sensor uses a radio frequency (RF) signal for measuring an optical wavefront having a state of phase that differs throughout its aperture. It comprises a lens, optical fiber and optical frequency shifter arrangement to develop a reference optical wavefront having substantially the same phase throughout and that is shifted in frequency by an amount corresponding to the RF. A beam combiner interferometrically combines the optical wavefront and the shifted reference optical wavefront into a heterodyne optical signal at the RF frequency, each subaperture of which has a phase that corresponds to the state of phase of a like subaperture of the optical wavefront. An array of lenses focuses the optical heterodyne signal into an array of photodetectors, each responsive to a subaperture and that produces an electrical signal at the optical beat frequency corresponding to the RF and having a phase corresponding to the state of phase of the corresponding subaperture of the optical wavefront. A plurality of circuit chains, each including a digital divider, responds to the RF signal and a corresponding one of the array of electrical signals and with an exclusive OR circuit and an integrating circuit develops a like plurality of output signals that comprise voltages that linearly correspond to the state of optical phase of a subaperture of the optical wavefront. The composite output signals represent the optical state of phase of the incoming optical wavefront.
The foregoing and additional features and advantages of this invention will become apparent from the detailed description and accompanying drawing figures. below. In the figures and the written description, numerals indicate the various elements of the invention, like numerals referring to like elements throughout the drawing figures and the written description.
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Brosnan Stephen J.
Heflinger Donald G.
Heflinger Lee O.
Font Frank G.
Keller Robert W.
Natividad Phil S.
TRW Inc.
Yatsko Michael S.
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