Hughes Aircraft Company

Peter J DeGroot

David H Kittell

Guy H Hayes

Optical metrology apparatus, specifically a laser probe (1), includes a frame (10) comprised of a material selected to have a predetermined coefficient of thermal expansion. A beamsplitter (36) is coupled to the frame for generating a sample beam optical path (D) and a reference beam optical path (C). The beamsplitter is optically coupled to an optical fiber (12) that delivers radiation to and conveys radiation from the frame. A piezoelectric stack (48) has an excitation signal coupled thereto and includes a mirror (26) for phase modulating the reference beam optical path length in response to the excitation signal. The laser probe includes a first strain gauge (58) that is coupled to the piezoelectric stack and a second strain gauge (60) that is coupled to the frame. A closed loop control system (A1, A2, A3, A4, VR) varies the excitation signal in accordance with the detected strains so as to maintain the reference beam optical path length in a predetermined relationship to a path length of the sample beam optical path. This athermalizes the probe, in that any expansion or contraction of the frame is matched by the piezoelectric stack, yielding a net zero change in the non-common beam path lengths. The apparatus includes an optical fiber support (14) that is rotatably coupled to the frame for positioning the optical fiber at a desired angular position.

Apparatus for coupling an optical fiber to a structure at a desired angle

Metrology method and apparatus (10) for determining a characteristic of an object's surface. The apparatus includes a multiple wavelength source (12) having an optical output. The source is comprised of a multimode laser diode (18) for simultaneously providing a plurality of wavelengths from which a synthetic wavelength is derived for metrology purposes. The apparatus further includes a beamsplitter (30), preferably a polarizing beam splitter, coupled to the optical output for providing a reference beam (32) and a measurement beam (34), the measurement beam impinging on the surface and the reference beam being phase modulated. The apparatus also includes optical elements and devices for detecting a phase difference between the reference beam and a reflected measurement beam for determining the characteristic of the surface. A diffraction grating (60) is blazed for a nominal characteristic output of the laser diode, such as 785 nanometers, and spatially resolves the various wavelengths of the returned laser light. The resolved wavelengths are provided as two beams by a lens system (62) and are detected by a pair of photodetectors (64, 66). A data processor ( 68) coupled to an output of the photodetectors determines an interferometric phase at the two wavelengths and the surface characteristic.

Interferometric laser profilometer including a multimode laser diode

Compact laser probe for profilometry

Method and apparatus for determining if a surface of an object, such as a ball bearing (12), has a radius of curvature within a predetermined tolerance. A test system (10) includes an interferometer having a reference beam (20a) and a measurement beam (20b). The measurement beam is disposed for illuminating a surface of the object while the object is in motion. A cylindrical lens (24) is provided for forming the measurement beam into a linear, stripe-like pattern that illuminates the surface. The measurement beam reflects from the surface of the object to a beamsplitter (20) that combines the reflected measurement beam with a reflected reference beam. The test system further includes an imager (30), optically coupled to an output of the interferometer, for recording a two dimensional interferogram that results from a change in an optical path length of the measurement beam relative to an optical path length of the reference beam. The test system further includes a processor (32), coupled to an output of the imager, for comparing the interferogram recorded by the imager to a reference interferogram previously recorded from a surface of an object known to have a radius of curvature within the predetermined tolerance.

Interferometric ball bearing test station

The phase ambiguity of conventional interferometers may be removed by using two laser diodes of different optical frequency to generate a synthetic wavelength. However, the stability requirements for a two-color interferometric laser gauge that must provide unambiguous determination of the optical fringe order over a large distance can be severe. The invention determines upper limits on the optical wavelength uncertainity and expresses same as a function of optical path difference between object and reference beams, phase measurement errors and the synthetic wavelength. A wavelength stabilization arrangement involves simultaneous servo control of two laser diodes (10, 12) with a single Fabry-Perot etalon (36). An embodiment of the invention demonstrates its effectiveness for long-term stabilized two-color interferometry over a distance of 250 mm, with a 15 .mu.m synthetic wavelength and a repeatability of 40 nm. For periods of less than 1000 seconds, the repeatability is eight nm.

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Belleview, WA, United States of America

Peter J DeGroot