Optics: measuring and testing – By light interference – Having polarization
Reexamination Certificate
1999-03-15
2001-01-30
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having polarization
C356S511000
Reexamination Certificate
active
06181430
ABSTRACT:
The invention relates to the art of interferometry, and more particularly to multi-color interferometry for measuring the position of the surface of a solid or liquid object.
BACKGROUND OF THE INVENTION
Interferometers are widely used for measuring the profiles of smooth surfaces. In these devices, interfering beams are reflected from different areas on a solid surface and interferometric techniques are used to detect variations in surface height by measuring the path differences between the two interfering beams as the beams are moved across the solid surface. The interferometry transforms phase information encoded on a beam of coherent light into an intensity modulation that can be detected by conventional “square-law” detectors. The encoded information is typically a representation of fringes, in time or space, whose repeat frequency is determined by the period of one cycle of the optical wavelength. The fringe separation can be scaled somewhat by the geometry of the apparatus, but in general one fringe cannot be distinguished from another. As a result, it is not practical to identify a single fringe uniquely. This problem can be partially remedied when the bandwidth of the light is selected to give the highest contrast in the “zero-order” fringe, and which the contrast is reducing in either direction. If the contrast of the fringe reduces rapidly enough to give unambiguous identification, the distance over which a useful measurement can be discriminated is proportionally altered (i.e. reduced or increased).
In addition to the fringe detection problems, typical interferometers are notoriously difficult to keep stable. Since the optical paths of light in different routes are the source of the desired measurement, any differences arising from effects other than that to be measured are ruinous to the accuracy. These include mechanical vibration; thermal expansion; intensity modulation; birefringence effects; spatial and temporal coherence reduction; optical, mechanical and/or electronic drift; and/or other unanticipated operational problems.
Previous interferometers have been able to measure relative surface displacements over a limited range. This has made it impossible to contour surfaces containing steps or other discontinuities. Previous interferometers have also used bulk optics for guiding and mixing the light. Bulk optics suffer from alignment, stability and fringe contrast problems.
In view of the deficiencies of present interferometers used to measure relative surface displacement, there is a need for an interferometer that is simple to use, overcomes past stability and fringe contrast problems, and can measure relative surface displacements over a larger range than heretofore.
SUMMARY OF THE INVENTION
The present invention relates to an improved apparatus and method of measuring surface characteristics of solids and liquids, and more particularly, to a multi-color interferometer that measures the absolute displacement and contouring of a solid or liquid.
The invention overcomes the problems associated with alignment, stability and fringe contrast, and provides useful performance in a range of applications and environments not formerly accessible. The invention significantly expands the applicability of an interferometer by overcoming the restricted dynamic range arising from the inability to distinguish between adjacent “fringes”, either in time or space. The dynamic range is typically restricted to the physical parameter represented by a single fringe in any given geometry. By shifting the fringe in time or space, the directional ambiguity can be resolved and the dynamic range can be extended by keeping count of fringes as they pass, thus increasing the relative dynamic range. However, fringe shifting alone cannot overcome the problems resulting from the temporary loss of a fringe which occurs by a discontinuity in the measured quantity, i.e. a step in the surface of a solid object.
In accordance with one aspect of the present invention, there is provided an interferometer that utilizes a plurality of wavelengths to measure the surface characteristics of a solid or liquid such as, but not limited to, the position of the surface of the solid or liquid. The dynamic range of the interferometer is increased by the inclusion of multiple colors of monochromatic radiation into the same optical path. Preferably, three colors are used; however two colors are sufficient to exploit advantages of the improved interferometer, and more than three colors can produce various advantages in certain applications. The use of a plurality of colors enables the interferometer to obtain absolute displacement measurements. Preferably, the colors used in the interferometer are formed by three single longitudinal mode lasers of three different coherent emission wavelengths. As can be appreciated, other types of lasers can be use (i.e. the use of one or more modulated lasers that generate a plurality of wavelengths. When three or more wavelengths of light are used, two of the wavelengths preferably have a similar, but not exact, frequency and at least one of the remaining wavelengths has a wavelength frequency that is significantly different from the two wavelengths of light having a similar frequency. By selecting such a wavelength frequency relationship, the dynamic range of the interferometer is significantly increased. Preferably the wavelengths of light are selected from the visible range; however, one or more wavelengths can outside the visible light range.
In accordance with another aspect of the present invention, there is provided an interferometer that is able to detect the change in the direction of displacement of the surface of a solid or liquid as the solid or liquid is passed through an interference node. The detected change in direction of displacement is accomplished without the need of sophisticated and expensive acoustic optic modulators. The interferometer is designed to determine the direction of motion by taking the ratio of the intensities of the fringes of the different colors of light used by the interferometer. By selecting wavelengths that are not integer multiples of one another, the relative amount of intensity for each wavelength of light will have a different proportion on each side of the fringe node that arises from each wavelength of light. In addition, not all of wavelengths of light simultaneously pass through the intensity node at the same time. The fringes for each wavelength of light are thus formed by the light which is returned from the surface and then mixed with light of the same wavelength from the coherent laser which produced the particular wavelength.
In accordance with still another aspect of the present invention, there is provided an interferometer that includes the use of optics to improve the detection of fringe contrast. In one preferred embodiment, monomode optics are incorporated into the interferometer and at least partially function as low-pass spatial filters. The monomode optics allow the fringe contrast between light and dark fringes to approach their theoretical limit. One preferred monomode optic that can be used, but is not limited to, are single-mode optical fibers.
In accordance with yet another aspect of the present invention, there is provided an interferometer that includes a diffraction grating. The diffraction grating allows multiple wavelengths to be launched from optics onto a fluid or solid surface. In one preferred embodiment, the diffraction grating is transparent. In another preferred embodiment, the diffraction grating causes light passing through the diffraction grating to be approximately co-extensive and propagate in substantially the same direction. The light is preferably collimated light and directed at certain angles into the diffraction grating. The angle of the collimated light is chosen in connection with the period and blaze properties of the grating so that the collimated light will be approximately coextensive and propagate in substantially the same direction. In yet another preferred embodiment, the dif
Meyer William V.
Smart Anthony E.
Ohio Aerospace Institute
Turner Samuel A.
Vickers Daniels & Young
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