Device for interferometric detection of surface structures

Details Device for interferometric detection of surface structures Device for interferometric detection of surface structures

1991-08-13

1994-03-08

Turner, Samuel A.

Optics: measuring and testing

By polarized light examination

With birefringent element

356349, 356359, 356376, 356445, G01B 1124

Patent

active

052932150

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The invention relates to a device for interferometric detection of surface structures by measurement of the phase difference in laser speckle pairs in the measuring points of this surface.


BACKGROUND

Such a device and a corresponding method are extensively described in U.S. Pat. No. 4,652,131, FERCHER/CARL ZEISS, and corresponding German Published, Non-Examined Patent Application DE-OS 33 18 678 or in the reference book "Laser Speckle". J. C. Dainty, Springer Verlag, Berlin, Heidelberg, New York, 1975. In the known device, the light emitted by a laser is split into a reference and a measuring beam, and a device shifts the frequency of the reference beam. The measuring beam is reflected on the rough surface which is to be measured and is then shown in the interferogram plane, together with the reference beam. With rough surfaces of this kind, a speckle pattern is obtained in the interferogram plane, instead of the customary interferogram. It is then possible to draw interferences as to the geometric shape of the rough surface by determining the phase difference such as described in more detail in the aforementioned prior art.
A complicated optical system with lenses, mirrors and prisms, as well as a frequency converter, not further described, is used for generating the different wavelengths or frequencies of the two beams. However, not only is such a system complicated and expensive to manufacture, but also hard to adjust to the required exactness. Furthermore, this arrangement requires such a large space that it is hard to contain it in a small and handy device.


Advantages of the Invention

In contrast to this, the device in accordance with the invention has the advantage that it is possible to use a very simple arrangement of optical fiber-conduits for beam splitting and frequency conversion. Optical couplers and splitters for such fiber conduits are available in the form of common components. Because of this, beam guidance is variable and does not need any adjustment, at least in this area. Frequency shifting is achieved by a predetermined lengthening of a fiber line and frequency modulation or wave length modulation, because of which the "frequency converter" becomes very simple and cost-saving. Since the fiber lines can be placed very closely next to each other without any exactness being required, the entire arrangement not only becomes very cost-saving, but also very compact.
A particular problem in measuring consists in that, with the known methods, the height differences to be measured on the measured surface must be less than one-half of a wavelength of the measuring beam. Height differences which are one-half of a wavelength apart result in the the same signal, namely a phase difference of 360.degree.. To meet this problem, it is necessary with the known measuring devices to select the wavelength of the measuring beam appropriately, i.e. certain prior information or estimates regarding the roughness and structure of the surface to be measured are required. This problem recedes into the background when a plurality of coupled laser sources with different wavelengths is employed. The phase differences are determined in a known shape from the interferometrically superimposed signals separately from each laser source in the evaluation device and subsequently a comparison of the different phase differences detected is performed. In the course of subtraction, clear differences now appear in regard to whether the differences in the height structure are less than half a wavelength or whole number multiples. Adaptation of the wavelength to the appropriate surface to be measured is no longer necessary within a wide margin, so that it is also possible to examine very different surfaces with the same measuring device without special setting.
A simple structural embodiment is achieved in that the splitter device is provided with an optical coupler for the at least two optical fiber lines on the input side.
The wavelengths of the different laser sources differ only slightly. The exa

REFERENCES:
patent: 4652131 (1987-03-01), Fercher et al.
patent: 4681395 (1987-07-01), Lindsay et al.
patent: 4759627 (1988-07-01), Thylen et al.
patent: 4789237 (1988-12-01), Inoue et al.
patent: 4830486 (1989-05-01), Goodwin
Thierry Gys "A Simple Multimode Fiberoptic Vibrometer", Nov. 1985, pp. 79-82.
A. F. Fercher, H. Z. Hu. & U. Vry, "Rough Surface interferometer with a two-wave length heterodyne speckle interferometry", Jul. 1985, No. 14 pp. 2181-2188.
I. A. Massie, R. D. Nelson, & S. Holly, "High-Performance real-time heterodyne interferometry", Jun. 1, 1979, vol. 18, No. 11, pp. 1797-1803.
K. Leonhardt, K. -H. Rippert & H. J. Tiziani "Optische mikroprofilometrie und Rauheitsmessung".
"Optical microprofilometry and roughness measurement", 1987, No. 6, pp. 243-252.

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