Optical: systems and elements – Collimating of light beam
Utility Patent
1999-05-14
2001-01-02
Epps, Georgia (Department: 2873)
Optical: systems and elements
Collimating of light beam
C359S572000, C359S722000, C359S317000
Utility Patent
active
06169634
ABSTRACT:
TECHNICAL FIELD
The present invention relates to optical metrology, and, in particular, to an apparatus and methods for reducing or eliminating speckle noise.
BACKGROUND ART
Noise levels in optical systems, whether directed toward imaging or measurement of an illuminated object or scene, are detrimentally affected by speckle. Speckle arises because of interference effects due to the coherence of the illuminating light. Light may reach a given detector element via multiple paths. If the light rays reaching the detector from two points on the surface of the illuminated scene bear a defined phase relationship with respect to one another, the detected intensity will be a function of that phase, increasing when the rays are in phase, and decreasing when the rays are out of phase. While useful in certain applications, speckling generally degrades imaging, adding a noise component which may approach the level of the signal itself.
Thus, in order to despeckle the illumination, it is useful to decohere any coherent light source. Various methods are known, including the use of a fiber optic multichannel array, as described in U.S. Pat. No. 5,029,975. A survey of known speckle reduction techniques is provided by T. S. McKechnie, “Speckle Reduction,” in J. C. Dainty, ed.,
Laser Speckle and Related Phenomena
, Springer, Berlin, 1975, pp. 123-170, which is incorporated herein by reference.
One example of a metrological application that may be adversely affected by speckle is that of conoscopy, a form of holography that may be practiced using incoherent light. In ordinary holography, coherent light emanating from a source region is caused to interfere with a coherent reference beam in order to construct an interferogram in which the characteristics of the source region in two or three dimensions are encoded. Conoscopy is a distinct interferometric technique capable of determining the distance to one or more points within an object volume without employing a reference beam. Instead, light emanating from a source region is prepared in a defined state of polarization and then passed through an anisotropic optical element in which one polarization suffers phase retardation with respect to the other. The two polarization components emerging from the anisotropic optical element interfere with one another, producing a interferogram in the detector plane. Conoscopy is the subject of various patents, including U.S. Pat. Nos. 4,602,844, 4,976,504, 5,081,540, and 5,081,541. The source region may be illuminated with a grid (or “cloud”) of points, or with a line. However interference effects attributable to the coherence of the source are detrimental to the sensitivity of the measurement technique.
Accordingly, for this and other metrological techniques, it is desirable to overcome speckle.
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the present invention, there is provided a method for converting a beam of light that is substantially coherent and has a coherence length into substantially incoherent radiation. The method has the steps of:
a. collimating the beam for creating a substantially collimated beam;
b. delaying a first portion of the substantially collimated beam by a first optical delay by ballistic passage through a first cell of an optical element; and
c. delaying each of a plurality of portions of the substantially collimated beam different from the first portion of the substantially collimated beam by an optical delay different from the first optical delay by an amount exceeding the coherence length of the beam. Additionally, the step of delaying may include directing the plurality of portions of the substantially collimated beam through an optic having a plurality of cells of mutually different effective optical length.
In accordance with an alternate embodiment of the invention, a method is provided for reducing speckle in reflection from a scene illuminated by a beam produced by a substantially coherent source of light. The method has the steps of directing the beam of light through a wedge module so as to illuminate the scene and detecting light reflected from the scene. The wedge module has a first optically anisotropic wedge, a second optically anisotropic wedge inclined relative to the first optically anisotropic wedge, and an optical compensation plate having an interface parallel to a face of the first optically anisotropic wedge and a face of the second optically anisotropic wedge so as to compensate any change in optical path of the beam of light.
REFERENCES:
patent: 4521075 (1985-06-01), Obenschain et al.
patent: 4619508 (1986-10-01), Shibuya
patent: 4744615 (1988-05-01), Fan et al.
patent: 1 136 457 (1982-11-01), None
patent: WO 97/42891 (1997-11-01), None
Ripin, Barrett H.,Induced Spatial Incoherence Optical Delay Element, Navy Technical Disclosure Bulletin, vol. 10, No. 4, Jun. 1985 (1985-06), pp. 25-32 XP002112467.
McKechnie, T.S., “Speckle Reduction”, in Dainty, J.C.,Laser Speckle and Related Phenomena, XP002112684, Chapter 4, pp. 123-170, Berlin (1975).
Bromberg & Sunstein LLP
Epps Georgia
Lucas Michael A
Optimet, Optical Metrology Ltd
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