Optics: measuring and testing – By light interference – Having polarization
Reexamination Certificate
2001-11-02
2004-01-06
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having polarization
Reexamination Certificate
active
06674532
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to polarimetric instrumentation and associated methods for measuring the state of polarization of a light beam, and more particularly to a novel interferometric polarization interrogating filter assembly and method for measuring the spatially-varying, complete state of polarization across an image comprised of one or more optical wavefronts in a no-moving-parts, instantaneous manner.
A single optical wavefront or a plurality of such wavefronts, i.e., a wavefront ensemble, may carry with it information pertaining to the state of polarization, spectral content, radiance distribution, coherence and statistical composition. Polarization encoding of the wavefront ensemble occurs when light interacts with material media. By decoding the spatially-varying state of polarization across a partially-polarized image, it is possible to acquire additional information beyond the conventional irradiance image that enables one to discern heretofore-indiscernible characteristics of the original object.
There is a great diversity of naturally occurring and human-engineered polarization encoding schemes or “polarization signatures” in the physical world.
In astronomy, for example, it has been found that certain preferred states of polarization result when celestial dust grains scatter light from nearby stars. Some scientists speculate that such preferences in the state of polarization could help to explain why most living organisms show an overwhelming preference toward right-handed sugars and left-handed amino acids in the building of their cellular structure. See, e.g., Schneider, “Polarized Life, Astronomers Probe Orion to Answer one of Life's Mysteries”,
Scientific American
, Vol. 279, No. 4, October 1998, p. 24.
In the field of marine biology, it has been shown that certain cephalopods are sensitive to the azimuth of linearly polarized light by virtue of the orthogonal structure of photoreceptors in their retina. It has been further demonstrated that cuttlefish, i.e.,
Sepia officinalis
L., communicate by using polarized light. See, e.g., Shashar, Rutledge and Cronin, “Polarization Vision in Cuttlefish—A Concealed Communication Channel”,
The Journal of Experimental Biology
, Vol. 199, No. 9, September 1996, pp. 2077-2084.
Just as prolific, in the field of entomology, it has been shown that certain ants and bees use polarized sky light as a navigational compass. See, e.g., Wehner, “Polarized-Light Navigation by Insects”,
Scientific American
, Vol.235, No.1, July 1976, pp. 106-115.
In the semiconductor industry, ellipsometry is used to measure film thickness, refractive index and other material properties. A light beam with a controlled state of polarization whose spectral content is well defined is made to illuminate a test surface. By measuring the change in the state of polarization of the reflected beam, certain information concerning mechanical and material properties of the test surface can be inferred. See, e.g., Tompkins,
A User's Guide to Ellipsometry
, Academic Press, New York, 1993.
Terrestrial resource mapping and surveillance investigations conducted via satellite are growing fields of study. Contrast enhanced polarization images of objects on the ground, as viewed from a low-flying satellite, can provide additional information to definitively identify those objects.
In fiber-optic components and systems employed in the telecommunications industry, signal polarization plays a crucial role. Accordingly, it is desirable to measure the state of polarization of a time-varying signal exiting from a single-mode fiber in an instantaneous manner.
In the military, polarization of light may be used to identify vehicles with friendly or hostile intent under less-than-ideal viewing conditions. Aircraft, for example, have quasi-specularly reflecting surfaces, which give rise to unique polarization signatures. See, e.g., Duggin, “Imaging Polarimetry in Scene Element Discriminatiorn”, in Goldstein and Chenault (eds.), “Polarization: Measurement, Analysis and Remote Sensing II”,
Proc. Soc. Photo
-
Opt. Instrum. Eng.
, Vol. 3754, July 1999, pp. 108-117.
Traditional approaches for measuring the state of polarization of an arbitrary wavefront ensemble include (1) discrete time-sequential, (2) continuous time-sequential, i.e., polarization-modulation, (3) division-of-amplitude, (4) division-of-aperture, and (5) interferometric methods. See, e.g., Hauge, “Survey of Methods for the Complete Determination of a State of Polarization”,
Proc. Soc. Photo
-
Opt. Instrum. Eng.
, Vol. 88, 1976, pp. 3-10 and Hauge, “Recent Developments in Instrumentation in Ellipsometry”,
Surface Science
, Vol. 96, Nos. 1-3, June 1980, pp. 108-140.
Whereas all of these approaches have been utilized in the past for measuring the complete state of polarization in a point context, few are amenable to measuring the complete state of polarization across an image. Difficulties often arise in practice that limit the general usefulness of such methods for imaging polarimetry applications.
Arguably, the most common approach for measuring the spatially-varying state of polarization across an image employs a discrete or continuous time-sequential measurement modality in which a certain amount of time is required in between each step of the measurement process.
In the discrete time-sequential case, a set of polarization interrogating elements, i.e., polarizers, retarders or combinations thereof, are inserted one-by-one into the wavefront ensemble of interest and image acquisition is repeated for each element of the set. At least four separate elements and their corresponding images are required to compute the complete state of polarization across a partially-polarized image.
Walraven is generally accredited for being the first investigator to employ discrete time-sequential measurement methods using a 35 mm film-based camera. In his work, the transmission axis of a linear polarizer was manually oriented at 0, 45, 90 and 135 degrees respectively between sequential exposures of the same scene. See, e.g., Walraven, “Polarization Imagery”,
Optical Engineering
, Vol. 20, No. 1, January 1981, pp. 14-18. Although his work lacked determinacy with respect to the handedness of the polarization state across the image and his scene selection was severely restricted to static objects due to the time-sequential nature of his measurements, Walraven was successful in pointing out the importance of polarization image content.
In the continuous time-sequential case, one or more polarization interrogating elements are rotated in a continuous manner and the complete state of polarization is computed via Fourier analysis of the polarization-modulated signal. See, e.g., Berry, Gabrielse and Livingston, “Measurement of the Stokes Parameters of Light”,
Applied Optics
, Vol. 16, No. 12, December 1977, pp. 3200-3205.
As described by Aspnes in U.S. Pat. No. 3,985,447 and elsewhere by Azzam and others, conventional ellipsometric instrumentation frequently employs a continuous time-sequential measurement approach in a point context. Accordingly, in order to measure spatial variations in surface properties the point is methodically scanned across the surface and an “image” of the desired property is constructed by mathematically “stitching” the data together. Such scanning methods and numerical processing techniques are inherently time consuming for high-resolution images.
In polarization component metrology, Chipman has successfully employed continuous time-sequential measurement methods to compute the spatially-varying polarimetric properties of various optical components. See, e.g., Chipman and Somsin, “Mueller Matrix Imaging Polarimetry: An Overview”, in Yoshizawa and Yokota (eds.), “Polarization Analysis and Applications to Device Technology”,
Proc. Soc. Photo
-
Opt. Instrum. Eng.
, Vol. 2873, June 1996, pp. 5-12.
More often than not, spatially-varying polarimetric properties of optical components do not vary as a function of time. In such cases, a time-sequential measurement
Birdwell Janke & Durando, PLC
Connolly Patrick
Turner Samuel A.
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