Abstract: An interferometry method including: i) forming an optical interference image by combining different portions of an optical wave front reflected from a pair of surfaces; ii) recording an interference signal at different locations of the optical interference image in response to varying the relative position of the two surfaces over a range of positions; iii) transforming the interference signal for at least one of the locations to produce a spectrum having a peak at a spectral coordinate corresponding to the variation in the relative position of the two surfaces over a range of positions; iv) identifying the spectral coordinate of the peak; and v) for each location, extracting the spectral phase of the interference signal at the coordinate of the peak. For example, the method may further include, for each of the different locations, determining a surface profile of one of the surfaces based on the spectral phase of the interference signal at each of the multiple locations.

Abstract: A multi-axis interferometer includes a mounting block with first and second polarizing beam-splitter cubes contacting first and second faces of the mounting block. A beam-distribution system contacts a third face of the mounting block. The beam-distribution system is thus placed in optical communication with the first polarizing beam-splitter cube and the second polarizing beam-splitter cube.

Abstract: Conical surfaces (and other complex surface shapes) can be interferometrically characterized using a locally spherical measurement wavefront (e.g., spherical and aspherical wavefronts). In particular, complex surface shapes are measured relative to a measurement point datum. This is achieved by varying the radius of curvature of a virtual surface corresponding to a theoretical test surface that would reflect a measurement wavefront to produce a constant optical path length difference (e.g., zero OPD) between the measurement and reference wavefronts.

Abstract: The invention features an interferometry system that uses a small angular difference in the propagation directions of orthogonally polarized components of an input beam to an interferometer. The orthogonally polarized components define reference and measurement beams for the interferometer. The angular difference allows one to distinguish between the reference and measurement beam components of the input beam and facilitates the suppression of at least some of the cyclic errors caused by interferometer imperfections.

Abstract: An method for measuring an optical thickness of a test object, the method includes: interfering a first optical wave front from the test object and a second optical wave front from a reference surface to produce an interference signal; for a selected location on the test object, obtaining an interference pattern of the test object at a first wavelength &lgr;1; for the selected location, calculating a first estimate of the optical thickness from the interference pattern recorded at wavelength &lgr;1; for the selected location obtaining an interference pattern of the test object at a second wavelength &lgr;2; for the selected location, calculating a second estimate of the optical thickness from the interference pattern recorded at wavelength &lgr;2; and for the selected location, calculating a third estimate of the optical thickness by combining the first and second estimates of optical thickness.

Abstract: An interferometric surface profiler implements a lenslet array to accommodate a large field of view (FOV) without a corresponding loss in light efficiency. Using the lenslet array, the optical surface profiler multiplexes the measurement of an interference phase over multiple spots on a measurement surface, with each spot corresponding to an element of the lenslet array. The FOV of the profiler corresponds to the area of the measurement surface spanned by the spots, and each lenslet element provides a large numerical aperture for each spot, thereby improving light efficiency. The large FOV and increased light efficiency are useful in scanning white light interferometry, as well as other types of interferometric analysis of surface form and roughness such as phase shifting interferometry.

Abstract: Methods and apparatus that combine dispersion interferometry with refractometry to compensate for refractive index fluctuations in the measurement path of a dispersion interferometer over both short and long time periods. Dispersion and refractometry data are weighted over appropriate time intervals, and means and methods are also provided for initializing &Ggr;, the inverse dispersive power, so that the dispersion and refractometry data are self consistent. A refractometer is placed in close proximity to the measurement path of the dispersion interferometer to experience substantially the same air flow and act as a surrogate for obtaining information about the index of refraction.

Abstract: Methods and apparatus that combine dispersion interferometry with refractometry to compensate for refractive index fluctuations in the measurement path of a dispersion interferometer over both short and long time periods. Dispersion and refractometry data are weighted over appropriate time intervals, and means and methods are also provided for initializing &Ggr;, the inverse dispersive power, so that the dispersion and refractometry data are self consistent. A refractometer is placed in close proximity to the measurement path of the dispersion interferometer to experience substantially the same air flow and act as a surrogate for obtaining information about the index of refraction.

Abstract: Interferometric apparatus and method by which polarization effects and stage yaw and pitch are substantially reduced. A beam redirecting means selected from the group consisting of corner mirrors, prisms, diffractive elements, holographic elements, and combinations thereof are fixedly mounted on a body capable of rectilinear motion for movement therewith. The measurement beam path to and from the redirecting means is folded at least once so that incoming and outgoing beam segments are spatially separated and substantially parallel to one another and the reference beam.

Abstract: Interferometers utilizing polarization preserving optical systems by plane polarized beams are deviated through preselected angles without changing their linear state of polarization. The interferometers utilizing such optical systems have a variety of applications and are particularly suitable for use in the field of distance measuring interferometry (DMI) to enhance measurement accuracy by reducing undesirable polarization effects that can introduce errors associated with an otherwise present undesirable polarization rotation found in classical retroreflectors. Prismatic optical elements are preferably used to construct assemblies which can include polarization beam splitting coating arrangements and/or birefringent materials to enhance the extinction ratio between orthogonally polarized beams propagating through such systems.

Abstract: Polarization interferometric architectures, preferably plane mirror types, having first and second measurement legs are provided with retardation elements, preferably, at least one set of split waveplates, comprised of two segments, tilted slightly (e.g., 2 mrad) in opposite directions with respect to each other to reduce the effects of undesirable ghost beams that otherwise would travel along the same path as the principal beams to produce significant cyclic errors. With the use of the split waveplates, double reflection ghost beams have net tilts relative to the principal measurement beam, and therefore do not contribute to interference effects. This effectively eliminates waveplate ghost reflections as a source of error. Use of the such split, tilted waveplates may be made in a variety of polarization interferometers including, but not limited to, uncompensated plane mirror, high stability plane mirror, differential plane mirror, double differential plane mirror, and dual linear/angular types.

Abstract: The frequency separation between two light signals, one having a first and the other having a second frequency, is determined by first forming three light beams, each including both frequencies; directing the three beams through an interferometer in which they traverse three optical paths having three different known optical path lengths; determining the interferometric phases for each of the three optical paths at both frequencies; and determining the difference between the first and second frequencies from the interferometric phases and known optical path lengths. The light signals may be light from two lasers operating at different frequencies. The optical path lengths can be predetermined by calibration using two light sources of know frequencies. An apparatus for carrying out this method is also disclosed.

Abstract: A method and apparatus for distance measurement includes an optical interferometer, a two-color source characterized by a synthetic wavelength, and processing means for acquiring and analyzing phase information. The synthetic wavelength, which corresponds to the spatial period of phase coincidence in two-color interferometry, is commonly used in metrology to increase the unambiguous distance-measurement range. The invention provides a greatly enhanced unambiguous measurement range, for a given synthetic wavelength, over the unambiguous range of prior-art techniques. This extended range is achieved by analysis of a phase couple formed by a combination of the synthetic and optical phases. This phase couple repeats at spatial intervals which may be very much larger than the synthetic wavelength. Detailed computational methods for illumination source selection and for data analysis are provided, and several specific embodiments of the invention are described.

Abstract: An interferometer with a dual laser source having a tunable frequency separation between the laser emissions is used to measure absolute distance to weakly-reflecting targets. A dual-laser source is commonly characterized by a synthetic wavelength, which is equal to the speed of light divided by the frequency separation with time over a total frequency range that is small compared to the average frequency separation. Simultaneously, a high-speed phase modulator generates a signal whose amplitude is proportional to the interferometric fringe visibility, and data acquisition means are used to record the fringe visibility as a function of time during the synthetic wavelength chirp. Signal processing means are then used to extract the frequency and phase of the resultant quasi-periodic fringe-visibility curve. The optical path is then determined to high accuracy without the phase ambiguity problems of prior-art synthetic-wavelength techniques.

Abstract: A laser system for measuring dimensional aberrations across a target surface includes a laser diode for producing a diverging beam of laser emission. A mask spaced from the diode in the beam has an aperture therein and can be moved so as to translate the aperture to selected locations laterally with respect to the beam. The mask blocks emission from impinging on the target except for emission transitting the aperture. Emission reflected from a segment to the target returns through the aperture back into the laser diode. An AC current is added to the driver current to modulate the emission. A lock-in amplifier of a photodetector signal adds feedback current to the driver current to lock in phase angle, so that the feedback current is a measure of the aberrations.

Abstract: An optical transmitter includes at least one single-mode type laser diode for producing laser emission, and an external cavity with a partial reflector for reflecting a portion of the emission back into the laser. A DC current operates the laser in a first mode with a first frequency of emission. The laser is pulsed with a predetermined sequence of signal pulses each shifting the laser to a second mode with a second frequency different than the first frequency. The frequency shifting thereby constitutes information in a laser beam corresponding to the sequence of signal pulses. The laser system may be formed of a plurality of laser diodes in an array, so that the reflector reflects emission back into the diodes to lock them in the alternate modes of operation.

Abstract: An optical radar system for coherent ranging and velocimetry. Modulation of a laser diode by light backscattered from a target is used to determine distance and velocity of a target. An array of laser diodes are used to determine the velocity and topography of a target. Three laser diodes are used to determine speed and orientation of a rotating disk.