Wyko, Inc.

4d Technology Corporation

Ad Technology Corporation

James C Wyant

James E Millerd

Chiayu Ai

Neal Brock

John Hayes

Robert E Parks

Keith N Prettyjohns

Katherine Creath

Lian-Zhen Shao

A phase-difference sensor measures the spatially resolved difference in phase between orthogonally polarized reference and test wavefronts. The sensor is constructed as a pixelated phase-mask aligned to and imaged on a pixelated detector array. Each adjacent pixel of the phase-mask measures a predetermined relative phase shift between the orthogonally polarized reference and test beams. Thus, multiple phase-shifted interferograms can be synthesized at the same time by combining pixels with identical phase-shifts. The multiple phase-shifted interferograms can be combined to calculate standard parameters such as modulation index or average phase step. Any configuration of interferometer that produces orthogonally polarized reference and object beams may be combined with the phase-difference sensor of the invention to provide, single-shot, simultaneous phase-shifting measurements.

Pixelated phase-mask interferometer

The tilted relationship between the reference and test mirrors () of a Fizeau interferometer is used to spatially separate the reflections (R,T) from the two surfaces. The separate beams (R,T) are filtered through a spatial polarization element () that provides different states of polarization to the beams. The beams (R,T) are subsequently recombined to form a substantially collinear beam that is processed using a spatial-phase-shift interferometer () that permits quantitative phase measurement in a single video frame. Alternatively, two beams () with orthogonal polarization are injected into the Fizeau cavity () at different angles, such that after reflection from the reference and test optics () they are substantially collinear. Unwanted reflections are blocked at the focal plane through the use of a circular aperture (). Short coherence length light and a delay line () may be used to mitigate stray reflections, reduce measurement integration times, and implement temporal phase averaging.

Simultaneous phase-shifting fizeau interferometer

A polarizing point-diffraction plate is used to produce common-path test and reference wavefronts with mutually orthogonal polarizations from an input wavefront. The common-path test and reference wavefronts are collimated, phase shifted and interfered, and the resulting interferograms are imaged on a detector. The interference patterns are then processed using conventional algorithms to characterize the input light wavefront.

Common optical-path testing of high-numerical-aperture wavefronts

The tilted relationship between the reference and test mirrors of a Fizeau interferometer is used to spatially separate the reflections from the two surfaces. The separate beams are filtered through a spatial polarization element that provides different states of polarization to the beams. The beams are subsequently recombined to form a substantially collinear beam that is processed using a spatial-phase-shift interferometer that permits quantitative phase measurement in a single video frame. Alternatively, two beams with orthogonal polarization are injected into the Fizeau cavity at different angles, such that after reflection from the reference and test optics they are substantially collinear. Unwanted reflections are blocked at the focal plane through the use of a circular aperture. Short coherence length light and a delay line may be used to mitigate stray reflections, reduce measurement integration times, and implement temporal phase averaging.

Simultaneous phase-shifting Fizeau interferometer

A method and apparatus for measuring an absolute profile of a flat using an interferometer system that includes an interferometer adapted to support two flats, a detection system, and a computer adapted to compute the OPD (optical path difference) between surface of the two flats, wherein a first flat A! having a first surface and a second flat B! having a second surface are supported in the interferometer, with the second surface facing the first surface. The interferometer system measures the OPDs between the first and second surfaces for each pixel. The first flat A! then is rotated by a number of predetermined angles relative to its initial position and each time the OPDs are measured. The first flat A! is rotated to its initial position or 180.degree. therefrom. A third flat C! having a third surface is substituted for the second flat. The OPDs between the first and third surfaces are measured. The first flat A! is replaced by the second flat, with the second surface facing the third in an orientation mirror imaged to its original orientation. The interferometer system is operated to measure the OPDs. The computer solves first, second, and third equations to obtain the entire surface topographies of the first, second, and third surface, wherein each equation is expressed as a sum of even--even, even-odd, odd-even, and odd--odd parts so as to effectuate cancellation of terms, permitting solving of the equations for the surface topography.

Method and apparatus for absolute optical measurement of entire surfaces

The invention provides a technique for eliminating 'ripple' or ghost fringes from a wavefront transmitted by an optical window with a very small wedge angle, distortions in the wavefront being measured by an interferometer. A collimated beam produced by the interferometer is transmitted through the optical window, which is tilted so as to prevent direct reflections from entering a detector of the interferometer. The beam transmitted through the window is reflected by a return flat back through the window and transmitted to the detector. The return flat is tilted slightly in the direction of or opposite to the direction of tilt of the window, causing the re-incident angle of the returned ray to be different from the original incident angle of the collimated beam. This causes the multiple reflections within the window to be different and to be out of phase. The ghost fringes are cancelled by appropriately tilting the return flat.

Method for testing an optical window with a small wedge angle

An improved apparatus and method are described for accurately 'reconstructing' steep surface profiles, such as for aspheric surfaces, using improved two-wavelength phase-shifting interferometry, wherein single-wavelength precision is obtained over surfaces having departures of hundreds of visible wavelengths from a reference surface. The disclosed technique avoids cumulative summing of detector errors over a large detector array by computing the 'equivalent' phase for each detector independently of the intensities of other detectors. Inaccurate phase data points having an equivalent fringe contrast less than a predetermined threshold are eliminated from data from which contour maps of the aspheric surface are displayed or plotted.

Two-wavelength phase-shifting interferometer and method

An optical profiler includes a two-beam interferometer. An interference pattern produced thereby is focused onto an array of photocells. Phase shift in a reference beam of the interferometer is produced by accelerating a piezoelectric transducer supporting the interferometer mirror to a constant velocity. The velocity is maintained constant for at least 360.degree. of phase shift, during which four integrated buckets are obtained from each photocell. The outputs of the photodetector array are continuously integrated and effectively read out every 90.degree. of phase shift of the reference beam by a computer that computes a first value of phase corresponding to each photocell output from the first, second, and third integrated buckets produced by that photocell and a second phase value from the second, third, and fourth integrated bucket values obtained from that photocell. The two phase values are averaged, eliminating the effects of sinusoidal noise produced by inaccuracies in the reference beam phase at which integrated buckets are initiated and terminated. Data points at which intensity modulation produced by the reference beam phase variation is less than a noise threshold are masked from phase calculations. The intensity of the interferometer light source is automatically controlled by averaging intensity of the intereference pattern at angles that differ by 180.degree. to cancel out the effects of the interference and obtain an average intensity. The lamp voltage is automatically adjusted to maintain the average intensity.

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Tucson, AZ, United States of America

James C Wyant