James C. Wyant

Two-wavelength phase shifting interferometry

This paper describes a technique that combines ideas of phase shifting interferometry (PSI) and two-wavelength interferometry (TWLI) to extend the phase measurement range of conventional single-wavelength PSI. To verify theoretical predictions, experiments have been performed using a solid-state linear detector array to measure 1-D surface heights. Problems associated with TWLPSI and the experimental setup are discussed. To test the capability of the TWLPSI, a very fine fringe pattern was used to illuminate a 1024 element detector array. Without temporal averaging, the repeatability of measuring a surface having a sag of ~100 μm is better than 25-A (0.0025%) rms.

Multiple-wavelength phase-shifting interferometry

This paper describes a method to enhance the capability of two-wavelength phase-shifting interferometry. By introducing the phase data of a third wavelength, one can measure the phase of a very steep wave front. Experiments have been performed using a linear detector array to measure surface height of an off-axis parabola. For the wave front being measured the optical path difference between adjacent detector pixels was as large as 3.3 waves. After temporal averaging of five sets of data, the repeatability of the measurement is better than 25-A rms (λ = 6328 A).

Use of an ac heterodyne lateral shear interferometer with real-time wavefront correction systems

An analysis is performed to determine the accuracy with which an ac heterodyne lateral shear interferometer can measure wavefront aberrations if a white light extended source is used with the interferometer, and shot noise is the predominate noise source. The analysis shows that for uniform circular or square sources larger than a derived minimum size, the wavefront measurement accuracy depends only upon the radiance of the source and not upon the angular subtense of the source. For a 1-msec integration time, a 25-cm(2) collecting area, and a source radiance of 10 W/m(2)-sr the rms wavefront error is approximately 1/30 wave, assuming the signal is shot noise limited. It is shown that for both uniform circular and square sources an optimum shear distance is approximately (1/2) the aperture diameter required to resolve the light source. Comments are made on the optimum shear for nonuniform radiance distributions.

Improved vertical-scanning interferometry

We describe a method that combines phase-shifting and coherence-peak-sensing techniques to permit measurements with the height resolution of phase-shifting interferometry without the interval-slope limitation of lambda/4 per data sample of phase-shifting interferometry. A five-frame algorithm is used to determine both the best-focus frame position and the fractional phase from the best-focus frame of the correlogram acquired through vertical scanning. The two surface profiles retrieved from the phase and the modulation contrast of the correlograms are compared in the phase-unwrapping process to remove fringe-order ambiguity.

Testing Aspherics Using Two-Wavelength Holography

It is shown that both single exposure and double exposure two-wavelength holography provide a good method of using visible light to obtain an interferogram identical to what would be obtained if a longer nonvisible wavelength were used. Both techniques provide for the real-time adjustment of defocus and tilt in the final interferogram. When both hologram exposures are made simultaneously, the sensitivity to air turbulence is essentially the same as if the longer nonvisible wavelength were used. Results are shown for testing both lenses and mirrors at equivalent wavelengths at 6.45 micro, 9.47 micro, 14.20 micro, 20.22 micro, and 28.50 micro obtained by using an argon laser for the visible light source.

Analysis of a micropolarizer array-based simultaneous phase-shifting interferometer.

Recent technological innovations have enabled the development of a new class of dynamic (vibration-insensitive) interferometer based on a CCD pixel-level phase-shifting approach. We present theoretical and experimental results for an interferometer based on this pixelated phase-shifting technique. Analyses of component errors and instrument functionality are presented. We show that the majority of error sources cause relatively small magnitude peak-to-valley errors in measurement of the order of 0.002-0.005lambda. These errors are largely mitigated by high-rate data acquisition and consequent data averaging.

An Optical Profilometer for Surface Characterization of Magnetic Media

Conventional surface-characterization techniques either are not sophisticated enough to provide complete surface-topographical data or cannot be employed because of the relatively low hardness of magnetic media. An optical profilometer has been developed which provides a noncontact method of obtaining surface characteristics from a magnetic medium. The system consists of a standard Leitz reflection microscope, a Mirau interferometer controlled by a piezoelectric transducer, a linear array of photodiode detectors, and a microcomputer. The combination yields a system that measures the optical-height variations of surfaces to a high degree of precision. This height variation is processed by a computer to provide surface-topographical statistical parameters, which are useful to predict tribological and magnetic performances of the head-media interface. Sample data of magnetic media (tape, floppy disk, and rigid disk) are presented. Presented at the 38th Annual Meeting in Houston, Texas, April 24–28, 1983

White Light Interferometry

White light interferometry is an extremely powerful tool for optical measurements. This paper discusses the advantages and disadvantages of white light interferometry compared to laser light interferometry. Three different white light interferometers are discussed: 1) diffraction gratin interferometers; 2) vertical scanning or coherence probe interferometers, and 3) white light scatterplate interferometers.

Fringe modulation skewing effect in white-light vertical scanning interferometry

An interference fringe modulation skewing effect in white-light vertical scanning interferometry that can produce a batwings artifact in a step height measurement is described. The skewing occurs at a position on or close to the edge of a step in the sample under measurement when the step height is less than the coherence length of the light source used. A diffraction model is used to explain the effect.

Effect of piezoelectric transducer nonlinearity on phase shift interferometry.

If the nonlinearity of the motion of a piezoelectric transducer (PZT) can be described as a quadratic function, the integrated intensity of one frame in phase shift interferometry can be calculated using the Fresnel integral. For a PZT with smaller nonlinearity, the rms phase error is almost linearly proportional to the quadratic coefficient The effects of PZT nonlinearity on the three- and the four-bucket algorithms are compared.