Two-Wavelength Holography

Direct phase measurement of aspheric surface contours

  • Katherine Creath and James C. Wyant
  • Proceedings of SPIE, Vol. 645, page 101, 1986
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Two-wavelength holography and phase-shifting interferometry are combined to measure aspheric surface contours with variable sensitivity. In this technique, the surface is effectively tested at a synthesized longer equivalent wavelength leq=la lb/Abs[la – lb] using measurements made at wavelengths la and lb where the difference of the phases measured for la and lb yields modulo 2p phase at leq. A mask of point apertures is placed over the detector array in order to resolve closely spaced fringes. This technique has an rms repeatability of leq/100. Limits to this technique are discussed and results are shown.

Contouring aspheric surfaces using two-wavelength phase-shifting interferometry

  • Katherine Creath, Yeou-Yen Cheng, and James C. Wyant
  • Optica Acta, Vol. 32, page 1455, 1985
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Two-wavelength holography and phase-shifting interferometry are combined to measure the phase contours of deep wavefronts and surfaces, such as those produced by aspherics, with a variable sensitivity. When interference fringes are very closely spaced, the phase data contain high frequencies where 2p ambiguities cannot be resolved. In this technique, the surface is tested at a synthesized longer equivalent wavelength. The phase of the wavefront is calculated modulo 2p using phase-shifting techniques at each of two visible wavelengths. The difference between these two phase sets is the phase of the wavefront as it would be measured at leq =l1l2/|l1-l2|, assuming that 2p ambiguities can be removed at leq. This technique enables surfaces to be contoured to an accuracy of leq/l00.

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-Å rms (l = 6328 Å).

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 1024 element detector array. Without temporal averaging, the repeatability of measuring a surface having a sag of ~100mm is better than 25-Å (0.0025%) rms.

Testing aspherics using two-wavelength holography: use of digital electronic techniques

Two-wavelength holography has been shown to be quite useful for testing aspheric surfaces since it can produce interferograms with a wide range of sensitivities. However, TWH has the drawback that the accuracy attainable from measurements on photographs of the fringes is limited. It is shown how this limitation can be overcome by using electronic techniques to evaluate the phase distribution in the interference pattern.

Two-wavelength holographic interferometer

It has been demonstrated that a Bi12SiO20 crystal is a good holographic recording material. Using this crystal as a real-time recording device, a two-wavelength holographic interferometer has been constructed. The 488- and 514.5-nm lines of an Ar-ion laser were used as the source to yield an equivalent wavelength of 9.47 mm.

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 µ, 9.47 µ, 14.20 µ, 20.22 µ, and 28.50 µ obtained by using an argon laser for the visible light source.

 

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