Measurement of wedge angle of a transparent parallel plate using quasi-monochromatic light source and phase shifting interferometry

A polarization phase shifting interferometer using a cyclic path configuration for measurement of phase nonuniformities in transparent samples is presented. A cube beam splitter masked by two linear polarizers is used to split the source wavefront into two counter propagating linearly polarized beams that pass through the sample. At the output of the interferometer, the two orthogonally polarized beams are rendered circularly polarized in the opposite sense through the use of a quarter wave plate. Finally, phase shifting is achieved by rotating a linear polarizer before the recording plane. In a rectangular path interferometer, although the two counter propagating wavefronts are laterally folded with respect to each other in the interferometer arms, the beams finally emerge mutually unfolded at the output of the interferometer. This phenomenon is utilized to create a reference if the sample is introduced in one lateral half of the beam in any one of the interferometer arms. The polarization phase shifting technique is used to generate four phase-shifted interferograms, which are utilized to evaluate the phase profile of the phase sample. Experimental results presented validate the proposed technique.

Self-referenced rectangular path cyclic interferometer with polarization phase shifting

A novel interferometric method, called double shearing interferometry, is presented for profiling large-scale quasi-planar surfaces, such as semiconductor wafers, optical flats and x-ray mirrors. The surface profile is measured even if there is an inclination in the scanning stage. By adopting the common-path optical configuration and heterodyne detection, the proposed method achieves excellent resolution at the subnanometre scale and allows a robust measurement in the presence of unwanted disturbances in the measurement environment. A height resolution of 0.1 nm was achieved experimentally. The standard deviation of the analytical surface profiles was found to be 1.3 nm, even when using a conventional screw-lead scanning stage. The measured results show that the surface profile and stage inclination are determined separately. We confirmed that the resultant profile was quite consistent with that measured with a Fizeau interferometer.

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Sub-nanometre double shearing heterodyne interferometry for profiling large scale planar surfaces

A method is proposed for improving the accuracy of measuring the wedge angle of transparent plates using a lateral-shear holographic interferometer. Consecutive processing of a pair of interferograms with doubled number of interference fringes leads to reduction of the observational error by a factor of two compared to traditional interferometric methods. Data on the experimental validation of the method are presented.

Improvement of accuracy of interferometric measurement of wedge angle of plates

We present a new (to our knowledge) technique for introducing phase shifts between the laterally sheared emergent beam components of a cyclic path optical configuration (CPOC). The phase shifts are introduced by applying a small change in the angle of incidence of the incident beam due to the small angular rotation of the CPOC setup. Phase-shifting interferometry has been applied along with this phase-shifting technique for a CPOC with lateral shear to find the surface slope/profile of curved optical surfaces. Results for a spherical optical surface have been discussed. An optical setup for measurement of the surface profile of toroidal beam line mirrors of synchrotron radiation sources is proposed.

Measurement of the surface profile of a curved optical surface with rotation phase-shifting lateral shear cyclic path optical configuration.

A technique for determination of residual wedge angle of high optical quality transparent parallel plate using a reversal shear interferometer-based optical system has been discussed. In this technique the parallel plate to be tested is used to introduce angular tilt in the preset two-beam interference fringes of a reversal shear interferometer. The parallelism is calculated from the angular tilt of the fringes. The technique is more sensitive compared to techniques using Fizeau interferometer and is most suitable for highly parallel transparent plate with very low residual wedge angle.

Measurement of residual wedge angle with a reversal shear interferometer

The results of the development of a much simpler optical configuration for the long-trace profiler (LTP) are presented. The current technique employs a cyclic optical configuration to achieve zero optical path difference for the closely spaced, laterally separated laser beams. The accuracy of measurement is found to remain as good as that in the case of the widely used LTP, although the geometrical alignment problem is simplified significantly.

Y. Pawan Kumar

Papers

Measurement of residual wedge angle with a reversal shear interferometer

Long-trace profiler with cyclic optical configuration