Common-path Configuration Research Articles

Polarized Optical Heterodyne Profilometer

A quasi common path polarized optical heterodyne interferometer with an aluminum coated polarized beam splitter is proposed in order to sense the surface profile. The heterodyned P wave is generated by two acousto-optic modulated P waves with different frequencies in the Mach-Zehnder interferometer. So is the heterodyned S wave. Both heterodyned P wave and S wave propagate through quasi common optical path in the interferometer. An aluminum coated polarized beam splitter was used to conduct a quasi common path configuration to sense the surface profile. The stability of the interferometer was ±0.25°/0.5 h and 0.5% in phase and amplitude respectively. The height sensitivity was 2 Å and the lateral resolution was 2 µ m in the experiment. In case of a birefringence lens or a Wollaston prism is suggested to replace the aluminum coated polarized beam splitter, a novel polarized common path profilometer with better stability is expected.

Practical common-path interferometry for real-time thermal/fluid flow measurements

Interferometric technique can be a useful gross-field diagnostic tool for quantitative and qualitative evaluation of thermal/fluid flow phenomena. Here, we present a practical interferometric approach based on a common-path configuration of the interfering beams with some results for demonstration. It incorporates advances in the modern optical technology to fully exploit the potential of interferometric techniques for on-site applications. The results of the initial testing are encouraging. The approach can allow one to build a simple and compact system insensitive to external disturbances for real-time monitoring with good measurement accuracy. The system errors contributed by low-quality optics can be eliminated by employing a moire technique. The approach based on the numerical moire, which utilizes the images directly captured by a solid-state camera to avoid wet chemical processing, appears to be promising.

Effect of elliptical birefringence on the measurement of the phase retardation of a quartz wave plate by an optical heterodyne polarimeter

The elliptical birefringence of a quartz phase-retardation plate was precisely measured and discussed. A polarized optical heterodyne interferometer with a common path configuration was proposed and set up. It was used to measure the linear phase retardation of λ/4 quartz wave plate, taking the elliptical birefringence into consideration. The ratio of the amplitude of the heterodyned P and S waves versus the azimuth angle of the quartz plate was more sensitive to elliptical birefringence than was the phase retardation. The experimental data, which were well fitted to the theoretical curve, showed evidence of the existence of small elliptical birefringence in the tested λ/4 quartz wave plate. However, the results of the experiment confirm the general belief that the phase difference of the wave plate is not sensitive to the elliptical birefringence effect.

Deformation mapping and surface inspection of historical monuments

Electronic speckle pattern interferometry (EPSI) in various configurations (reference beam or shear) as well as double exposure speckle photography are applied for deformation monitoring in historical monuments. On-the-site measurements are made possible by a rigid construction of the optical head, common path configurations and portable image processing equipment. The delicate objects particularly call for non-intrusive measurement. Basic features of the techniques, their implementation and results from historical sites are presented. Inhomogeneities in plaster, paint or natural stones were detected and related to crack formation. The reversibility of deformations under mechanical load was checked. In frescos, debonds from the carrier wall could be discovered and the detrimental effects of crystalline efflorescences were studied. Correlation analysis in ESPI images showed the influence of moisture on a rough stone surface and exposed microbiological activity. Thus, valuable deformation data could be obtained that assist in the diagnostics of deterioration processes and provide information in the design of remedies.

Double focus interferometers and errors the influence of their sample setting errors

Double Focus interferometers are attractive for reducing the influence of ambient conditions due to their common-path configuration. This paper describes the influence of sample setting errors in these interferometers. They have been divided into two types, i.e. the normal illumination type and the oblique illumination type. For each type, an equation of the measurement error due to sample setting error has been derived with Gaussian optics. The measurement error of the normal illumination type becomes a parabolic function of the position on the sample surface. The derived error equations have been verified by ray tracing and by experiment. The error of the oblique type is smaller than that of the normal type.