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Optical Interference
Published in Rajpal S. Sirohi, Optical Methods of Measurement, 2018
Two-beam interferometers using division of wavefront have been used for the determination of the refractive index (Rayleigh interferometer), determination of the angular size of distant objects (Michelson stellar interferometer), deformation studies (speckle interferometers), and so on. Interferometers based on division of amplitude are used for the determination of wavelength (Michelson interferometer), the testing of optical components (Twyman–Green interferometer, Zygo interferometer, Wyco interferometer, and shear interferometers), the study of microscopic objects (Mirau interferometer), the study of birefringent objects (polarization interferometers), distance measurement (modified Michelson interferometer), and so on. The Michelson interferometer is also used in spectroscopy, particularly in the infrared region, and offers Fellgett’s advantage. The Michelson–Morley experiment, which used a Michelson interferometer to show the absence of the ether, played a great part in the advancement of science. Gratings are also used as beam-splitters and beam-combiners. Because of their dispersive nature, they are used in the construction of achromatic interferometers.
Mounting Multiple Lens Assemblies
Published in Paul Yoder, Daniel Vukobratovich, Opto-Mechanical Systems Design, 2017
The tolerances assigned to the design represented in Figure 5.96 are as indicated in the right-hand column of Table 5.2. Meeting the limiting tolerances entails great expense compared with the typical values applied to more common assemblies. In order to achieve ±2 µm maximum air space thickness errors between elements, the location of each lens must be controlled with respect to its associated mechanical mount to ±1 µm. Alignment of subassemblies is performed for the objective of Figure 5.96 using a Mirau interferometer as indicated in Figure 5.97. The location of the flat surface of the mount is determined by placing an optical flat on the annular knife edge and moving the interferometer so that its focus is on the flat surface. The flat is then removed, and the lens subassembly to be measured is placed on the knife edge. The interferometer focus is readjusted to the vertex of the lens. Note that the interferometer can be tilted in two directions to ensure beam propagation along the lens’s surface normal. The distance labeled “Δh” in the figure is then measured to an accuracy of ±200 nm and compared to the design requirement. Subassemblies within tolerance are accepted for use in production of the objectives.
Synthetic wavelength scanning interferometry for 3D surface profilometry with extended range of height measurement using multi-colour LED light sources
Published in Journal of Modern Optics, 2023
Priyanka Mann, Vishesh Dubey, Azeem Ahmad, Ankit Butola, Dalip Singh Mehta
Synthetic wavelength scanning interference-based optical profilers are one of the best non-contact techniques for areal topographical measurement of various objects. Here, we successfully demonstrated the profilometry of a standard object having step height beyond the phase ambiguity using a synthesized multicolour LED light source. We are capable to provide a range of synthetic wavelengths using the proposed light source and hence we have an extended range of height measurement within the coherence length range of the LEDs. The proposed LED-based light source has low spatial and temporal coherence which reduces the possibility of speckle generation and multiple coherent reflections. This increases the signal-to-noise ratio of the proposed system. Here, we have used a Mirau interferometer-based objective lens which is a nearly common-path geometry, and it helps to enhance the phase sensitivity of the proposed system. By utilizing the phase shifting interferometry we have further reduced the measurement error. We measured the height profile of a standard step-like object and compared the results with a Stylus profilometer. Our system is also capable to map the surface roughness of specimens over full FOV. In conclusion, we have developed a non-contact surface topography measurement system using commercially available multi-colour LED light sources which can explore optical profiling as well as surface roughness measurement. The proposed system can be used in a manufacturing environment by simply replacing the light source and microscope objective from any conventional bright-field microscope. The proposed system provides a non-contact measurement which prevents any kind of contamination and damage to the sample surface under measurement. As per many developing countries low cost and easy accessibility are the major factors in technology transfer sectors. In addition, with the low-cost LED source, the proposed system utilize multiple synthetic wavelengths, incoherent light source and phase shifting algorithm simultaneously which makes the system multi-modal and provides full field multispectral extended range of height measurements.