Digital Holographic Interferometry for the Measurement of Symmetrical Temperature Fields in Liquids

Optical thickness measurement of occluded samples by lens-less Fourier transform digital holography, thermal loading, and machine learning

Thickness measurements of objects, especially transparent and semi-transparent objects, are essential for their characterization and identification. However, in the case of occluded objects, the optical thickness determination becomes difficult, and an indirect way must be devised. Thermal loading of the objects changes their opto-thermal properties, which will be reflected as a change in their optical thickness. The key to quantifying such occluded objects lies in collecting these opto-thermal signatures. This could be achieved by imaging the changes occurring to a probe wavefront passing through the object while it is being thermally loaded. Digital holographic interferometry is an ideal tool for observing phase changes, as it can be used to compare wavefronts recorded at different instances of time. Lens-less Fourier transform digital holographic imaging provides the phase information from a single Fourier transform of the recorded hologram and can be used to quantify occluded phase objects. Here we describe a technique for the measurement of change in optical thickness of thermally loaded occluded phase samples using lens-less Fourier transform digital holography and machine learning. The advantage of the proposed technique is that it is a single shot, lens-less imaging modality for quasi-real-time quantification of phase samples behind thin occlusions.

Advances in Digital Holographic Interferometry

Holographic interferometry is a well-established field of science and optical engineering. It has a half-century history of successful implementation as the solution to numerous technical tasks and problems. However, fast progress in digital and computer holography has promoted it to a new level of possibilities and has opened brand new fields of its application. In this review paper, we consider some such new techniques and applications.

Characterization of Supersonic Compressible Fluid Flow Using High-Speed Interferometry

This paper presents a very effective interference technique for the sensing and researching of compressible fluid flow in a wind tunnel facility. The developed technique is very sensitive and accurate, yet easy to use under conditions typical for aerodynamic labs, and will be used for the nonintrusive investigation of flutter in blade cascades. The interferometer employs a high-speed camera, fiber optics, and available “of-the-shelf” optics and optomechanics. The construction of the interferometer together with the fiber optics ensures the high compactness and portability of the system. Moreover, single-shot quantitative data processing based on introducing a spatial carrier frequency and Fourier analysis allows for almost real-time quantitative processing. As a validation case, the interferometric system was successfully applied in the research of supersonic compressible fluid discharge from a narrow channel in a wind tunnel. Density distributions were quantitatively analyzed with the spatial resolution of about 50 μm. The results of the measurement revealed important features of the flow pattern. Moreover, the measurement results were compared with Computational Fluid Dynamics (CFD) simulations with a good agreement.