From speckle pattern photography to digital holographic interferometry [Invited]

Phase derivative estimation in digital holographic interferometry using a deep learning approach

In digital holographic interferometry, reliable estimation of phase derivatives from the complex interference field signal is an important challenge since these are directly related to the displacement derivatives of a deformed object. In this paper, we propose an approach based on deep learning for direct estimation of phase derivatives in digital holographic interferometry. Using a Y-Net model, our proposed approach allows for simultaneous estimation of phase derivatives along the vertical and horizontal dimensions. The robustness of the proposed approach for phase derivative extraction under both additive white Gaussian noise and speckle noise is shown via numerical simulations. Subsequently, we demonstrate the practical utility of the method for deformation metrology using experimental data obtained from digital holographic interferometry.

Applications of digital speckle pattern shearing interferometry in characterization of fluids

Digital speckle pattern shearing interferometry is a robust optical technique mostly used for measuring small deformations in solid objects. In this paper we focus on applications of this technique in characterization of liquid and gaseous samples. We demonstrate this by a few examples: measurement of the density (temperature) profiles inside a candle flame and around a hot wire in air. Also, we study the diffusion process in a binary mixture and measure the diffusion coefficient. The results of these studies confirm that this highly sensitive, nondestructive, and real-time technique is ideal for investigation of fluid specimens.

Thermally responsive polymer-dispersed liquid crystal diffusers fabricated using laser speckle pattern irradiation

This study examined the thermal response of polymer-dispersed liquid crystal (PDLC) diffusers, patterned using a two-lens imaging system. Optical modulation was achieved by modifying the PDLC transmittance using temperature-induced changes to liquid crystal (LC) orientation. PDLCs with controllable scattering properties were obtained by irradiating LC-polymer composites with laser speckle patterns. The variation of the scattering characteristics of the PDLCs with temperature, average speckle size, and LC orientation order was analyzed to determine the most suitable parameters for a diffuser for smart window solar-ray control applications. The findings of these experiments demonstrate that using speckle patterns, a one-time laser exposure process, can provide a simple fabrication method of novel optical devices.

Segmented fast linear canonical transform

Investigation of the discrete and fast linear canonical transforms is becoming one of the hottest research topics in modern signal processing and optics. How to handle and obtain the linear canonical frequency spectrum of very large input data based on equipment with limited memory space is one of the key problems. To focus on this problem, a new kind of segmented fast linear canonical transform has been proposed in this paper. First, the large data is segmented into short data. Thereby, the proposed algorithms can calculate very large input data and simultaneously keep the ideal frequency resolution. Second, the complexity of the derived algorithms has been analyzed in detail for different kinds of signals. Their performance with regard to resolution and precision are compared with the existing fast linear canonical transforms. Finally, experimental results are presented to verify the correctness of the results obtained.

Simultaneous acquisition of 3D shape and deformation by combination of interferometric and correlation-based laser speckle metrology

A metrology system combining three laser speckle measurement techniques for simultaneous determination of 3D shape and micro- and macroscopic deformations is presented. While microscopic deformations are determined by a combination of Digital Holographic Interferometry (DHI) and Digital Speckle Photography (DSP), macroscopic 3D shape, position and deformation are retrieved by photogrammetry based on digital image correlation of a projected laser speckle pattern. The photogrammetrically obtained data extend the measurement range of the DHI-DSP system and also increase the accuracy of the calculation of the sensitivity vector. Furthermore, a precise assignment of microscopic displacements to the object's macroscopic shape for enhanced visualization is achieved. The approach allows for fast measurements with a simple setup. Key parameters of the system are optimized, and its precision and measurement range are demonstrated. As application examples, the deformation of a mandible model and the shrinkage of dental impression material are measured.

On the speckle number of interferometric velocity and distance measurements of moving rough surfaces

The minimum achievable systematic uncertainty of interferometric measurements is fundamentally limited due to speckle noise. Numerical and physical experiments, regarding the achievable measurement uncertainty of Mach-Zehnder based velocity and position sensors, are presented at the example of the laser Doppler distance sensor with phase evaluation. The results show that the measurement uncertainty depends on the number of speckles on the photo detectors. However, while the systematic uncertainty due to the speckle effect decreases, the random uncertainty due to noise from the photo detector increases with increasing speckle number. This results in a minimal total measurement uncertainty for an optimal speckle number on the photo detector, which is achieved by adjusting the aperture of the detection optics.