Effects of random vibration in high-speed phase-shifting speckle pattern interferometry

The influence of random vibrations on a dynamic phase shifting speckle pattern interferometer, in which phase difference evaluation is performed using temporal phase shifting and temporal phase unwrapping, is investigated by means of experiments and numerical simulations. A well-defined velocity spectral density function, typical of the spectra found under non-vibration-isolated conditions, is used throughout. Five phase-shifting formulae are studied, with camera framing rates (1,2 and 4 kHz) typical of current dynamic speckle pattern interferometers. Two main aspects were evaluated: firstly the unwrapping reliability, and secondly the noise induced in the phase maps by the vibration. The former was found to be a significant constraint, even for peak velocities well below the Nyquist velocity limit of the interferometer, and is therefore likely to be more important than the latter in many applications. Three analytical criteria for determining the expected unwrapping success rate are proposed and their predictions compared with the measured values. It is demonstrated that shorter sampling windows and higher framing rates are preferred in order to increase the unwrapping success rate, but that longer windows reduce the root mean square error in the phase change maps due to the vibration

High-NA lensless coherent imager as a building block for a synthetic aperture interferometry array

In digital holography, the field of view (FOV) and lateral resolution are limited by the pixel pitch and sensor dimensions, respectively. A large numerical aperture can be synthesized to increase the FOV and spatial resolution by coherently combining low resolution holograms obtained for different illumination and/or observation directions. This is known as Synthetic Aperture Interferometry (SAI) and in this work we describe the design, construction, calibration and testing of high numerical aperture compact coherent imagers (CI) which constitute the optical building block of a multi-sensor SAI array. The CIs consist of a photodetector array, a highly divergent reference beam close to it and an aperture that acts as a spatial filter to prevent aliasing of the digital holograms. We explore different optical designs to produce a highly divergent reference beam close to the sensor, including bulk optics, micro-optics, and ion beam milled optical fibres. An optimization approach is used to characterize the reference wavefront for accurate digital reconstructions of the scattered field first at the aperture plane and then at the object plane. The performance of a compact CI is demonstrated by reconstructing an object 76 mm wide at 80 mm from the sensor, which corresponds to a numerical aperture NA>0.5

An experimental technique for tracking dolphins in the vicinity of a trawl net

The system described in this paper provides a means of tracking one or more echolocating cetaceans (dolphins, porpoises and whales) in three-dimensional space. The aim of the research is to enable the behaviour of cetaceans to be observed passively in the wild in the vicinity of a large trawl net and to evaluate the effectiveness of deterrents intended to prevent the by-catch of these animals