Coherence length of single laser pulses as measured by CCD interferometry

A novel interferometric method for the direct, real-time measurement of the complete temporal coherence function of a pulsed laser is presented. A Michelson interferometer is modified by replacing one mirror with an inclined diffraction grating to observe interference fringes as a function of path-length difference on a single pulse. Computerized data acquisition and methods of extending the range of wavelengths to the infrared are discussed

Nature of intensity and phase modulations in stimulated Brillouin scattering

The nature of stimulated Brillouin scattering (SBS) temporal modulations for a focused beam in a finite-length cell with homogeneous medium is examined numerically. The finite phonon lifetime produces deterministic oscillations at the threshold while the inclusion of the random noise as an initiation source of SBS leads to stochastic fluctuations in Stokes intensity and phase. A unified study of both modulations under different parameters is presented. The results indicate a large useful parameter space for excellent Stokes beam quality.Shahraam Afshaarvahid, Vladimyros Devrelis, and Jesper Munc

Fidelity of optical phase conjugation using stimulated brillouin scattering / Vladimyros Devrelis.

Includes author's previously published articles.Includes bibliographical references.v, 504 leaves : ill. ; 30 cm.Thesis (Ph.D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 199

The use of statistical mixture models to reduce noise in SPAD images of fog-obscured environments

Navigating through fog plays a vital part in many remote sensing tasks. In this paper, we propose an Expectation- Maximization (EM) algorithm for fitting a mixture of lognormal and Gaussian distributions to the probability distributions of photon returns for each pixel of a 32x32 Single Photon Avalanche Diode (SPAD) array image. The distance range of the target can be determined from the probability distribution of photon returns by modeling the peak produced due to fog scattering with a lognormal distribution while the peak produced by the target is modeled by a Gaussian distribution. In order to validate the algorithm, 32x32 SPAD array images of simple shapes (triangle, circle and square) are imaged at visibilities down to 50.8m through the fog in an indoor tunnel. Several aspects of the algorithm performance are then assessed. It is found that the algorithm can reconstruct and distinguish different shapes for all of our experimental fog conditions. Classification of shapes using only the total area of the shape is found to be 100% accurate for our tested fog conditions. However, it is found that the accuracy of the distance range of the target using the estimated model is poor. Therefore, future work will investigate a better statistical mixture model and estimation method

Impact of water quality on Single Photon Avalanche Diode direct time-of-flight imaging

A detailed study on the effect of chlorophyll and sediment, two of the main constituents of ocean water on the image quality of a Single Photon Avalanche Diode (SPAD) direct time-of-flight (dToF) imaging system is conducted. This system consists of a 532nm laser and a 32x32 SPAD time-of-flight sensor. The degradation of laser power and the volume scattering function (VSF) are measured and the image quality of underwater objects imaged by the SPAD 32x32 time-of-flight sensor is examined. Classification accuracy of simple geometric shapes is used as an indicator of the quality of the SPAD images. Under lab conditions, controlled amounts of sediment and chlorophyll are added into a water tank for these experiments. The laser's output to input power ratio is found to be exponentially decreasing with increasing path length travelled by the beam. The laser's beam attenuation coefficient is calculated and found to increase linearly with increasing concentration of these constituents. Likewise, the volume scattering function of the beam is found to be larger when these two constituents are present in the water. All of these experimental results are in accordance with the predictions of the Beer-Lambert Law. Furthermore, the shape classification accuracy is shown to decrease with increasing concentrations of sediment. Overall, these results confirm that the quality of underwater images taken by the SPAD flash imaging system will rapidly degrade with increasing chlorophyll and sediment concentration

Through thick and thin : imaging through obscurant using SPAD array

Preliminary work on 3D image collection and classification of targets in the presence of obscurant using a Flash LiDAR system is discussed in this paper. The system is based around a DST designed 32 x 32 Single Photon Avalanche Diode (SPAD) array to image either targets or silhouettes of targets. The collected data included military targets that were obscured either by camouflage nets or fog. For camouflage net, the target was detected using an algorithm implemented on the Nvidia Jetson TX2. Targets obscured by fog are detected and classified where the classification accuracy is 100% for fog visibility down to 17.3m and 89.5% for 14.1m. This algorithm was not implemented on the TX2 but its simplicity shows potential for it in the future. This initial approach opens the road to eventually operate SPAD based systems for real-time classification through dust or smoke