Single-photon detection techniques for underwater imaging
Abstract
This Thesis investigates the potential of a single-photon depth profiling system for
imaging in highly scattering underwater environments. This scanning system measured
depth using the time-of-flight and the time-correlated single-photon counting (TCSPC)
technique. The system comprised a pulsed laser source, a monostatic scanning
transceiver, with a silicon single-photon avalanche diode (SPAD) used for detection of
the returned optical signal.
Spectral transmittance measurements were performed on a number of different water
samples in order to characterize the water types used in the experiments. This identified
an optimum operational wavelength for each environment selected, which was in the
wavelength region of 525 - 690 nm. Then, depth profiles measurements were performed
in different scattering conditions, demonstrating high-resolution image re-construction
for targets placed at stand-off distances up to nine attenuation lengths, using average
optical power in the sub-milliwatt range. Depth and spatial resolution were investigated
in several environments, demonstrating a depth resolution in the range of 500 μm to a
few millimetres depending on the attenuation level of the medium. The angular
resolution of the system was approximately 60 μrad in water with different levels of
attenuation, illustrating that the narrow field of view helped preserve spatial resolution
in the presence of high levels of forward scattering.
Bespoke algorithms were developed for image reconstruction in order to recover depth,
intensity and reflectivity information, and to investigate shorter acquisition times,
illustrating the practicality of the approach for rapid frame rates. In addition, advanced
signal processing approaches were used to investigate the potential of multispectral
single-photon depth imaging in target discrimination and recognition, in free-space and
underwater environments. Finally, a LiDAR model was developed and validated using
experimental data. The model was used to estimate the performance of the system under
a variety of scattering conditions and system parameters.