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Title: Optical property variability in the underwater optical wireless channel
Author: Johnson, Laura J.
ISNI:       0000 0004 5924 1407
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2015
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This thesis details an investigation into the variability of optical properties within the aquatic medium and its consequence for optical wireless communication systems. The principle aim is to aid the optimisation of optical wireless for underwater links through the application of oceanographic light propagation models, where optical property variations that occur within common communication ranges are emphasised. This kind of approach is not typical within the underwater optical wireless community where variability between different natural waters has been considered but, so far, not within a single underwater optical wireless link. As part of this thesis, relevant underwater optical properties are surveyed and their variability quantified, where the importance of depth-dependent variations are established. A unique model is developed which characterises changing optical attenuation with depth based on two inputs; transmitted wavelength and turbidity at the surface of the water column. From this model, an investigation began into the impact of link location and orientation on link design and in advanced channel models. Select wavelengths are found to perform optimally, these are 410 nm, 490 to 500 nm, 540 nm, and 560 nm and greater; discretisation is attributed to the attenuation-wavelength profile shape and led to the advent of multi-wavelength transmitter designs. Meanwhile, a Monte Carlo modelling scheme, suitable for multi-layered media, predicted discrepancies between the overall attenuation and an average attenuation found by the depth-dependent model. Latterly, such knowledge is implemented in an experimental investigation in which a laser is transmitted down a turbid inland water column, to a maximum depth of 7.5 metres, then redirected back up. In addition to attenuation, changes in refractive index with depth are considered. With refractive changes occurring from pressure, salinity and temperature gradients, this research recognises that beams transmitted with a vertical component undergo some amount of refraction. Through ray tracing links transmitted at different angles, the maximum distance between a straight-path and the true beam location after propagating 200 metres was 0.3 metres. This is expected to be compensated by the natural widening of the beam.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council ; Thales Research and Technology
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TK Electrical engineering. Electronics Nuclear engineering