de Miranda, Erasmus Couto B; (1996) Atmospheric laser propagation at near infrared. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

The main aim of this project is to investigate the characteristics of the propagation of laser beams through the atmosphere at the wavelengths of 0.83 and 1.55[mu]m across a 4km path in central London, a densely urbanised terrain. The thesis reports practical measurements of the propagation characteristics in clear air and in also in rain and mist conditions. Meteorological measurements of rainfall rate and wind speed were also made to supplement the propagation measurements. Measurements were also made at 1.55[mu]m with the transmitter modulated at 155Mbits/s. The analysis of the results is divided into four sections. (1) The statistical and spectral analysis of amplitude scintillations and angle of arrival in dry weather, (2) in rain, (3) modelling of the rainfall induced attenuation of the received optical power and (4) the performance evaluation of the prototype of a 155Mbits/s digital free space optical communications system operating at 1.55[mu]m. Results for studies (1) and (2) indicate that the gamma is the best distribution for the received amplitude scintillations for varying turbulence conditions in clear air and also in rain conditions. The normal distribution is the best fit for angle of arrival data, regardless of the strength of turbulence and weather. Level crossing statistics of amplitude scintillations are presented for clear air conditions. Spectral analysis of the scintillations and angle of arrival showed results that confirm theoretical assumptions in dry but not in rain conditions. No evidence of saturation of scintillations was found. In study (3), cumulative distributions of attenuation and rainfall for a year of measurements are presented and the rainfall induced attenuation is evaluated. Statistical comparison of the performances of both wavelengths in rain are shown and indicate that the 0.83[mu]m system has a slightly better overall performance. For study (4), reliability and quality analysis results are presented. Availability figures show values as high as 94.5[percent]. Analysis of bit error rates using level crossing statistics showed an approximate overall 92[percent]-95[percent] probability of error-free operation for the system.

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