Luthra, Davinder; (1995) Atmospheric infra-red wave propagation at 0.83μm and 1.55μm. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

This works seeks to characterise scintillations from two links at 1.55 ?m and 0.83 ?m operating on a 4 km path across Central London. The lognormal and Gaussian distributions were the best fits to the pdf's of intensity and angle-of-arrival scintillations. The aperture averaging function was best described by the weak turbulence and large inner scale model. In order to investigate localised turbulence production the Cn2 path weighting function was biased to selected parts of the path. The salient features of the path were highlighted. For example, the Middlesex Hospital, where Cn2 ranged between 2.3 and 25.0 times greater than any other part of the path because of its air conditioning and incineration plants. This technique may benefit a free space optical communication system by utilising the most advantageous propagation terrain of an inhomogeneous path. A model is developed which seeks to predict the attenuation at 1.55 ?m. The model includes losses due to beam spreading, fog, rainfall and intensity scintillations. The model shows good agreement between the real and predicted received power levels based on meteorological measurements. These include a fading margin of up to 36 dB and rainfall rates of between 0 and 25 mmhr-1. The visibility conditions varied between clear skies, light mist and thick haze, corresponding to losses of up to 3.5 dBkm-1. The 1.55 µm transmitter was modulated at 155 Mbit/s. A significant effect in clear air was found to be intensity scintillations which result in burst errors. In order to quantify error rates in the presence of intensity fluctuations a Schottky diode was connected to the clock output and indicated whether the clock and hence the data had been recovered or lost. It is shown that if performed over shorter paths, the amount of burst error would be substantially lower.

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