We consider the difficult problem of ground-based propagation of 90 femtosecond laser pulses through the turbulent atmosphere in the range of 1 mile, where the atmospheric properties change significantly according to weather conditions. Our experimental data indicates that use of femtosecond laser pulses for optical communications at up to 1.35 Gb/sec is significantly advantageous compared to CW lasers, offering much reduced error rates and higher consistency in transmission in diverse atmospheric conditions. We develop a new model using Gaussian beamlets to simulate the effects of propagation in different refractive index conditions within the beam. The results are quite consistent with the experimental observations indicating significantly less scintillation and small- scale fluctuations in received data when fs laser pulses at 1540 nm are used. In the new model, we simulate the transmitter as a coherent sum of Gaussian beamlets (5-500) and propagate them with slightly different index of refraction, corresponding to the normal variations in average index of refraction due to changing atmospheric conditions. For instance, when transmitting over one mile of air, a temperature difference within the beam of only 0.02 °C causes the Gaussian beamlets to arrive at the receiver with time delays differing by about 200 fs, exceeding the coherence time of the 90 fs laser pulses, thereby causing incoherent summation at the detector.
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