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.
These experiments report the first qualitative observation of dependency
of typical fog transmission on pulse length and the first systematic measurement
of scattering amplitude and pattern for fs-pulse propagation through strongly
scattering (fog) medium. Also, we include the first reported measurement of
enhancement of fs-pulse propagation in real FSO environment. We also provide a classical analysis of the Mie-Scattering predictions,
and experiments which demonstrate a different transmission behavior than is
predicted. This proves only in part that the enhanced fs performance is due to a
different scattering formula than is expected; absorption is normally a value
which is measured by default. Once the transmission and scattering properties of
a stable substance are known (usually determined in two different
measurements), the difference is the absorption. This is particularly hard to do in
simulated fog, as the simulation itself is unstable, as is real fog. However, there is
no current measurement or claim which indicates this enhanced propagation is
related to different physics of absorption of Ultrafast Pulses by the atmosphere.
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