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Free Space Optics (FSO) is a growing up technology offering a higher bandwidth with fast and cost-effective deployment compared to fiber technology associated to a lack of regulation unlike radio frequencies. Multiple applications are envisioned including but not limited to campus-scaled network, substitution for a fiber network after a disaster (e.g., earthquake, attack, etc.) or connecting a drone as a relay in white spot. Despite that, FSO performances are restricted by atmospheric phenomena (e.g., turbulence, fog or scattering). Therefore, the operating wavelength is an important parameter that has to be chosen wisely so as to reduce the impact of the environmental parameters. In order to improve the FSO availability, performance and range, the investigation of the cross relation between the climatic conditions and the wavelength is highly required.
The main scope of this study is to evaluate and compare the availability and the performance of an FSO link of a few kilometers in urban environment for two infrared wavelengths matching the appropriate atmospheric windows. To do so, we computed the transmission rate under various atmospheric conditions, including fog, both at 1.55-µm (standard telecom wavelength) and 4-µm (mid infrared) for which the attenuation of the beam intensity is smaller whatever the extinction coefficient.
Using the transmission rate of an FSO link extracted from our radiative transfer software MATISSE the link budget is derived for a simple direct detection system design. The source and detector components characteristics are also considered to compute the reception induced noise and estimate the Bit Error Rate (BER) of the FSO link for the two wavelengths with respect to the visibility. Assuming error correction and the corresponding BER limit, it is possible to derive the minimum of visibility below which the efficiency of the optical link is strongly affected. Thus, for a 50 mW source, the minimum visibility required to get an effective FSO link is respectively 600 m at 1.55 µm and 400 m at 4 µm. A weather visibility database is then used to obtain the theoretical availability of the FSO system during a whole year. As an example the availability at Vélizy-Villacoublay (France) weather station throughout the entire year 2017 will be displayed.
This paper presents the results of the transmission simulation, the development of the link budget calculation, and the contrast between laser and background. We describe the different detection noises and discuss about the availability of our FSO link during a year for the two wavelengths in urban environment.
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