Underwater visible optical communications become very important for their high velocity and more data rate. But the optical is suffering from the high water attenuation. For optical communication. Pure water is the best of the ten water types with wavelengths λ = 455 and 486 mm. The Nile river water is a pure water without salinity (fresh water). The temperature of the water is daily changes and so the performance of optical communication underwater becomes temperature-dependent. A simplified expression very good accuracy of Egypt Nile water to determine the water refractive index, water dispersion, water attenuation, received optical power, and SNR as direct temperature dependence is done. The optical channel loss model is used to determine the received optical power and the ray trace model is used to define optical radiation pattern. The equation of optical received power by ray trace is the same as that by using the optical channel loss model except for the transmitter gain of them are different. For λ = 486nm with water temperature varying from 4oC to 30oC, the corresponding refractive index decreases from 1.3399 to 1.3379 (so, the optical velocity under fresh water increases from 2.239*108m/s to 2.2423*108 m/s), dispersion decreases from 0.6492 to 0.6459 (ps/m nm), attenuation factor decreases from 0.0378 m− 1 to 0.0345m− 1 and so the required transmitted optical power due to attenuation for 800 m long shrinks to 7.4 %. The Effect of temperature becomes more evident with more link distance. The required transmitted power to achieve the required SNR increases with the more data rate. To overcome the unavailability of the link due to water temperature, the transmitted power must be controlled by the daily water temperature. In this study, the temperature dependence of the performance of the optical link and a simulation proposed example design is done.

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