**4.6 Conclusion**

70 Optical Communications Systems

Fig. 17. Loss at Beam Center (dB) versus the Link Range (km).

Fig. 18. Beam Wander (m) versus the Link Range (km).

In this chapter, we focused on haze, rain and turbulence effects on FSO systems. Mie scattering occurs in hazy days and it depends on wavelength. The scattering coefficient on hazy days is determined by using Beer's Law. From the results analysis and data in the Table 5.1 the fog and haze represent the most important atmospheric scatters. Their attenuation, which can reach about 17.6 dB at 1.8 km low visibility in Yemen and 163.5 dB (corresponding to very thick fog), at 0.05 km low visibility is in Taiz city. This attenuation value affects the performance of a FSO link for distances as small. Wavelength 1550 nm is less scattered from the wavelengths 850 nm & 780 nm and it is not harmful to the human eyes.

Rain does not introduce a significant attenuation in FSO systems links in Yemen. This is due to the rainfall affect mainly radio and microwave systems that use a longer wavelengths and attenuation at heavy rain 5.77 mm/hr in Yemen about 0.69 dB, is very small compared with attenuation due to fog. Therefore the effect of rain is neglected in Yemen. Atmospheric turbulence will change in refractive index structure of air from one area to another. Atmospheric turbulence fluctuates intensity of the laser beam. Scintillation is wavelength and distance dependent. We can reduce the effect of the turbulence by enlarging the diameter of the receiver's aperture or setting tracking system at the receiver. The results indicate that the attenuation depends on weather conditions which are uncontrollable and transmission range which can be controlled; hence, it is considered an important element in the design of FSO system. So, to improve the performance of FSO system, we must reduce the transmission range and use wavelength 1550 nm.

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