**2.2 FSO system**

FSO system refers to the transmission of modulated visible or infrared (IR) beams through the atmosphere to obtain broadband communications. This technique requires clear line of site between the transmitter and the receiver. FSO system provides higher bandwidth at faster speed. The elements of FSO designed which must be considered by a prudent user are wavelength, beam divergence angle, aperture diameter and range.

## **2.2.1 Wavelength**

To select the best wavelength to use for free-space optical communication systems, you must consider several factors, such as availability of components, eye safety considerations, required transmission distance, price, and so on. The availability of components is light sources and detectors [4]. Eye safety is one of the most important restrictions to the optical power level emitted by a wireless IR transmitter. Lasers of much higher power can be used more safely with 1550 nm systems than with 850 nm and 780 nm systems. This is because wavelengths is less than about 1400 nm focused by the human cornea into a concentrated spot falling on the retina as shown in Fig. (3), which can cause eye damage.

Fig. 3. Penetration of Light into Eyeball.

The allowable safe laser power is about 50 times higher at 1550 nm. This factor, 50 is important as it provides up to 17 dB additional margin, allowing the system to propagate over longer distances, through heavier attenuation, and to support higher data rates [11]. However, 1550 nm systems are at least 10 times more expensive than 850 nm systems [16]. The highest data rate available with commercial 850 nm systems is 622 Mbps, and 2.5 Gbps for 1550 nm systems. Table (1) illustrates the maximum Permissible Exposure (MPE) limited for unaided viewing, in case of 850 nm and 1550 nm [11].


Table 1. Maximum Permissible Exposure Limited for "unaided viewing".
