## 2 FSO Communication System
### Transmitter Side
#### 2.1.2 Transmission Safety  
In FSO communication systems, the primary optical source is typically a semiconductor LD. However, some manufacturers opt for high-power LEDs equipped with beam collimators as an alternative. The chosen optical source must be capable of delivering relatively high optical power across a wide range of temperatures. Additionally, it should feature a long mean time between failures (MTBF), ensuring reliable performance over extended periods. The components associated with the optical source should also have a small physical footprint and consume minimal power [1; 3; 18]. To meet these stringent requirements, Vertical-Cavity Surface-Emitting Lasers (VCSELs) are predominantly used for operations around the 850 nm wavelength. For operations at the 1550 nm wavelength, Fabry-Perot (FP) and Distributed Feedback (DFB) lasers are commonly employed. These types of lasers are selected for their efficiency, reliability, and suitability for the specified wavelengths, ensuring optimal performance of the FSO systems [1; 18].  
Safety considerations are critical when dealing with laser Txs, with the primary concern being the potential risk to the eyes from laser exposure. To mitigate these risks, various safety standards have been established that regulate the transmitted optical power of lasers. These standards take into account factors such as the intensity of the laser beam, its wavelength, proximity to the laser source, peak power in a pulse, and the average power over extended periods [1; 3; 18; 23].  
Lasers emit light at different wavelengths, and only certain wavelengths, particularly those in the near-IR (between approximately 400 nm and 1550 nm), have the potential to penetrate the eye with enough energy to harm the retina. Light outside this range is generally absorbed by the cornea and lens before reaching the retina, making it less likely to cause damage. The eye's tolerance to laser power varies with wavelength, primarily  
Figure 5: A typical block diagram of an FSO Tx.  
due to the differential absorption of light by water, which is a major component of the eye. Specifically, the absorption coefficient at the front of the eye is significantly higher for longer wavelengths (greater than 1400 nm) compared to shorter wavelengths. Consequently, lasers operating at 1550 nm can have a higher allowable transmission power, which makes them suitable for long-distance applications [1; 3; 18; 19; 23].  
Additionally, the risk associated with laser exposure differs between high-power pulses and low-power continuous beams. A single high-power laser pulse, even if it lasts less than a microsecond, can cause irreversible damage if it directly enters the eye. In contrast, lower-power continuous beams are hazardous primarily with prolonged exposure. Furthermore, the risk of eye damage diminishes with increased distance from the laser source, as the power density of the laser beam decreases with distance [23]. So, when selecting the operating wavelength for FSO, it is crucial to consider eye-safety regulations and standards. This means that the chosen wavelength must ensure that the optical signals used in the FSO system do not pose a risk to human eyes, either through direct exposure or accidental reflections. Compliance with eye-safety requirements helps prevent potential health hazards and ensures the safe deployment and operation of the FSO system in various environments [21].