# From Mirrors to Free-Space Optical Communication--Historical Aspects in Data Transmission
## 3 FSO Communication System Constructions  
The general idea of Free-Space Optics relies on the transmission of invisible, eye-safe light beams. They are transmitted by a laser with light focused onto a highly sensitive photon detector, which serves as a receiver, equipped with a telescopic lens. In fact, the FSO operation is somewhat similar to an infrared TV remote controller. Free-Space Optical links have quite a simple construction, typically consisting of two identical heads enabling duplex data transmission, as shown in Figure 4. These heads are connected via interfaces directly to computers or with a telecommunications network. Physically, these connections are made by fiber optic cables (single and multimode) or Bayonet Neill-Concelman British Naval Connector/Spanning-Tree Protocol (BNC/STP) coaxial cables. BNC connects the computer with the coaxial cable. Generally, BNC is a commonly used plug and socket for audio, video, and networking applications that provides a tight connection. The block diagram of the transceiver head of a typical FSO link is presented in Figure 5. It includes two main blocks: a transmitter and a receiver of the optical radiation. The source of the optical radiation may be a light-emitting diode, a semiconductor laser, or several lasers [40]. LEDs and semiconductor lasers are used in wireless optical links for short distances (e.g., inside buildings) [41]. On the other hand, FSO links for longer distances mainly use vertical-cavity surface-emitting lasers [42]. Lasers provide more power and faster transmission speeds than LEDs [43]. The advantage of surface radiating lasers, compared to classic edge lasers, is the symmetrical spatial characteristic of the radiation. The data transmission speed on the FSO link is strictly defined by the manufacturer of a given device. The telecommunications protocol is selected depending on the data stream transmission rate; for example, Non-Return-to-Zero (NRZ) and Return-to-Zero (RZ) modulations are used in FSO links [44; 45; 46]. The optical signal generated by the radiation source is spatially formed by a transmitting lens that creates a beam of radiation with a defined diameter. In commercial systems, the beam divergence  
Figure 4: The scheme of the data transmission via Free Space Optics (FSO) technology between two heads. Figure 5: Block diagram of the FSO system. angle is between 2 and 10 mrad. Beams with a small divergence angle can only be used in FSO systems equipped with beam targeting and stabilization systems. In simpler solutions, beams with a greater divergence angle are used. This type of solution facilitates the adjustment of the system and enables operation during vibrations of the supporting structure and when the refraction of the atmosphere is changing (e.g., due to a variable temperature or air density). A large advantage of this simplicity is also the reduction of costs of the FSO setup and maintenance. However, the divergence of the beam means that the beam spreads out quicker and hence loses power at a distance, meaning that FSO communication using such a beam is limited in range. The existence of beam divergence causes the diameter of the optical radiation beam to increase with an increasing distance from the source. The power density distribution, in the plane that is perpendicular to the radiation propagation direction, is usually described by the Gaussian distribution. This beam creates an illuminated area in the receiver plane, with an area generally much larger than the effective receiving area. This results in a radiation power loss resulting from the ratio of these surfaces. The atmosphere causes an additional power loss because it absorbs and scatters the transmitted radiation.