# From Mirrors to Free-Space Optical Communication--Historical Aspects in Data Transmission
## 3 FSO Communication System Constructions
The power of the radiation reaching the receiver can be determined from the following formula [47]:  
\[P_{rec}=P_{trans}\frac{D_{rec}^{2}}{\left(\theta R\right)^{2}}\tau \tag{1}\]  
where \(P_{trans}\) is the radiation power emitted by the transmitter, \(D_{rec}\) is the diameter of the receiver optics, R is the distance between the transmitter and receiver, \(\Theta\) is the beam divergence angle, and \(\tau\) is the atmospheric transmission coefficient. For the FSO link to operate with an acceptable bit error rate, the \(P_{rec}\) power must be higher than the noise. Having the defined minimum received power \(P_{recmin}\), the link range can be determined from the Formula (1):  
\[R=\sqrt{\frac{P_{trans}}{P_{recmin}}}\frac{D_{rec}}{\theta}\sqrt{\tau}. \tag{2}\]  
Thus, a formed and modulated optical signal reaches the opposite optical radiation receiver, which consists of a receiving lens, an optical filter, a photodiode with a power supply and cooling system, a low-noise preamplifier, and a signal processing system. The optical system is used to focus the radiation onto the surface of the detector (avalanche or pin photodiode). For larger surfaces of the optical system, an optical signal with a higher intensity is obtained. The use of optical systems with larger surfaces is advantageous due to the possibility of using photodiodes with smaller active surfaces and due to the mitigation of the effect of photon flux fluctuation (scintillation). With a larger aperture of the receiving system, \"aperture averaging\" of the photon flux fluctuation also takes place. With a sufficiently large aperture, the fluctuating beam is within the lens surface, so the fluctuations will be averaged according to:  
\[D_{rec}\approx\sqrt{R\lambda}. \tag{3}\]  
Formula (3) shows that the diameter of the lens should match the planned range of the link on a given wavelength. Thus, for a link with a range of 1 km, \(\lambda\) = 10 \(\upmu\)m, the diameter of the receiving lens should be less than 10 cm. The use of a receiving lens with a larger diameter will increase the weight of the FSO head and its cost, while not bringing any large benefits to the data transmission rate. In addition to the useful signal, the background radiation reaches the receiver of the FSO system, causing an increase in noise. As a consequence, the ratio of the signal power to noise power decreases [48]. The power of the background radiation reaching the optical system of the receiver depends on the field of view of the lens and its active surface (aperture). To improve the ratio of the signal power to the noise power, the receiver is equipped with optical filters, which narrow its spectral range down to the \(\Delta\lambda\) band covering the transmitted signal band. Another technique used for the reduction of noise is the direct isolation of noise sources. For example, the FSO receiver set up in the shade will be protected from noise-inducing sunlight, which is especially prominent in the near-infrared band. In the signal processing circuit, the received signal is amplified, demodulated, and then forwarded to the interface. FSO transceivers are placed on masts, buildings, and other structural elements, so they are susceptible to strong gusts of wind. To avoid a break in communication, multibeam systems, or systems for directing and stabilizing the position of the laser beam (so-called Automatic Tracking (AT) systems) are used [49]. Using an AT system, the FSO head changes its position to search for the strongest possible signal from the transmitter. For this purpose, photoreceivers with position-sensing diode detectors and other special articulations are used to enable the positioning of the head in three planes. This is of particular importance in portable FSO systems [50]. In FSO links, systems for heating the transmitter/receiver optics can be used [51].