# From Mirrors to Free-Space Optical Communication--Historical Aspects in Data Transmission
## 4 Laser Wavelengths Versus FSO Links  
A huge effort has been made to understand the physical phenomena related to the propagation of optical radiation in the Earth's atmosphere [57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75]. Most of the conducted research focuses on the phenomena related to the absorption and scattering of radiation by atoms and molecules that make up the atmosphere, as well as by natural and artificial pollutants: fog, rain, smoke, pollen, and dust. Indeed, the presence of rain and snow can influence the FSO links [76], while fog and water vapor droplets significantly lower data transmission efficiency, especially in the tropics [77]. The presence of water vapor molecules in the air contributes to the absorption of some light particles (photons)  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**FSO system** & **FlightStrata 52E** & **CableFree G2000** & \begin{tabular}{c} **SONAbeam** \\ **10G-E+** \\ \end{tabular}  & \begin{tabular}{c} **Koruza Bridge** \\ **10 Gbps** \\ \end{tabular}  \\ \hline Description & \begin{tabular}{c} Multiple Beam \\ System with Auto \\ Tracking— \\ full duplex \\ \end{tabular}  & \begin{tabular}{c} Multiple Beam System with \\ Automatic Transmit Power \\ Control (ATPC)— \\ full-duplex \\ \end{tabular}  & \begin{tabular}{c} Multiple Beam \\ System—full duplex \\ \end{tabular}  & \begin{tabular}{c} Single beam \\ unit—full duplex \\ \end{tabular}  \\ \hline Bit Rate & Up to 155 Mbps & Up to 1.5 Gbps & Up to 10 Gbps & Up to 10 Gbps \\ \hline Distance & Up to 5600 m & Up to 2000 m & Up to 1000 m & \(<\)150 m, \\ \hline Optical Transmitter & VCSEL & VCSEL & \begin{tabular}{c} InGaAsP Laser \\ Diode \\ \end{tabular}  & \ \begin{tabular}{c} SC SEP WDM Bi-Di \\ module with DDM \\ \end{tabular}  \\ \hline Wavelength & 850 nm & 780 nm & 1550 nm & 1270 nm/1310 nm \\ \hline Optical Transmit & \(-20\) dBm & \(+19.0\) dBm & \(+26\) dBm & \(+0.5\) dBm \\ \hline Optical Receiver & Si APD & APD & APD & \begin{tabular}{c} SC SEP WDM Bi-Di \\ module with DDM \\ \end{tabular}  \\ \hline Unit Weight & 11.1 kg & 9 kg & 8 kg & 2 kg \\ \hline Operating & \(-25\)\({}^{\circ}\)C to \(60\)\({}^{\circ}\)C & \(-20\)\({}^{\circ}\)C to \(60\)\({}^{\circ}\)C & \(-40\)\({}^{\circ}\)C to \(50\)\({}^{\circ}\)C & \(-40\)\({}^{\circ}\)C to \(60\)\({}^{\circ}\)C \\ \hline Power & \(20\) W & 45 W & 40 W & 6 W \\ Consumption Max. & \(20\) W & \(45\) W & \(40\within the light beam due to absorption, refraction, and scattering. This causes an overall loss in power density in the received signals. This is similar to the impact that rain-fading has on Radio Frequency (RF) wireless systems, which contributes to signal attenuation. FSO may also be temporarily blocked by moving objects such as flying birds, Unmanned Aerial Vehicles (UAVs), or aircraft. Permanent blocking may also be caused by stationary objects such as trees or tall buildings [78]. Since the physics of the atmosphere cannot be changed, the wavelengths of the lasers used in FSO are selected to operate in so-called \"atmospheric transmission windows\". In Figure 6, an example of the characteristics of the atmosphere transmission coefficient for a distance of 1 km is presented. The spectral ranges in which the atmosphere has relatively low absorption of radiation are noticeable. Commercial systems mainly use spectral ranges corresponding to transmission windows of around 0.8 \(\upmu\)m and 1.5 \(\upmu\)m.  
The wavelength of the laser radiation used in FSO links depends on the transmission properties of the atmosphere, particularly fog, as well as the availability and parameters of the lasers [79]. The attenuation is classified into three groups: Rayleigh scattering (molecule scattering, when the molecule is smaller than the light wavelength), Mie scattering (aerosol scattering, when the size of the molecule is comparable to the wavelength), and nonselective scattering (geometric scattering, when the size of the molecule is larger than wavelength size) [80].