*3.3.1. Absorption*

**Visibility S (Line of Sight) (km)**

174 Contemporary Issues in Wireless Communications

minima and maxima. The visibility *V* is defined by:

**3.3. Atmospheric attenuation**

Beer's law equation [14]:

where,

Source:

**λ = 800 nm (dB/km) λ = 2500 nm (dB/km)**

(6)

0.5 32.5 30.8 0.7 23 21 0.9 18 16 1.1 14.5 12.5 1.3 12 10 1.5 10 8.33

**Table 3.** Variation in atmospheric attenuation due scattering based on visibility (data obtained from [7,12]).

I Max + I min

interfere with the principal beam, inducing modulations of the detected signal [11].

scattering and attenuation may be caused more in low visibility conditions [13].

τ=exp(-

V = I Max - I min

When the length difference between the two optical paths varies, the energy passes through

The visibility depends on the degree of coherence of the source, on the length difference between the paths as well as on the location of the detector with respect to the source. The coherence between the various beams arriving at the detector also depends on the crossed media: for example the diffusing medium can reduce the coherence. For links referred to as "in direct sight" links, coherent sources can be used, provided that parasitic reflections do not

Low visibility will decrease the effectiveness and availability of FSO systems, and it can occur during a specific time period within a year or at specific times of the day. Low visibility means the concentration and size of the particles are higher compared to average visibility. Thus,

Atmospheric attenuation is defined as the process whereby some or all of the electromagnetic wave energy is lost when traversing the atmosphere. Thus, atmosphere causes signal degra‐ dation and attenuation in a FSO system link in several ways, including absorption, scattering, and scintillation. All these effects are varying with time and depend on the current local conditions and weather. In general, the atmospheric attenuation is given by the following

βL

), (7)

Absorption is caused by the beam's photons colliding with various finely dispersed liquid and solid particles in the air such as water vapor, dust, ice, and organic molecules. The aerosols that have the most absorption potential at infrared wavelengths include water, O2, O3, and CO2 Absorption has the effect of reducing link margin, distance and the availability of the link [15].

The absorption coefficient depends on the type of gas molecules, and on their concentration. Molecular absorption is a selective phenomenon which results in the spectral transmission of the atmosphere presenting transparent zones, called atmospheric transmission windows [11], shown in Fig. 7, which allows specific frequencies of light to pass through it. These windows occur at various wavelengths. The Atmospheric windows due to absorption are created by atmospheric gases, but neither nitrogen nor oxygen, which are two of the most abundant gases, contribute to absorption in the infrared part of the spectrum [7].

It is possible to calculate absorption coefficients from the concentration of the particle and the effective cross section such as [16,17]:

$$
\beta\_{\text{abs}} = \alpha\_{\text{abs}} N\_{\text{abs}} \left[ \frac{1}{\text{km}} \right] \tag{9}
$$

Where:

αabs : is the effective cross section of the absorption particles [km2 ].

Nabs: is the concentration of the absorption particles [1/km3 ].

An absorption lines at visible and near infrared wavelengths are narrow and generally well separated. Thus, absorption can generally be neglected at wavelength of interest for free space laser communication. Another reason for ignoring absorption effect is to select wavelengths that fall inside the transmittance windows in the absorption spectrum [18].

**Figure 7.** Atmospheric transmittance window with absortion contribution.
