**2.2.2 Beam divergence**

46 Optical Communications Systems

photodiode (APD) because they have good quantum efficiency and are made of

The performance characteristics indicate how a detector responds to an input of light energy. They can be used to select an appropriate detector for a particular application. To understand the descriptions of detector performance and to be able to pick a detector for a specific application, one should understand these detector characteristics. In general, the

FSO system refers to the transmission of modulated visible or infrared (IR) beams through the atmosphere to obtain broadband communications. This technique requires clear line of site between the transmitter and the receiver. FSO system provides higher bandwidth at faster speed. The elements of FSO designed which must be considered by a prudent user are

To select the best wavelength to use for free-space optical communication systems, you must consider several factors, such as availability of components, eye safety considerations, required transmission distance, price, and so on. The availability of components is light sources and detectors [4]. Eye safety is one of the most important restrictions to the optical power level emitted by a wireless IR transmitter. Lasers of much higher power can be used more safely with 1550 nm systems than with 850 nm and 780 nm systems. This is because wavelengths is less than about 1400 nm focused by the human cornea into a concentrated

semiconductors that are widely available commercially [10].

A high response at the wavelength to be detected.

A small value for the additional noise is introduced by the detector.

wavelength, beam divergence angle, aperture diameter and range.

spot falling on the retina as shown in Fig. (3), which can cause eye damage.

**Features of detector** 

following properties are needed:

Sufficient speed of response.

Fig. 3. Penetration of Light into Eyeball.

**2.2 FSO system** 

**2.2.1 Wavelength** 

Beam divergence purposely allows the beam to diverge or spread. The advantage using narrow beam in FSO system generates much higher data rates and increases the security. Laser generated with extreme narrow light can be easily modulated with voice and data information. The beam spread is dependent on the beam divergence angle and transmission range. Typically, 1 mrad to 8 mrad beam divergence spreads 1 to 8 m at distance of 1 km. To avoid spreading of a large beam, it is better to use narrow beam divergence such as 1 mrad [12-14].

## **2.2.3 Aperture diameter**

In FSO system a smaller diameter of transmitter and a larger diameter of receiver aperture are needed to establish high data rate communication links. The diameter aperture of the transmitter and receiver must be adequate for the weather conditions. When the laser beam propagates through atmosphere, the beam is spreading, at a distance L from the source, due to the turbulence. If the turbulence cell is larger than the beam diameter, and the diameter of receiver aperture is small, then the beam bends and it can cause the signal to complete missing the received unit. A large size of diameter aperture of receiver is able to reduce turbulence effect on FSO [12][15]. Two particular design specifications are made in Table (2) due to particular implementation especially based on the existing product available in the industry [12].


Table 2. Diameter of Transmitter and Receiver Aperture of an FSO System.

#### **2.2.4 Range**

Distance between a transmitter and a receiver impacts the performance of FSO systems in three ways. First, even in clear weather conditions such as scintillation, the beam diverges and the detector element receives less power. Second, the total transmission loss of the beam

Effect of Clear Atmospheric Turbulence on

**3.2 Visibility runway visual range (RVR)** 

clouds can affect visibility in the same way [19].

**3.3 Atmospheric attenuation** 

Where:

**3.4 Absorption** 

by the following Beer's law Eq. (1) [22]:

�: is the atmospheric attenuation.

�: is the total attenuation coefficient given as:

�: is the distance between (T�) and (R�) in kilometer.

����: is the molecular and aerosol absorption. �����: is the molecular and aerosol scattering.

Quality of Free Space Optical Communications in Western Asia 49

Visibility defined as (Kruse model) means of the length where an optical signal of 550 nm is reduced to 0.02 of its original value. It is characterized by the transparency of the atmosphere, estimated by a human observer. Visibility is a useful measure of the atmosphere containing fog, smog, dust, haze, mist, clouds and other contaminating particles. Thick fog can reduce visibility down to a few meters; and maritime mist and

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, scattering and attenuation may be caused more in low visibility conditions [20]. Attenuation can reach hundreds of dB per km for low visibility values, and is higher at shorter wavelength [21]. Low visibility and the associated high scattering coefficients are the

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 degradation and attenuation in a FSO system link in several ways, including absorption, scattering, and scintillation. All these effects are time-varying and will depend on the current local conditions and weather. In general, the atmospheric attenuation is given

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, O�, O�, and CO�. Absorption has the effect of reducing link margin, distance and the availability of the link [4].

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

� � �������) (1)

������ ������� (2)

most limiting factors for deploying FSO systems over longer distances [4].

increases with increasing distance. Third, scattering and absorption effect accumulates with longer distances. Therefore, the value for the scintillation fading margin in the overall power budget will increase to maintain a predefined value for the BER. Most commercially available FSO systems are rated for operation between 25–5000 m, with high-powered military and satellite systems capable of up to 2000 km. Most systems rated for greater than 1 km incorporate three or more lasers operating in parallel to mitigate distance related to issues. It is interesting to note that in the vacuum of space, FSO can achieve distances of thousands of kilometers [4].
