**Driver**

Driver circuit of a transmitter transforms an electrical signal to an optical signal by varying the current flow through the light source.

#### **Optical source**

Optical source may be a laser diode (LD) or light emitting diode (LED), which used to convert the electrical signal to optical signal.

A laser diode is a device that produces optical radiation by the process of stimulated emission photons from atoms or molecules of a lasing medium, which have been excited from a ground state to a higher energy level. A laser diode emits light that is highly monochromatic and very directional. This means that the LD's output has a narrow spectral width and small output beam angle divergence. LDs produce light waves with a fixedphase relationship between points on the electromagnetic wave. There are two common types of laser diode: Nd:YAG solid state laser and fabry-perot and distributed-feedback laser (FP and DFB) [3].

## **Laser source selection criteria for FSO**

The selection of a laser source for FSO applications depends on various factors. They factors can be used to select an appropriate source for a particular application. To understand the descriptions of the source performance for a specific application, one should understand these detector factors. Typically the factors that impact the use of a specific light source include the following [4]:


## **Transmitter telescope**

The transmitter telescope collects, collimates and directs the optical radiation towards the receiver telescope at the other end of the channel.

## b. FSO channel

44 Optical Communications Systems

Transmitter transforms the electrical signal to an optical signal and it modulates the laser beam to transfer carrying data to the receiver through the atmosphere channel. The transmitter consists of four parts as shown in Fig. (2): laser modulator, driver, optical source

Laser modulation means the data were carried by a laser beam. The modulation technique can be implemented in following two common methods: internal modulation and external

**Internal modulation:** is a process which occurs inside the laser resonator and it depends on the change caused by the additive components and change the intensity of the laser beam

**External modulation:** is the process which occurs outside the laser resonator and it depends

Driver circuit of a transmitter transforms an electrical signal to an optical signal by varying

Optical source may be a laser diode (LD) or light emitting diode (LED), which used to

A laser diode is a device that produces optical radiation by the process of stimulated emission photons from atoms or molecules of a lasing medium, which have been excited from a ground state to a higher energy level. A laser diode emits light that is highly monochromatic and very directional. This means that the LD's output has a narrow spectral width and small output beam angle divergence. LDs produce light waves with a fixedphase relationship between points on the electromagnetic wave. There are two common types of laser diode: Nd:YAG solid state laser and fabry-perot and distributed-feedback

The selection of a laser source for FSO applications depends on various factors. They factors can be used to select an appropriate source for a particular application. To understand the descriptions of the source performance for a specific application, one should understand these detector factors. Typically the factors that impact the use of a specific light source

on both the polarization phenomena and the refractive dualism phenomenon.

**a. Transmitter** 

modulation [2].

**Driver** 

**Optical source** 

laser (FP and DFB) [3].

include the following [4]:

Modulation capabilities

Eye safety

and transmit telescope. **Laser modulator** 

according to the information signal.

the current flow through the light source.

convert the electrical signal to optical signal.

**Laser source selection criteria for FSO** 

Transmission power and lifetime

Price and availability of commercial components

Physical dimensions and compatibility with other transmission media.

For FSO links, the propagation medium is the atmosphere. The atmosphere may be regarded as series of concentric gas layers around the earth. Three principal atmospheric layers are defined in the homosphere [5], the troposphere, stratosphere and mesosphere. These layers are differentiated by their temperature gradient with respect to the altitude. In FSO communication, we are especially interested in the troposphere because this is where most weather phenomena occur and FSO links operate at the lower part of this layer [5].

The atmosphere is primarily composed of nitrogen (N2, 78%), oxygen (O2, 21%), and argon (Ar, 1%), but there are also a number of other elements, such as water (H2O, 0 to 7%) and carbon dioxide (CO2, 0.01 to 0.1%), present in smaller amounts. There are also small particles that contribute to the composition of the atmosphere; these include particles (aerosols) such as haze, fog, dust, and soil [6].

Propagation characteristics of FSO through atmosphere drastically change due to communication environment, especially, the effect of weather condition is strong. The received signal power fluctuates and attenuates by the atmospheric obstacles such as rain, fog, haze and turbulence in the propagation channel. The atmospheric attenuation results from the interaction of the laser beam with air molecules and aerosols along the propagation. The main effects on optical wireless communication are absorption, scattering, and scintillation [7].

c. Receiver

The receiver optics consists of five parts as shown in Fig. 2: receiver telescope, optical filter, detector, amplifier and demodulator.

## **Receiver telescope**

The receiver telescope collects and focuses the incoming optical radiation on to the photo detector. It should be noted that a large receiver telescope aperture is desirable because it collects multiple uncorrelated radiation and focuses their average on the photo detector [8].

#### **Optical filter**

By introducing optical filters that allow mainly energy at the wavelength of interest to impinge on the detector and reject energy at unwanted wavelengths, the effect of solar illumination can be significantly minimized [6].

#### **Detector**

The detector also called photodiode (PD) is a semiconductor devices which converts the photon energy of light into an electrical signal by releasing and accelerating current conducting carriers within the semiconductors. Photodiodes operate based on photoconductivity principals, which is an enhancement of the conductivity of p-n semiconductor junctions due to the absorption of electromagnetic radiation. The diodes are generally reverse-biased and capacitive charged [9]. The two most commonly used photodiodes are the pin photodiode and the avalanche

Effect of Clear Atmospheric Turbulence on

**2.2.2 Beam divergence** 

**2.2.3 Aperture diameter** 

[12-14].

**2.2.4 Range** 

for unaided viewing, in case of 850 nm and 1550 nm [11].

Quality of Free Space Optical Communications in Western Asia 47

The allowable safe laser power is about 50 times higher at 1550 nm. This factor, 50 is important as it provides up to 17 dB additional margin, allowing the system to propagate over longer distances, through heavier attenuation, and to support higher data rates [11]. However, 1550 nm systems are at least 10 times more expensive than 850 nm systems [16]. The highest data rate available with commercial 850 nm systems is 622 Mbps, and 2.5 Gbps for 1550 nm systems. Table (1) illustrates the maximum Permissible Exposure (MPE) limited

Wavelength Maximum Permissible Exposure (MPE)

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

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].

Design 1 18 cm 18 cm Design 2 3.5 cm 20 cm

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

Design Diameter of transmitter aperture Diameter of receiver aperture

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

850 nm 2 mW/cm2 1550 nm 100 mW/cm2

Table 1. Maximum Permissible Exposure Limited for "unaided viewing".

photodiode (APD) because they have good quantum efficiency and are made of semiconductors that are widely available commercially [10].

#### **Features of detector**

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 following properties are needed:

