Model of a Laser Transceiver Used in an Fso System


Figure 3.4 Model of a Laser transceiver used in an FSO system

3.4 Channel characteristics in FSO system

3.4.1. Types of attenuation in FSO transmission medium

The transmission channel of the FSO system involves the transmission, absorption and scattering of light by the earth's atmosphere. The atmosphere interacts with light depending on the composition of the air, which under normal conditions consists of various types of gas molecules and suspended particles. The interaction produces various optical phenomena: selective absorption, scattering, and flickering of the received light.

Selective absorption : of radiation transmitted in light wavelengths is created by interactions of photons and molecules, atoms (H 2 O, CO 2 , N 2 , O 2 , H 2 , O 3 …). This leads to the disappearance of transmitted photons, signal attenuation and increase in ambient temperature. This phenomenon depends on the composition of the air and the wavelength of the light used. There are wavelength regions where the transmission is almost transparent (no absorption) called frequency windows.


Figure 3.5 Environmental impact on FSO line

Scattering : the air environment is the result of partial interaction of light and elements (dust, water particles in the air) in the transmission medium. It only changes the radiation direction of the interacting component without changing the wavelength. Scattering occurs when the size of particles in the air is similar to the wavelength of the transmitted light. And in real conditions, it is mainly caused by fog and drizzle.

Flicker phenomenon : in FSO is the change of signal under the influence of temperature change inside the transmission medium, the random distribution of air layers on the transmission line is created. These layers have variable distance (10cm - 1km) and different temperatures, creating different refractive coefficients which are the cause of scattering, multi-path, and angle of incidence change. The received signal changes rapidly with a frequency range of 0.01 - 200 Hz. The changing wave front creates convergence and divergence of the light beam.

In addition, other impacts also greatly affect the transmission line such as obstacles arising during use: growing trees, flying creatures, movement of buildings or towers installing equipment, ground vibrations that deflect light rays. These types of occurrence are very low and we can also eliminate them.


Summary diagram of environmental impact on FSO system:


The influence of the atmosphere on the propagation of light fields

Signal degradation effects

Effect of reflection coefficient

Random air turbulence changes the reflection coefficient.

Light loss, due to slow change of reflection coefficients

Scattering


+ Rayleigh scattering (by electron resonance)

+ Mie scattering (due to airborne particles)

Absorb


+ Absorption lines of gas molecules

+ Continuous absorption;

+ Absorbed by solid and liquid elements.

Attenuation in space

Wavefront distortion

Expand the light beam

Flicker

Change the angle of incidence


3.4.2 Effect of atmospheric changes on signal quality

The change in the properties of the air causes a spatial and temporal variation in the signal intensity at the receiver. The reason is that this change causes the refractive index to change and the air acts like a lens that deflects the beam from the main direction to the receiver. The time of this change is the time the beam is transmitted through space and it depends on the wind speed. In practice, if the fluctuations are weak, the signal intensity distribution function is proportional to the logarithmic function. For space optics using horizontal propagation, this change is stronger, so the received intensity distribution function follows an exponential law.

The parameter commonly used to measure air change rate is the structural parameter.

refractive index . It is directly related to wind speed. The change of can be used to predict the change in signal strength at the receiver.

(3.2)


Where: is the variance of the signal intensity change. is the refractive structure parameter ( .


k is the wave propagation constant (rad/m). L is the distance (m).

From the expression we see:

The air change intensity is inversely proportional to the wavelength used (a system operating at 780 nm has about a two-fold change at 1550 nm).

The effect of change is proportional to the distance.

The effect of the change is illustrated in Figure 3.6. The figure shows the receiver mouth with randomly distributed black and white spots. The spot size is proportional to (λR) 1/2 .

At long wavelengths the speckle is larger at the receiver mouth. This is not good for the system operation because there are few speckles at the receiver mouth. If the receiver mouth receives only one spot, the transmitter is required to increase the power to ensure the BER when the spot is a black spot.

Size is proportional to the square root of distance. Longer distances adversely affect the system.

Figure 3.6 Spots and average mouth size

3.5 Factors affecting, evaluating and improving the quality of wireless optical lines

3.5.1 Parameters affecting the quality of the line

3.5.1.1 Transmission equation of the line

The transmission equation of the space optical system in simple form (ignoring transmitter optical efficiency, receiver noise, etc.):

(3.3)

In there:


is the receiver surface area (m 2 )

Div is the beam divergence angle (radians).


α is the air attenuation coefficient.


P transmit is the transmitter power (W).

exp(-α.Range) is the exponential function with base e of the product of the attenuation coefficient and the distance.

The received power is directly proportional to the transmitted power and the receiver area. Inversely proportional to the square of the product of the beam divergence angle and the transmission distance. Inversely proportional to the exponential function of the air attenuation coefficient and the distance.

Looking at the equation the variables that can be changed are: transmit power, receiver size, beam divergence angle and distance. The attenuation coefficient is not controllable, depends on external environmental conditions and can be independent of wavelength in a severely attenuated medium.

It is found that the received power depends greatly on the product of the attenuation coefficient and the distance. Figure 3.7 illustrates this.

This means that in bad weather conditions, even if the designer increases the transmit power, the receiver size, or installs a very narrow beam, the received power will not change. The only variable parameter is the distance, which must be short enough to ensure that the attenuation factor does not dominate the equation.

Figure 3.7 Received power depends on the product of attenuation coefficient and distance


The x-axis represents the distance R (m) of the line. The y-axis is the value of the multiplier


exp(-α.Range) (natural logarithmic function) and the multiplier in the expression


(3.3).

Through this section we see the great influence of the attenuation coefficient in the environment.

weather conditions in the transmission equation compared to other quantities. However, it is possible to achieve efficient, optimal, reliable and economical designs under these constraints.

3.5.1.2 Air attenuation

The parameter that affects the transmission quality is mainly air loss. Let us analyze the influence of this parameter.

The laser power attenuation through air is defined by the Beers-Lambert law:


In there:


is the transfer function at distance R. P(R) is the power at distance R. P(0) is the power at the source.

α is the attenuation coefficient (1/Km).

(3.4)

Common attenuation factors: dry air = 0.1 (0.43dB/Km), haze = 1 (4.3dB/Km) and fog = 10 (43dB/Km).

The attenuation coefficient is caused by the absorption and scattering of laser photons by gas molecules in the air. Since the wavelengths commonly chosen for use (785 nm, 850 nm, 1550 nm) are within the transmission window, the influence of the absorption coefficient is small compared to the total attenuation. Therefore, the influence of the attenuation coefficient caused by transmission line scattering is dominant.

The type of scattering is determined by the specific particle size relative to the transmission wavelength.

It is described by a dimension number called the dimension parameter α :

(3.5)


In which: r is the scattering particle radius,

λ is the laser wavelength.


Table 3.2 Radius of scattered particles in air and corresponding size parameters of laser wavelengths 785 nm and 1550 nm.

Type

Radius (µm)

Dimensional parameters α

785 nm

1550 nm

Air molecules

0.0001

0.0008

0.0004

Fog

0.01 – 1

0.08 – 8

0.04 – 4

Steam

1 – 20

8 – 160

4 – 80

Rain

100 – 10000

800 – 80000

400 – 40000

Snow

1000 – 5000

8000 – 40000

4000 – 20000

Hail

5000 – 50000

40000 – 800000

20000 - 400000

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Rayleigh scattering occurs when air particles are smaller than the laser wavelength (785 nm and 1550 nm) due to gas molecules (Cox, Nox…) in the air. The attenuation coefficient varies according to λ -4 . The influence of this scattering on the total attenuation coefficient is very small.

As the particle size approaches the laser wavelength, the radiation of the particle scatters in the direction opposite to the propagation direction. This scattering is Mie scattering caused by small fog particles. With Mie scattering, the wavelength exponent in relation to the attenuation coefficient changes from -1.6 to 0.

Figure 3.8 Scattering particle size parameters

Figure 3.8 shows the scattering particle size parameters in Table 3.2 at laser wavelengths of 785nm and 1550nm for Rayleigh, Mie and non-selective scattering, respectively.


The third common scattering occurs when the particle size is larger than the wavelength. For size parameters greater than 50, this scattering is called geometric or non-selective scattering (since there is no dependence of the attenuation coefficient on wavelength and the exponent of the wavelength in the attenuation coefficient is zero). These scattering particles are large enough that the angle of the scattered radiation can be described by geometric optics. Falling rain, snow, and heavy fog will cause this scattering.

The amount of air scattering for short communication links is either wavelength dependent (Mie scattering) or wavelength independent (geometric or non-selective scattering).

3.5.2 Parameters for evaluating the quality of the line

3.5.2.1 Route usability

Line usability is a key factor to consider when installing the system.

Affected by many factors:

Equipment reliability. This factor can be ensured by selection, calculation, and design.

The air attenuation statistics are unknown. They are collected through specialized equipment such as visibility and rainfall meters. These devices are often installed with the space optical system. From the data of these devices, we can calculate the air attenuation coefficient. And we can accurately estimate the usability of the line over a transmission distance.

3.5.2.2 Bit error rate BER and data rate over transmission distance

We consider the effect of fog based on the visibility range. The distant building is about 300m away from the photographed building. The left image shows dry air, the attenuation coefficient is about 6.5dB/Km (2000m visibility) measured by a nephelometer (turbidity measuring device) mounted on the camera. During the fog, the attenuation coefficient is about 150dB/Km (113m visibility) shown in the middle image, the building is still visible. In the right image, the attenuation coefficient is 225dB/Km (75m visibility) and the building is completely lost.

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