Free Space Optics

Abstract

When we design an optical system, we need to consider that the transmitting power is appropriate to reach the receiving end, if this power is less than required, it will be difficult to separate the data from the noise, which in turn, will causes errors. In this chapter, a new method is presented to improve the received power in wireless optical communications by introducing an adaptive power method that adjusts the amplifier’s power according to weather conditions. The simulation was done using OptiSystem programming environment and MATLAB environment. Results have shown improvement in the received power, bit error rate, quality factor, and eye diagram, in which the quality factor was improved by a rate ranging from 3.6 to 44.45% in different weather conditions.

1. Introduction

Free Space Optics (FSO) technology is very promising in the world of optical communications where the signal is transmitted from the transmitter to the receiving end through free space and using the laser beam.

This technology has many advantages such as high data rates, faster installation, unlicensed spectrum data, high security, and low cost [1, 2, 3].

The most prominent applications of FSO are satellite communications, data center networks, underwater communications, and mobile backhaul (Figure 1) [4, 5, 6, 7].

Figure 1.

Wireless optical communications.

The main challenge of FSO system is the different weather conditions that affect the optical link. Attenuation is caused by weather changes such as rain, snow, fog, and dust, which in turn, affects the received power, that is reflected in BER and quality factor [8].

2. Photo transmitters

The light sources convert the electrical signal into a light signal with the provision of sufficient power to overcome the damping to reach the receiving end with a level of power higher than the sensitivity of the receiver. The light source is chosen according to the communication system to be established, that is according to the transmission rate and the distance to be covered. The adjustment is made directly or indirectly (external rate).

Semiconductor laser sources have been adopted in modern optical communications because of their many advantages compared to other laser transmitters, including [9]:

• Small size: These transmitters are built on a single chip that includes all the components needed to emit laser light, which allows them to be easily combined with other equipment.

• Great efficiency: The efficiency of the laser transmitter is up to 50%, which allows it to be driven with low electrical power compared to other transmitters.

• It is driven directly and with a small current: that is, conventional transistor circuits can be used to power the laser transmitter.

• High reliability.

However, these transmitters are not without some disadvantages, including high sensitivity to heat, and therefore the issue of cooling is a fundamental issue that must be taken into consideration.

2.1 Basic parameters of laser transmitters

• Transmitter output power: It is measured in mW or dBm.

• The wavelength of light emitted by the transmitter, as the laser transmission takes place at a wavelength that represents one of the transmitter windows.

• Spectral width of the source: it is the width of the output pulse and is usually denoted by Δλ.

• Primary page.

3. Optoelectronic modifiers

3.1 Photoelectric effect

The photoelectric effect occurs when an electric field is applied to a transparent medium, where the optical properties of this medium (the refractive index) change according to the changes of the field applied to the transparent medium [10].

The applied electric field causes changes in the lengths and directions of the axes of the ellipse by the crystal’s refractive index. This leads to a change in the polarization of the light wave passing through the crystal. Thus, the optical intensity at the output will change according to the applied voltage, and this is known as the photoelectric effect.

3.2 Photo modulation mechanism

The optical intensity of the laser transmitter can be modified in one of two ways [11]:

3.2.1 A direct modification

This is done by feeding the laser transmitter directly with an electrical information signal. Despite the simplicity of this modification, it is rarely used because of the frequency chirping resulting from the direct modification of the source.

3.2.2 Indirect modification (external)

The semiconductor laser is fed with a continuous electric current to generate a continuous light wave, and the optical power is modified by an external modifier, such as the Mach Zehnder rate, and Figure 2 shows the external modulation.

Figure 2.

External modification.

3.2.2.1 Mach-Zehnder modulator

This rectifier consists of a planar waveguide on a substrate with a pair of electrodes, to which modulating electrical forces are applied. The instantaneous electric field at the output Eout(t) is given in terms of the instantaneous electric field at the input Ein(t) in relation (1) [11]:

$$\mathit{E}\mathit{\text{out}}\left(\mathit{t}\right)=\frac{1}{2}\left[exp\left(\mathrm{j}\frac{\mathit{\pi }}{{\mathit{V}}_{\mathrm{\pi }}}\mathit{V}1\left(\mathrm{t}\right)\right)+exp\left(\mathrm{j}\frac{\mathit{\pi }}{{\mathit{V}}_{\pi }}\mathit{V}2\left(\mathrm{t}\right)\right)\right]{\mathit{E}}_{\mathit{\text{in}}}\left(\mathrm{t}\right)$$E1

$${\mathit{V}}_{\pi }={\mathit{V}}_1+{\mathit{V}}_2$$E2

In order to obtain a direct relationship between the two optical intensities at the input and output, voltages V1 and V2 are chosen as follows:

$$\mathit{V}1\left(\mathrm{t}\right)=\mathit{V}\left(\mathrm{t}\right)-\frac{{\mathit{V}}_{\mathit{\pi }}}{2}$$E3

$$\mathit{V}2\left(\mathrm{t}\right)=-\mathit{V}\left(\mathrm{t}\right)+\frac{{\mathrm{V}}_{\mathit{\pi }}}{2}$$E4

Thus, the ratio of the electric fields at the input and output is:

$$\frac{\text{Eout}\left(\mathrm{t}\right)}{\mathrm{Ein}\left(\mathit{t}\right)}=\frac{sin\left(\mathrm{\pi }.\mathit{V}\left(\mathrm{t}\right)\right)}{{\mathit{V}}_{\mathit{\pi }}}$$E5

Thus, the optical intensity (the square of the electric field) is modified according to the voltages applied to the two rectifier paths. Figure 3 shows the rate structure and the main intervention:

Figure 3.

Mach-Zehnder Modulator.

Figure 4.

A typical high-speed digital signal with an eye diagram.

3.2.3 Photoreceptors

The photoreceptor converts the optical signal into an electrical signal. There are several types of these receivers depending on their physical structure, but the most common are the photodiode (PIN) and the diode scattering (APD). These elements are characterized by their small size, good sensitivity, fast response, and low noise.

3.3 Basic parameters of photoreceptors

The photodetector has the following properties:

• The sensitivity of the photodetector: It is defined as the ratio of the output current of the detector (estimated in mA) to the power of the input light (estimated in mW).

• Dark current: One of the specifications of photoreceptors is that they allow the passage of electric current even in the absence of the optical signal (darkness), which leads to the appearance of a dark current that enters as a systematic error in any of the measurements taken by this detector. The cause of this current is the thermal effect generated charge carriers.

• Responsiveness: It is the ratio of the detector’s output current to the input light power, measured in A/Watt, and given for a specific wavelength.

• Response speed: It determines the maximum possible transmission speed for a particular photodetector.

• Spectral response: It determines the relationship of the detector’s response with wavelength.

4. Photodetector

The primary function of the photodetector is to convert the light signal falling on it into an electrical signal from which the transmitted information can be extracted using suitable electronic circuits. Semiconductor photodetectors depend in their work on the phenomenon of semiconductor absorption of photons, which leads to the generation of free charge carriers that cause the passage of an electric current, and this phenomenon is called the photoelectric effect. If a reverse bias is applied to the P-N junction and without light, a small current will pass through it called the dark current. On the other hand, when the light ray falls on the diode, it absorbs the energy of a photon and an electron-hole pair is formed. If such carriers are formed in the depletion region or near it, it will travel across the junction as a result of the effect of the electric field, and this movement of charge carriers across the junction will lead to the flow of a current in the external circuit of the diode that is directly proportional to the capacity of the light absorbed by the diode, and thus the light is converted into an electric current.

It is a mechanism that gives us analysis of high-speed digital signals. This diagram is formed from the digital form of the signal by folding the part of the signal corresponding to each bit on one graphic curve so that its horizontal axis is time and its vertical axis is the amplitude of the signal, and by repeating this process for many wave samples, and the resulting curve will present the statistical average of the signal, which is similar in shape to the shape of the eye, where Figures 1–12 shows the ideal shape of the eye chart and the practical form of the eye chart.

5. Eye chart basics

Figure 5 shows the measurements that can be obtained from an eye chart.

Figure 5.

Practical high-speed digital signal with eye diagram.

Figure 6.

Scheme of the eye with the measurements that we can read on it.

One level:

The one level in the eye chart expresses the mean value of the one logical amplitude.

Zero level:

The zero level in the eye chart expresses the mean value of the zero logical amplitude.

Eye amplitude:

It is the difference between the one level and the zero level, and based on the amplitude of the eye, the logical circuits at the receiving end will determine if the receiving bit is 1 or 0.

Eye height:

It is a measure of the vertical aperture of the eye chart, the eye height measurement will be equal to the eye amplitude measurement, but for a true eye chart noise will affect the eye height and cause the eye to close, as a result, the eye height measurement determines the degree of eye closure because of the noise, the SNR indicates directly the degree of eye closure.

Eye crossing percentage:

The eye crossing ratio gives an indication of the distortion of the duty cycle or problems in the symmetry of the pulses at high speeds. The best value for eye crossing percentage is 50% where the duration of the positive pulse is equal to the duration of the negative pulse.

Figure 7 shows the distortion in the duty cycle and its reflection on the eye chart.

Figure 7.

Format of a received digital signal and the resulting eye diagram with a crossover ratio of 75.

6. Effect of weather factors on FSO link

One of the challenges facing the FSO channel is atmospheric attenuation due to absorption, scattering, and scintillation (which may cause signal loss and optical link failure occur), and the effect of these factors changes with time and depends on local conditions and distance. Water molecules and carbon dioxide mainly cause absorption of light signals, while fog, rain, snow, and cloud cause scattering of light signals traveling in free space. This scattering leads to the deviation the light beam that is transmitted from the sender to the receiver. Atmospheric attenuation is given by Beer law, which is the ratio between the power of the transmitted signal and the power of the received signal [8, 12]:

$$\mathrm{\tau }={\mathrm{P}}_{\mathrm{L}}={\mathrm{P}}_0.exp\left(-\mathrm{\alpha }.\mathrm{L}\right)$$E6

Where

P_L: is laser power at the distance L.

P₀: is laser power at source.

α: is air attenuation coefficient.

Where the air attenuation coefficient is calculated by the Kim model.

$$\mathrm{\alpha }=\frac{3.91}{\mathrm{V}}.{\left(\frac{550\mathrm{nm}}{\mathrm{\lambda }}\right)}^{-\mathrm{q}}$$E7

Where

λ: is wavelength in nm.

V:is visibility.

q: is size distribution of the scattering particles.

The attenuation of the transmitted signal can be obtained from the previous model for different weather conditions using q which takes the following values depending on visibility [4, 5]:

q = 1.6 for visibility (V > 50 km).

q = 1.3 for visibility (6 km < V < 50 km).

q = 0.16 V + 0.34 for visibility(1 km < V < 6 km).

q = V − 0.5 for visibility (0.5 km < V < 1 km).

q = 0 for visibility (V < 0.5 km).

7. Optical amplifiers

The importance of optical amplifiers is that they restore optical power without any conversion electro-optical, according to which the principle of the optical amplifier works depends on the induced emission mechanism. There are many types of optical amplifiers, the most important of which are semiconductor amplifiers and similar fiber amplifiers. EDFA-like fiber amplifiers are of great importance in contemporary applications due to their important properties, and they have many work patterns that we will use in our research. This power control mode considers the value of the output power to control the amplifier performance (Power Control). The specified amplifier output power (${\mathrm{P}}_{\text{speci}}$) is [13, 14, 15]:

$${\mathit{P}}_{\mathit{\text{speci}}}=\mathit{G}\times \sum _{\mathit{\lambda }}{\mathit{P}}_{\mathit{\text{in}}}\left(\mathit{\lambda }\right)-{\int }_{-\infty }^{+\infty }{\mathit{S}}_{\mathit{ASE}}\left(\mathit{f}\right)\mathit{df}$$E8

where

G: the amplifier gain.

$S_{\mathit{ASE}}\left(f\right)$: the spectral density of amplified spontaneous emission integrated on the optical frequency f.

8. The proposed FSO system

When designing an optical system, it must be taken into account that the transmitting power is sufficient to reach the receiving end, and in the event that the receiving power is less than the required limit, it will be difficult to separate the data from the noise, and therefore this matter will cause errors, and to design a well-functioning optical system, considerations must be taken following:

• The received power must be large enough to keep the BER at a low value.

• The receiving power must be small enough to avoid damage to the receiver.

• The transmitted power must be the lowest value required for the system to avoid excessive power consumption.

It was found through studies that when the received power decreases, the bit error rate becomes bad and thus this problem is solved either by increasing the transmitter power or by using an optical amplifier, however if the power increases by a fixed amount, we will have two additional problems, first one is the excessive consumption of power since the weather factors to which the light link is exposed are temporary factors that continue for specific hours of the day, and during certain times of the year. The second is the damages the receiver as a result of permanent exposure to high power.

Hence the idea of this chapter is to suggest an adaptive method of transmitting power according to the weather conditions to which the optical link is exposed. The proposed method ensures a balance between the receiving power being large enough to maintain the BER at a low value in addition to the receiving power being small enough to avoid damaging the receiver.

If the received signal is less than -30 dBm, it sends a control signal from the receiver to the EDFA amplifier which adjusts its power to obtain a good signal reception as shown in Figure 8.

Figure 8.

Proposed method.

Figure 9.

The environment of simulation used.

9. Results and discussion

The simulation was done using OptiSystem and MATLAB.

OptiSystem is a powerful software for experimenting and controlling fiber optic networks, and optical wireless communication, and has good features for this work.

In this section, numerical results of performance of FSO link based on the proposed system model above according to the parameters shown in Table 1 are presented for different attenuation and distances.

Parameter	Typical value
Wavelength	1550 nm
Beam divergences	1.7 mrad
Transmission power	5 mW
Transmitter and receiver loss	0.01 dB
Attenuation	(3.1–0.47)dB/km
Gain amplifier	40 dB
Dynamic range	Up to10000 m
Transmitter hole diameter	5 cm
Receiver hole diameter	30 cm
Optical amplifier noise number	4 dB

Table 1.

Simulated parameters used in the designed optical link.

Performance was evaluated by the received power, bit error rate, and quality factor as shown in Eqs. (9)-(12) [16, 17].

$${\mathrm{P}}_{\mathrm{R}}={\mathrm{P}}_{\mathrm{T}}\frac{\mathrm{d}{2}^2}{{\left(\mathrm{d}1+\mathrm{D}.\mathrm{R}\right)}^2}{10}^{\left(-\mathrm{\tau }.\mathrm{R}/10\right)}$$E9

Where

${\mathrm{P}}_{\mathrm{R}}$: is received power.

${\mathrm{P}}_{\mathrm{T}}:$ is transmitted power.

d1: transmit aperture diameter.

d2: receive aperture diameter.

D: beam divergences.

R: range.

$\mathrm{\tau }$: atmospheric attenuation factor.

$$\mathit{B}\mathrm{ER}=\frac{erfc\left(\frac{\mathrm{Q}}{\sqrt{2}}\right)}{2}$$E10

Where

BER: is bit error rate.

$Q:$ is quality Factor.

Erfc: is Gauss error function.

$$\mathit{Q}=\frac{\mathit{SNR}\sqrt{2\mathit{TB}}}{1+\sqrt{1+2\mathit{SNR}}}$$E11

Where

TB: is the bit period.

SNR: signal-to-noise ratio.

$$\mathrm{SNR}=10{log}_{10}\frac{{\mathrm{p}}_{\mathrm{s}}}{{\mathrm{p}}_{\mathrm{n}}}$$E12

Ps: is signal power.

Pn: is signal noise.

The results show a clear improvement in the received power values, bit error rate, quality factor, and eye diagram. For attenuation value 0.47 dB / km the received power value improved to −30.424 dBm at the distance 6500 m (compared to the value −31.658 dBm before improvement). At the distance 10,000 m the value of the received power improved to −30.945 dBm (compared to the value −37.031 dBm before improvement) as shown in Figure 10.

Figure 10.

Received power before and after applying the proposed method for 0.47 dB/km attenuation.

For attenuation value 3.1 dB/km the power value improved to −25.997 dBm at the distance 3000 m (compared to the value −31.232 dBm before improvement). At the distance 10,000 m the value of the received power improved to −31.094 dBm (compared to the value −63.331 dBm before improvement as shown in Figure 11).

Figure 11.

Received power before and after applying the proposed damping method of 3.1 dB/km.

For attenuation value 0.47 dB / km the BER value improved to 1.71825e-014 at the distance 6500 m (compared to the value 3.63577e-009 before improvement). At the distance 10,000 m the value of the BER improved to 7.0694e-012 (compared to the value (1) before improvement) as shown in Figure 12.

Figure 12.

BER before and after applying the proposed method for damping 0.47 dB/km.

For attenuation value 3.1 dB/km the BER value improved to 6.15722e-089 at the distance 3000 m (compared to the value 1.0669e-010 before improvement). At the distance 10,000 m the value of the BER improved to 3.13463e-011 (compared to the value (1) before improvement) as shown in Figure 13.

Figure 13.

BER before and after applying the proposed method for damping 3.1 dB/km.

For attenuation value 0.47 dB/km the Q factor value improved to 7.578 at the distance 6500 m (compared to the value 5.781 before improvement). At the distance 10,000 m the value of the BER improved to 6.754 (compared to the value (0) before improvement) as shown in Figure 14.

Figure 14.

Quality factor before and after applying the proposed method for damping 0.47 dB/km.

For attenuation value 3.1 dB / km the Q factor value improved to 19.958 at the distance 3000 m (compared to the value 6.348 before improvement). At the distance 10,000 m the value of the BER improved to 6.534 (compared to the value (0) before improvement) as shown in Figure 15.

Figure 15.

Quality factor before and after applying the proposed method for damping 3.1 dB/km.

The results also show an improvement in the opening of the eye after using the proposed method, for attenuation value 3.1 dB/km the eye height value improved to 4.83267e-006 at the distance 3000 m (compared to the value (0) before improvement). At the distance 10,000 m the value of the eye height improved to 2.7099e-006 (compared to the value (0) before improvement) as shown in Figures 16 and 17.

Figure 16.

Eye diagram before and after the application of the proposed method at damping 3.1 and the distance 6500 m.

Figure 17.

Eye diagram before and after the application of the proposed method at damping 3.1 and the distance 10,000 m.

10. Conclusions

In this chapter, a new method was presented to improve the performance of the wireless optical link through the suggestion of adaptive power control, by modifying the power of the optical amplifier based on changing weather conditions. We obtain an acceptable BER and a good quality factor, while maintaining the minimum power required for the transmitter.The proposed system was tested on attenuation values (0.47, 3.1) dB/km and results have shown a clear improvement in the received power, bit error rate, quality factor, and eye diagram after using the proposed method.