One of the representatives of wireless connections is Free Space Optics, which is based on the application of a laser for communication. Currently, FSO links operate using lasers with wavelengths of — nm and 1. Already with these parameters, they revolutionized data transfer.
The selection of radiation sources should be done according to two main factors, i. The maximum ranges of these links are determined by the properties of the transmitting and receiving systems, as well as by weather conditions.
Others include using systems to regenerate the shape of impulse waveforms, elements for optical systems heating, adaptive optics systems, and multibeam systems [ ]. Still, the optimal structure of an FSO link depends on a compromise between the price, range, and data transfer rate concerning the intended applications. A practical hint to FSO link designers is that they must specify the requirements for the components of the individual link. This is extremely important to ensure the required transmission speed and the appropriate power of the optical signal.
Other essential factors include the selection of an appropriate data stream transm ission protocol in the optical channel, and the application of an automatic tracking procedure of the relative positions of the transmitting and receiving systems. A significant drawback of the available systems is the fact that they are sensitive to harsh weather conditions, and in particular to fog.
This entry is adapted from Scholarly Community. Submitted successfully! Thank you for your contribution! Check Note. Read Edit History Comment Lists. Table of Contents. Topic review. Subjects: Computer Science, Information Systems. View times: Submitted by: Agnieszka Pregowska. Introduction People have always needed to communicate. Historical Background The first long-distance messages were sent via a runner to warn of an invasion of some other danger, but that was too often too slow.
Figure 3. A scheme of a heliograph. Figure 5. Block diagram of the FSO system. Having the defined minimum received power P recmin , the link range can be determined from the Formula 1 : 2 Thus, a formed and modulated optical signal reaches the opposite optical radiation receiver, which consists of a receiving lens, an optical filter, a photodiode with a power supply and cooling system, a low-noise preamplifier, and a signal processing system.
With a sufficiently large aperture, the fluctuating beam is within the lens surface, so the fluctuations will be averaged according to: 3 Formula 3 shows that the diameter of the lens should match the planned range of the link on a given wavelength.
FSO Advantages and Disadvantages Free-Space Optical Communication is the answer to the worldwide demand for broadband communications offering flexible network connections [ 88 ]. The Communication Networks in the Future The crucial issue in Free-Space Communication is to expand the maximum usable distance between transceivers [ ]. Conclusions The need to improve data transfer drives the research and application of new technologies. References Wang, B. Safety informatics as a new, promising and sustainable area of safety science in the information age.
Information Age. Chuan, N. Sun, X. Publications Office of the European Union. Cong, L. IEEE Commun. Chowdhury, M. Tom, S. History of Wireless Communication. RBIS , 15, 37— Uysal, M. Sorrentino, R. Parida, S. Kaushal, H. Klotzkin, D. Khalighi, M. Khan, M. Bouchet, O. Malik, A. Huang, H. Althunibat, S. IEEE Trans.
Ghassemlooy, Z. Free Space Optical Communications. Darrigol, O. Agrawal, G. Optics in Our Time. Holzmann, G. Data Communications: The First Years. Chappe, I. In French Dawson, K. Electromagnetic telegraphy: Early ideas, proposals and apparatus.
In History of Technology; Hall, R. Oersted, H. Experiments on the effect of a current of electricity on the magnetic needles. Burns, R. Galvin, K. Bell, A. On the production and reproduction of sound by light. Forge, S. Singer, J. Masers; John Wiley and Sons Inc. Gould, R. Essiambre, R.
Capacity Limits of Optical Fiber Networks. Kolker, M. Laser Communications. Cable Free Solutions. Available online: www. Davis, C.
Flexible optical wireless links and networks. Nisar, S. Laser Appl. Rappaport, T. The wireless revolution. Caplan, D. Laser communication transmitter and receiver design. Fiber Commun. Zafar, F. Tyagi, S. Energy Watch News. Ramirez-Iniguez, R. Razavi, B. Bloom, S. Rogalski, A. Bielecki, Z. Maximisation of signal-to-noise ratio in infrared receivers.
Opto Electron. Understanding the performance of free-space optics. Al-Akkoumi, M. A tracking system for mobile FSO. Light Pointe Wireless. Teramile Company. Siegel, T. Wasiu Popoola, Z.
Srivastava, D. Kolwas, M. Vavoulas, A. Islam, A. Optik , , — Xu, G. Influence of atmospheric turbulence on FSO link performance. Ummul, K. Grover, M. Fu, H. Performance analysis of a PPM-FSO communication system with an avalanche photodiode receiver over atmospheric turbulence channels with aperture averaging.
Chaman-Motlagh, A. A modified model of the atmospheric effects on the performance of FSO links employing single and multiple receivers. Mirhosseini, M. Free-space communication through turbulence: A comparison of plane-wave and orbital-angular-momentum encodings. Krishnan, P. Gurdeep, S. Leitgeb, E. Impact of atmospheric effects in Free Space Optics transmission systems. Anshul, V. Analysis of free space optical link in turbulent atmosphere.
Rahman, A. Impact of rain weather over free space optic communication transmission. Prokes, A. Atmospheric effects on availability of free space optics systems. Jasmine, S. Investigation on free space optical communication for various atmospheric conditions. Al-Gailani, S. Single and multiple transceiver simulation modules for free-space optical channel in tropical malaysian weather.
Enhancement of free space optical link in heavy rain attenuation using multiple beam concept. On June the 3rd he conducted the first wireless telephone call in history using this method. The invention of lasers in the 's revolutionised free space optics and led us to the modern products.
Modern laser links allows the transmission of data through the air by modulating the wavelength of the laser light, these methods allow for high bandwidths to be achieved, typically Mbps or 1Gbps although there have been 10Gbps products on the market in the past - and rumours of another 10Gbps product in development at the moment.
The most common use of FSO links is to link 2 building together as a bridge between 2 LAN segments — although I know of a deployment in a fleet of police cars in the USA for fast data download from the cars internal cameras to the station when the cars park in designated bays.
FSO links are also used as a backup to traditional Fibre networks. Modern laser links can provide 1Gbps throughput at distances of up to M when deployed in the correct environment. As with any technology there are pros and cons to it. This specific feature can cause a problem when the output power has to be coupled efficiently into a fiber and external optics such as cylindrical lenses are used to increase the coupling efficiency.
Surface-emitting diodes typically produce less power output. However, the beam pattern is close to being symmetrical or round. A typical value for the beam divergence angle is 12 degrees. This feature is beneficial for coupling light into a round optical fiber. A lens is a piece of glass or other transparent material that refracts light rays in such a way that they can form an image. Lenses can be envisioned as a series of tiny refracting prisms, and each of these prisms refracts light to produce its own image.
When the prisms act together, they produce an image that can be focused at a single point. Lenses can be distinguished from one another in terms of their shape and the materials from which they are made.
The shape determines whether the lens is converging or diverging. The material has a refractive index that determines the refractive properties of the lens. The horizontal axis of a lens is known as the principal axis. A converging convex lens directs incoming light inward toward the center axis of the beam path. Converging lenses are thicker across their middle and thinner at their upper and lower edges.
When collimated Make rays of light or particles accurately parallel: as adjective collimated a collimated electron beam. Converging and diverging lenses. The focal length f of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance.
The focal length f is then given by. After illustrating the basic concepts of FSO, we return to the important definitions related to the laser power reduction due to atmospheric channel effects phenomena. These definitions are considered as the core principle of FSO transmission channel turbulence namely atmosphere, aerosol, absorption, scattering, and radiance etc.
Absorption and scattering are related to the loss and redirection of the transmitted energy. The majority of these definitions will be discussed in detail in the case study of this chapter section 4. An atmosphere is a layer of gases surrounding a planet or other material body material of sufficient mass that is held in place by the gravity of the body. An atmosphere is more likely to be retained if the gravity is high and the atmosphere's temperature is low.
Earth atmospheric, which is mostly nitrogen, also contains oxygen used by most organism for respiration and carbon dioxide used by plants, algae and cyanobacteria for photosynthesis, also protects living organisms from genetic damage by solar ultraviolet radiation.
Another definition of an atmosphere is the envelope of gases surrounding the earth or another planet. An aerosol is defined as a colloidal system of solid or liquid particles in a gas. An aerosol includes both the particles and the suspending gas, which is usually air.
This term describes an aero-solution, clouds of microscopic particles in air. According to the literature, the size range of aerosol particles to be only from 0. Another definition of aerosol is extremely-fine liquid droplets or solid particles that remain suspended in air as fog or smoke. Fog is a thick cloud of tiny water droplets suspended in the atmosphere at or near the earth's surface that obscures or restricts visibility to a greater extent than mist; strictly, reducing visibility to below 1 km.
Smoke is a visible suspension of carbon or other particles in air, typically one emitted from a burning substance. Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. Dust is a fine powder made up of very small pieces of earth or sand. Absorption of the light is the decrease in intensity of optical radiation light as it passes through a material medium owing to its interaction with the medium.
In the process of absorption, the energy of the light is converted to different forms of internal energy of the medium; it may be completely or partially re-emitted by the medium at frequencies other than the frequency of the absorbed radiation.
Light scattering is a form of scattering in which light is the form of propagating energy which is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media.
Deviations from the law of reflection due to irregularities on a surface are also usually considered to be a form of scattering. When these are considered to be random and dense enough that their individual effects average out, this kind of scattered reflection is commonly referred to as diffuse reflection. Radiance erasures of the quantity of radiation that passes through or is emitted from a surface and falls within a given solid angle in a specified direction.
Radiance is also used to quantify emission of neutering and other particles. Attenuation is the gradual loss in intensity of any kind of flux through a medium.
Attenuation affects the propagation of waves and signals transmission media. Scintillation is a flash of light produced in a transparent material by the passage of a particle an electron, an alpha particle an ion, or a high-energy photon. The process of scintillation is one of luminescence whereby light of a characteristic spectrum is emitted following the absorption of radiation. The emitted radiation is usually less energetic than that absorbed. Scintillation is an inherent molecular property in conjugated and aromatic organic molecules and arises from their electronic structures.
Scintillation also occurs in many inorganic materials, including salts, gases, and liquids. According to reference [ 5 ], certain high-power laser beams used for medical procedures can damage human skin, but the part of the human body most susceptible to lasers is the eye. Like sunlight, laser light travels in parallel rays. The human eye focuses such light to a point on the retina, the layer of cells that responds to light. Like staring directly into the sun, exposure to a laser beam of sufficient power can cause permanent eye injury.
For that reason, potential eye hazards have attracted considerable attention from standards writers and regulators. The standards rely on parameters such as laser wavelength, average power over long intervals, peak power in a pulse, beam intensity, and proximity to the laser. Laser wavelength is important because only certain wavelengths—between about nm and 1, nm—can penetrate the eye with enough intensity to damage the retina.
The amount of power the eye can safely tolerate varies with wavelength. This is dominated by the absorption of light by water the primary component in the eye at different wavelengths.
The vitreous fluid of the eye is transparent to wavelengths of —1, nm. Thus, the focusing capability of the eye causes approximately a ,to-1 concentration of the power to be focused on a small spot of the retina. However, in the far infrared 1, nm and higher , such light is not transmitted by the vitreous fluid, so the power is less likely to be transferred to the retina. Although damage to the corneal surface is a possibility, the focusing capabilities of the eye do not lead to large magnification of power densities.
Therefore, much greater power is required to cause damage. The relevance of this is that lasers deployed in FSO that utilize wavelengths greater than 1, nm are allowed to be approximately times as powerful as FSO equipment operating at nm and still be considered eye safe.
Also, lasers that operate at such wavelengths are more costly and less available. Nevertheless, at least one FSO manufacturer has overcome these obstacles and currently offers equipment deploying multiple 1, nm lasers.
As such, damage from the ultraviolet radiation of sunlight is more likely than from long wavelength infrared. Eye response also differs within the range that penetrates the eyeball nm—1, nm because the eye has a natural aversion response that makes it turn away from a bright visible light, a response that is not triggered by an invisible infrared wavelength longer than 0.
Infrared light can also damage the surface of the eye, although the damage threshold is higher than that for ultraviolet light. High-power laser pulses pose dangers different from those of lower-power continuous beams. A single high-power pulse lasting less than a microsecond can cause permanent damage if it enters the eye. A low-power beam presents danger only for longer-term exposure.
Distance reduces laser power density, thus decreasing the potential for eye hazards. FSO contains three components: transmitter, free space transmitted channel line of sight, and receiver. FSO link is demonstrated as in Fig.
The selection of a laser source for FSO applications depends on various factors. It is important that the transmission wavelength is correlated with one of the atmospheric windows. As noted earlier, good atmospheric windows are around nm and nm in the shorter IR wavelength range.
In the longer IR spectral range, some wavelength windows are present between 3—5 micrometers especially 3. However, the availability of suitable light sources in these longer wavelength ranges is pretty limited at the present moment.
In addition, most sources need low temperature cooling, which limits their use in commercial telecommunication applications. Other factors that impact the use of a specific light source include the following:. Block diagram of an optical wireless link showing the front end of an optical transmitter and receiver [ 7 ].
The transmitter, which consists of two part main parts: an interface circuit and source driver circuit, converts the input signal to an optical signal suitable for transmission. The drive circuit of the transmitter transforms the electrical signal to an optical signal by varying the current follow through the light source. Transmitter function is to project the carefully aimed light pulses into the air. This optical light source can be of two types:. The information signal modulates the field generated by the optical source.
The modulated optical field then propagates through a free-space path before arriving at the receiver. In the receiver side, transmitted data realizes inverse operations i. Received signal converted back into fiber or cooper and connected to the network. Reverse direction data transported the same way full duplex. We can see, anything that can be done in fiber can be done with FSO. Equation 5 illustrates the data rate of FSO system:.
Small angles — divergence angle and spot size between transmitter and receiver are presented in Fig. Small angles — divergence and spot size between transmitter and receiver. Transmitter and receiver aperture diameters are quantifiable parameters, and are usually specified by manufacturer.
Table 1 illustrates the relation of divergence in mrad , range in km , and spot diameter in inches or feet. The atmospheric attenuation is one of the challenges of the FSO channel, which may lead to signal loss and link failure. The atmosphere not only attenuates the light wave but also distorts and bends it.
Transmitted power of the emitted signal is highly affected by scattering and turbulence phenomena. Attenuation is primarily the result of absorption and scattering by molecules and particles aerosols suspended in the atmosphere.
Distortion, on the other hand, is caused by atmospheric turbulence due to index of refraction fluctuations. Attenuation affects the mean value of the received signal in an optical link whereas distortion results in variation of the signal around the mean.
Aerosols are particles suspended in the atmosphere with different concentrations. They have diverse nature, shape, and size. Aerosols can vary in distribution, constituents, and concentration. As a result, the interaction between aerosols and light can have a large dynamic, in terms of wavelength range of interest and magnitude of the atmospheric scattering itself.
Above the boundary layer, aerosol concentration rapidly decreases. At higher elevations, due to atmospheric activities and the mixing action of winds, aerosol concentration becomes spatially uniform and more independent of the geographical location. Scattering is the main interaction between aerosols and a propagating beam.
Because the sizes of the aerosol particles are comparable to the wavelength of interest in optical communications, Mie scattering theory is used to describe aerosol scattering [ 8 ].
Such a theory specifies that the scattering coefficient of aerosols is a function of the aerosols, their size distribution, cross section, density, and wavelength of operation. The different types of atmospheric constituents' sizes and concentrations of the different types of atmospheric constituents are listed in Table 2 [ 7 , 9 ].
Visibility was defined originally for meteorological needs, as a quantity estimated by a human observer. It defined as Kruse model means of the length where an optical signal of nm is reduced to 0. However, this estimation is influenced by many subjective and physical factors. The essential meteorological quantity, namely the transparency of the atmosphere, can be measured objectively and it is called the Runway Visual Range RVR or the meteorological optical range [ 11 ].
Some values of atmospheric attenuation due to scattering based on visibility are presented in Table 3. Variation in atmospheric attenuation due scattering based on visibility data obtained from [ 7 , 12 ]. When the length difference between the two optical paths varies, the energy passes through minima and maxima. The visibility V is defined by:.
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. 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 [ 13 ]. 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 varying with time and depend on the current local conditions and weather. L is the distance between transmitter and receiver unit: km ;. The aerosols that have the most absorption potential at infrared wavelengths include water, O 2 , O 3 , and CO 2 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. 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 ]:.
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 ].
Atmospheric transmittance window with absortion contribution. Scattering is defined as the dispersal of a beam of radiation into a range of directions as a result of physical interactions. The scattered light is polarized, and of the same wavelength as the incident wavelength, which means that there is no loss of energy to the particle [ 10 ].
There are three main types of scattering: 1 Rayleigh scattering, 2 Mie scattering, and 3 non-selective scattering. Figure 8 illustrates the patterns of Rayleigh, Mie and non-Selective scattering. Patterns of Rayleigh, Mie and Non-selective scattering. The scattering process for different scattering particles present in the atmosphere is summarized in Table 4 [ 21 ]. It is possible to calculate the scattering coefficients from the concentration of the particles and the effective cross section such as [ 16 ]:.
Rayleigh scattering refers to scattering by molecular and atmospheric gases of sizes much less than the incident light wavelength. The Rayleigh scattering coefficient is given by [ 16 ]:. The result is that Rayleigh scattering is negligible in the infrared waveband because Rayleigh scattering is primarily significant in the ultraviolet to visible wave range [ 10 ]. Mie scattering occurs when the particle diameter is equal or larger than one-tenth the incident laser beam wavelength, see Table 4.
Mie scattering is the main cause of attenuation at laser wavelength of interest for FSO communication at terrestrial altitude. Transmitted optical beams in free space are attenuated most by the fog and haze droplets mainly due to dominance of Mie scattering effect in the wavelength band of interest in FSO 0.
The attenuation levels are too high and obviously are not desirable [ 22 ]. The Mie scattering coefficient expressed as follows [ 10 ]:. Although their concentration is closely related to the optical visibility, there is no single particle dimension distribution for a given visibility [ 24 ].
V : is the visibility Visual Range [ k m ]. Since we are neglecting the absorption attenuation at wavelength of interest and Rayleigh scattering at terrestrial altitude and according to Eq. Rain is formed by water vapor contained in the atmosphere. It consists of water droplets whose form and number are variable in time and space. Their form depends on their size: they are considered as spheres until a radius of 1 mm and beyond that as oblate spheroids: flattened ellipsoids of revolution [ 11 ].
Rainfall effects on FSO systems:. The laser is able to pass through the raindrop particle, with less scattering effect occurring. The haze particles are very small and stay longer in the atmosphere, but the rain particles are very large and stay shorter in the atmosphere. This is the primary reason that attenuation via rain is less than haze [ 24 ]. An interesting point to note is that RF wireless technologies that use frequencies above approximately 10 GHz are adversely impacted by rain and little impacted by fog.
This is because of the closer match of RF wavelengths to the radius of raindrops, both being larger than the moisture droplets in fog [ 14 ]. The rain scattering coefficient can be calculated using Stroke Law [ 25 ]:. N a : is the rain drop distribution, c m - 3.
Q s c a t : is the scattering efficiency. The raindrop distribution N a can be calculated using equation following:. V a : is the limit speed precipitation. Limiting speed of raindrop [ 25 ] is also given as:. For more details about several weather conditions and the corresponding visibility at various wavelengths readers can refer to references [ 26 , 27 ].
Clear air turbulence phenomena affect the propagation of optical beam by both spatial and temporal random fluctuations of refractive index due to temperature, pressure, and wind variations along the optical propagation path [ 28 , 29 ]. Atmospheric turbulence primary causes phase shifts of the propagating optical signals resulting in distortions in the wave front. These distortions, referred to as optical aberrations, also cause intensity distortions, referred to as scintillation.
Moisture, aerosols, temperature and pressure changes produce refractive index variations in the air by causing random variations in density. These variations are referred to as eddies and have a lens effect on light passing through them. When a plane wave passes through these eddies, parts of it are refracted randomly causing a distorted wave front with the combined effects of variation of intensity across the wave front and warping of the isophase surface [ 30 ].
The refractive index can be described by the following relationship [ 31 ]:. P : is the atmospheric pressure in [ m b a r ]. T : is the temperature in Kelvin [ K ]. If the size of the turbulence eddies are larger than the beam diameter, the whole laser beam bends, as shown in Fig. If the sizes of the turbulence eddies are smaller than the beam diameter and so the laser beam bends, they become distorted as in Fig.
Small variations in the arrival time of various components of the beam wave front produce constructive and destructive interference and result in temporal fluctuations in the laser beam intensity at the receiver see Fig. Laser beam Wander Due to turbulence cells that are larger than the beam diameter.
Scintillation or fluctuations in beam intensity at the receiver due to turbulence cells that is smaller than the beam diameter. Refractive index structure parameter C n 2 is the most significant parameter that determines the turbulence strength. Clearly, C n 2 depends on the geographical location, altitude, and time of day.
Close to ground, there is the largest gradient of temperature associated with the largest values of atmospheric pressure and air density. Therefore, one should expect larger values C n 2 at sea level. As the altitude increases, the temperature gradient decreases and so the air density with the result of smaller values of C n 2 [ 8 ]. In applications that envision a horizontal path even over a reasonably long distance, one can assume C n 2 to be practically constant.
However, a number of parametric models have been formulated to describe the C n 2 profile and among those, one of the more used models is the Hufnagel-Valley [ 32 ] given by:. The most important variable in its change is the wind and altitude. Turbulence has three main effects ; scintillation, beam wander and beam spreading. Scintillation may be the most noticeable one for FSO systems. Light traveling through scintillation will experience intensity fluctuations, even over relatively short propagation paths.
Where the Eq. Expressions of lognormal field amplitude variance depend on: the nature of the electromagnetic wave traveling in the turbulence and on the link geometry [ 8 ]. Beam spreading describes the broadening of the beam size at a target beyond the expected limit due to diffraction as the beam propagates in the turbulent atmosphere.
Here, we describe the case of beam spreading for a Gaussian beam, at a distance l from the source, when the turbulence is present. Then one can write the irradiance of the beam averaged in time as [ 33 ]:.
P o : is total beam power in W. To quantify the amount of beam spreading, describes the effective beam waist average as:. As seen in other turbulence figure of merits, T depends on the strength of turbulence and beam path. Particularly, T for horizontal path, one gets:. Geometric loss is the ratio of the surface area of the receiver aperture to the surface area of the transmitter beam at the receiver.
Since the transmit beams spread constantly with increasing range at a rate determined by the divergence, geometric loss depends primarily on the divergence as well as the range and can be determined by the formula stated as [ 2 ]:.
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