OPTICAL COMMUNICATION BY SENIOR PDF
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Perhaps surprisingly for a glassy substance, the fibers may a]80 be bent to quite smalJ radii or twj. F urthermore, cable s tr uctu res have been developed see Section 4. Taking the size and weight advantage into account, these optical fiber cables are generally superior in terms of storage, transportation, handling and installation than corresponding copper cables whilst exhibiting at least compar able strength and durability.
Fu rthermore, the reliability of the optical component s j s no longer a problem with predicted lifetimes of years now quite common.
Optical Fiber Communications 4th Ed Gerd Keiser
Both- these factors also tend to reduce maintenance time and costs. As yet this potential has not been fully realized because of the sophisticated, and therefore expensive, processes required to obtain ultra-pure glass, a nd the lack of production volume.
At present, optical fiber cabJe is reasonably competitive with coaxial cable, but not with simple copper wires e. However, it is likely that in the future it will become as cheap to use optical fibers with their superior performance than almost any type of electrical conductor, Moreover, overall system costs when utilizing optical fiber communication on long-haul links are generally reduced to those for equivalent electrical line systems because of the 10w loss and wideband properties of the optical transmission medium.
As indicated in f' , the requirement for intermediate repeaters and the associated electronics is reduced.. However, although this cost ben efi t gives a net gai n for 1 ong - ha u I links this is not usually the case in short-haul applications where the additional cost incurred, due to the electrical-optical conversion and vice versa , may be a deciding factor..
Nevertheless, there are other possible cost advantages in relation to shipping, handling, installation and maintenance, as well as the features indicated in c and d which may prove significant in the system choice.. The Jow cost potentia] of optical fiber communications not only provides Itrong competition with electrical line transmission systems, but also with microwave and millimeter wave radio transmission systems.
Although these. The fundamental principles giving rise to these enhanced performance characteristics, together with their practical realization, are described in the following chapters. However, a general understand ing 0 r the basic nature and properties of I ight is assumed.
If this is lacking, the reader is directed to the many excellent texts encompassing the topic, a few of which are indicated in Refs. Ben, "Selen ium and the p hotophone ', The E lect ric ian, pp. Platt, 'Survey or near infra-red communication systems" J. Mairnan, "Stimulated optical radiation in ruby" Nature.. Kraemer, -i Free-space optical cornmun ications', Signal, pp, , Kao and G. Hartman, J. Dymen L, C. Hwang and H. K uhn, 'Contin uous operation of Ga.
Optical Fiber Communications-3rd Edition
As lasers with consistently low degradation rates at room temperature', Appl. Russer, 'Introduction to optical communication', M. Howes and D. Morgan Eds. WoJf Ed. Jenkins and H. Z aj ac, Optics. J ntroduction to Modern Optics 2nd cdn.
Geometrical and Physical Optics, 3 rd cdn. Smith and J. Thomson, Optics, John WileY1 Lipson and H. Optical Physics, 2nd edn. Nevertheless, interest in the application of dielectric optical waveguides in such areas as optical imaging and medica] diagnosis c. Th is structure is illustrated in Fig. I n essence, the ligh t energy travels in both t he core a nu the cl adding allowing the associated fields to decay to a negligible value at the cladding-air interface.
The invention of' the clad waveguide structure led to the first serious proposals by Kao and Hockham I Ref. Il fiber wavagulde showing the core of refractive index n1 surrounded by tn. This has resulted in improved conventional glass refining techniques giving fibers with losses of around 4.
Most of this work was focused on the However, as silica fibers were studied in further detail it became apparent that transmission at longer wavelengths ]. This produced a shift in optical fiber source and detector technology in order to provide operation at these longer wavelength s. Hence at longer wavelength s, es pee ially a rau nd ].. The following sections will therefore outline the transmis sion of light in optical fibers prior to a more detai1ed discu ssion of the v ariou s.
I n Section 2. Furthermore, this provides a basis for the disc ussion of electromagnetic wave propagation presented in Section 2. Finally, Section T he refractive index of a mcdi U In is defined as the ratio or the velocity of light in a vacu urn to the velocity of ligh t in the rued i u rn.
A ray of 1ight travels more slowJy in an optically dense medium than in one that is less dense, and the refractive index gives a measure of this effect. When a ray is u. As nl is greater than n2. High i nLlt: From Eq. Hence it may be observed in Fig. Figure 2. The light ray shown in Fig. It must also he noted that the light transmission illustrated in Fig. Since on] y rays with a sufficiently s ha1 low g razing angle i. The geometry concerned with launching a light ray into an optical tiber is shown in Fig.
This situation is also illustrated in Fig. If the fiber has a regular cross section i. It must be noted Lhat within this analysis, as with the previous discussion of acceptance angle, we are concerned with meridional rays. Figure 2,,5 shows a light ray incident on the fiber core at an angle 81 to the fiber axis which is less than the acceptance angle for the fiber 9.
Hence Eq.. Also in this limiting case 61 becomes the acceptance angle for the fiber Sa. Combining these limiting cases into Eq. Hence the N A is defined as: The numerical aperture may also be given in terms of the relative refractive index difference 6. They are independent of the fiber core diameter and will hold for diameters as small as 8 urn, However, for smaller dia meters they break down as the geometric optics approach is invalid.
This is because the ray theory model is only a partial description of the character of light It describes the direction a plane wave component takes in the fiber but does not take into account interference between such components.
When interference phenomcn a arc con sidercd it is found that only rays with certain discrete characteristics propagate in the fiber core.
Thus the fiber will only support a discrete number of guided modes. This becomes critical in small core diameter fibers which only support one or a few modes. Hence electromagnetic mode theory must be applied in these cases I Ref. NA fo r tho fiber; c the acceptance angle ina ir for the fi beL. S olution: Usi ng Eq. Using Eq. However, another category of ray exists which is transmitted without passing through the fiber axis, These rays, which greatly outnumber the meridional rays, follow a helical path through the tiber as illustrated in Fig.
It is not easy to visualize the skew ray paths in two dimensions but it may be observed from Fig. The amount of smoothing is dependent on the number 0 ' reflections encountered by the skew rays. A further possible advantage of the transmission of skew rays becomes apparen t when their acceptance condition s are con sidered, In order to calculate th e acceptance angle for a skew ray it is necessary to define t he direction of the ray in two perpendicular planes, The geometry of the situation is illustrated in Fig.
The ray is refracted at the air-core interface before travelling to the point B in the same plane. Hence Eq, 2. Hence substituting for sin a from Eq. It may be noted that the inequality shown in Eq. Thus the acceptance conditions for skew rays are: Hence as may be observed from Fig. However, they are complementary to meridional rays and increase the lightgathering capacity of the fiber. This increased light-gathering ability may be significant for large N A fibers, but for most communication design purposes the expressions given in Eqs.
Example 2. S otution: The accepta nee a ngle for meridional rays ls give n bv Eq. For a medium with zero conductivity these vector relationships may be written in terms of the electric field E, magnetic field H, electric flux density D and magnetic flux density B as the curl equations: The four field vectors arc related by the relation s: Subs tituting for D and Band iaki ng the cu r1 of Eq s..
The basic solution of the wave equation is a sinusoidal wave, the most important form of which is a uniform plane wave given by: We may assume it consists of a slab of dielectric with refractive index n f sandwiched bel ween two regions of lower refractive index n2' In order to obtain an improved model for opti cal propagation it is useful to consider the interference of pi ane wave components within this dielectric waveguide.
The conceptual transition from ray to wave theory may be aided by consideration of a plane monoch romatic wave propagating in the direction or the ray path within the guide see Fig When e is the an g le bet ween the wave propagation vector or the equivalent ray and the guide axis.. When the total This situation is illustrated in Fig..
However, it may he observed from Fig. Nevertheless the optical wave is etTectively confined within the guide and the electric fiel d distribution in the x direction docs not chan gc as the wa vc propagates in the z di rection. The sin usoidally vary ing electric field 1 n the z direction is also shown in Fig. In effect Eq 54 2. It should be noted that there is a phase shift on reflection of the plane wave at the interface a.
The phase shift on reflection at a dielectric interflce til dc. Hence the light propagating within the guide is fanned into discrete "modes each typified by a distinct value of 9.
These modes have a periodic z dependence of the form exp -J! If we now assume a time dependence for the monochromatic electromagnetic 1ight field with angular frequency OJ of cxp Urnt '! To visualize the dominant modes propagating in the z direction we may consider plane w aves correspond ing to rays at different specific angles in the planar guide..
These plane waves give constructive interference to form standing wave patterns across the guide following a sine or cosine formula. It may be observed that m denotes the number of zeros in this tran s verse field pat tern. I n this way m signifies the order of the mode and is known as the mode number. When light is described as an electromagnetic wave it consists of a periodically varying electric field E and magnetic fie1d H wh i ch are orientated "' Claduuig penet For plane waves these constant phase points form a surface which is referred to as a wavefront.
As a monochrornati c I ight wave propagates along a waveguide in the z direction these points of constant phase tr a vel at a phase velocity Pp given by: Often the situation exists where a group of waves with closely similar frequencies propagate so that their resultant forms a packet of waves. The envelope of the wave packet or group 01 waves travels at a grou p velocitv Vg. Equation 2. Using Eq.. In order to appreciate these p hcnomena it is necessary to use the wave theory model for total internal reflection at a planar interface.
This is illustrated in Fig. As the guide-cladding interface lies in the y-z plane and the wave is incident in the. Since the phase fronts must match at all points along the interface in the z direction, the three waves shown in Fig. When the components are resolved in this plane: The wave vectors of the incident. Thus the three waves in the waveguide indicated in Fig. Initially let us consider the TE field at the boundary.
When Eqs.. The expressions obtained in Eqs, 2. Under the conditions of tota] internal reflection Eq4 2. This is signified by OE which is given by: The curves of the amplitude reflection coefficient I rER I and phase shift on reflection, against angle of incidence 4t l' for TE waves incident on a glass-a ir interface are displayed in Fig" 2.
These curves illustrate the above results, where under the condition s of tota] in ternal reflection the reflected wave h as an equal amplitude to the incident wave, but undergoes a phase shift corresponding to OE degrees.
Again the expressions given in Eqs, 2. The second phenomenon of interest under conditions of total internal reflection is the form of the electric field in the ciaddi ng of the g uidc, Before the cri tical angle for total intern al reflection is reached and hen ce when there is only partial reflection, the field in the cladding is of the form given by Eq. A field of th is type stores energy and tran sports it in the directi on of propagation z but docs not tr ansport energy in the transverse di rection x , Nevertheless the existence of an evanescent field beyond the plane of reflection in the lower index medium indicates that optical energy is transmitted into the cladding.
The penetration of energy into the cladding underlines the importance of the choice of cladding mat eri al. It gives rise to the follow in g rcq uirernen ts: These effects degrade the reflection process by in tern cti on wi th the evanescent Held. Therefore the most widely used optical fibers consist of a core and cladding both made of gl ass.
The cl adding refractive index is thus hig her than would be the case with liquid or gaseous cladding giving a lower numerical aperture [or the fiber. Careful examination shows that the reflected beam is shifted laterally from the trajectory predicted by simple ray theory analysis as illustrated in Fig.
This lateral displacement is known as the Goos-Haenchen shift after its first observers..
The geometric reflection appears to take place at a virtual reflecting plane which is parallel to the dielectric interface in the lower index medium as indicated in Fig.
However, this concept provides an important insight into the guidance mechanism of d ielectric optical waveguides. In common with the planar guide Section 2. For the cylindrical waveguide we therefore refer to TE1m and TM1m modes.
These modes correspond to meridional rays see Section 2. DES 35 modes where E, and Hz are nonzero also occur within the cylindrical waveguide, These modes which result from skew ray propagation see Section 2. Fort un ately the analy sis may be simplified when considering optical fibers for communication purposes. This corresponds to small grazing angles a in Eq, These linearly polarized L.
P modes are not exact modes of the fiber except for the fundamental lowest order mode. However, as 6. Such modes are said to be degenerate. This linear combination of degenerate modes obtained from the exact solution produces a u sefu 1 sirnpl ification in the analysis of weakly guiding fibers. The mode subscripts I and m are related to the electric field intensity profile for a particul ar LP mode see Fig. T here are in general 21 field maxim a around the circumference of the fiber core and m field maxima along a radius vector.
Furthermore, it may be observed from Table 2. The electric field intensity profile for the lowest three LP modes, together with the electric field distribution of thci r constituent exact modes, are shown in Fig. Hence the origin of the term 'linearly polarized'.
Sol ution s of the wa vc equation for the cyl i ndrical fiber are separable, having the form: Hence the fiber supports a finite number of guided modes of the form of Eq, 2. In the core region the solutions are Bes sel function s denoted by J t: A graph of these grad u ally dam ped oscillatory functions with respect to r is shown in Fig. The electric field may therefore be given by: U and W which are the eigenvalues in the core and cladding respectively, are defined as: However, within this chapter there should be no confusion over this point.
Furthermore, using Eqs.. V is sometimes known as the normalized f lm thi c knes s as it re] ares to the thick ness 0 f the i u i de layer see Section J 1. It is also possible to define the normalized propagation constant b for a fiber in term s of the parameters of Eq, 2. Nevertheless, wave propagation does not cease abruptly below cutoff. I llEu 1[t:: Reproduced with permission from D. Glope, Appl.
The lower order modes obtained in a cylindrical homogeneous core waveguide are shown in Fig. In addition, the Bessel functions Jo and J1 arc plotted against the normalized frequency and where they cross the zero gives the cutoff point for the various modes.
However, the first zero crossing for J 0 is when the normalized frcq ucncy is 2. The electric field distribution of different modes gives similar distributions of light intensity within the fiber core. These waveguide patterns often called mode pattern s may give an indication of t he predominant modes propagating in the fiber.
The field intensity distributions for the three lower order L P modes were shown in Fig. I n Fig. Reproduced with permission from D, Gloge, Appl.
These will have the effect of coupling energy tra velling in one mode to another depcndin g on th e specific pertu rba tion.. Ray theory aids the u nderstan ding of th i s phenomenon as shown in Fig. In electromagnetic wave theory this corresponds to a change in the propagating mode for the 1 ight.
Thu s individ u al modes do not norm all y propagate throughout the length of the fiber without large energy transfers to adjacent modes even when the fiber is exceptionally good quality and not strained or bent by its surroundings. This mode conversion is known as mode coupling or mixing.
It is usually ana1yzed using coupled mode equations which can be obtained direct1y from Maxwell's equations. However, the theory is beyond the scope of this text and the reader is directed to Ref. Mode coupling affects the transmission properties of fibers in several important ways; a major one being in relation to the dispersive properties of fibers over long distances.
Message output The electrical forms of the message emerging from the signal processor are transformed into a sound wave or visual image. Sometimes these signals are directly usable when computers or other machines are connected through a fiber system. Advantages of Optical Fiber Communications 1. Wide bandwidth The light wave occupies the frequency range between 2 x Hz to 3. Thus the information carrying capability of fiber optic cables is much higher. Low losses Fiber optic cables offers very less signal attenuation over long distances.
This enables longer distance between repeaters. Immune to cross talk Fiber optic cables have very high immunity to electrical and magnetic field.
Since fiber optic cables are non-conductors of electricity hence they do not produce magnetic field. Thus fiber optic cables are immune to cross talk between cables caused by magnetic induction.
Interference immune Fiber optic cable immune to conductive and radiative interferences caused by electrical noise sources such as lighting, electric motors, fluorescent lights.
Light weight As fiber cables are made of silica glass or plastic which is much lighter than copper or aluminum cables. Light weight fiber cables are cheaper to transport. Small size 7. The diameter of fiber is much smaller compared to other cables, therefore fiber cable is small in size, requires less storage space.
More strength Fiber cables are stronger and rugged hence can support more weight. Security Fiber cables are more secure than other cables.
It is almost impossible to tap into a fiber cable as they do not radiate signals. No ground loops exist between optical fibers hence they are more secure. Long distance transmission Because of less attenuation transmission at a longer distance is possible.
Environment immune Fiber cables are more immune to environmental extremes. They can operate over large temperature variations. Also they are not affected by corrosive liquids and gases. Sage and easy installation Fiber cables are safer and easier to install and maintain. They are non-conductors hence there is no shock hazards as no current or voltage is associated with them. Their small size and light weight feature makes installation easier. Less cost Cost of fiber optic system is less compared to any other system.
High initial cost The initial cost of installation or setting up cost is very high compared to all other system. Maintenance and repairing cost The maintenance and repairing of fiber optic systems is not only difficult but expensive also. Jointing and test procedures Since optical fibers are of very small size. The fiber joining process is very costly and requires skilled manpower. Tensile stress Optical fibers are more susceptible to buckling, bending and tensile stress than copper cables.
This leads to restricted practice to use optical fiber technology to premises and floor backbones with a few interfaces to the copper cables. Short links Even though optical fiber cables are inexpensive, it is still not cost effective to replace every small conventional connector e. Fiber losses The amount of optical fiber available to the photo detector at the end of fiber length depends on various fiber losses such as scattering, dispersion, attenuation and reflection.
Applications of Optical Fiber Communications Applications of optical fiber communications include telecommunications, data communications, video control and protection switching, sensors and power applications. Telephone networks Optical waveguide has low attenuation, high transmission bandwidth compared to copper lines; therefore numbers of long haul co-axial trunks links between telephone exchanges are being replaced by optical fiber links.
Urban broadband service networks Optical waveguide provides much larger bandwidth than co-axial cable, also the number of repeaters required is reduced considerably. All these can be supplied over a single fiber optic link. Optical Fiber Waveguides In free space light ravels as its maximum possible speed i. When light travels through a material it exhibits certain behavior explained by laws of reflection, refraction. Electromagnetic Spectrum The radio waves and light are electromagnetic waves.
The rate at which they alternate in polarity is called their frequency f measured in hertz Hz. Infrared light covers a fairly wide range of wavelengths and is generally used for all fiber optic communications. Visible light is normally used for very short range transmission using a plastic fiber.
Ray Transmission Theory Before studying how the light actually propagates through the fiber, laws governing the nature of light m ust be studied.
These was called as laws of optics Ray theory. There is conception that light always travels at the same speed. This fact is simply not true. The speed of light depends upon the material or medium through which it is moving. In free space light travels at its maximum possible speed i. Reflection The law of reflection states that, when a light ray is incident upon a reflective surface at some incident angle 1 from imaginary perpendicular normal, the ray will be reflected from the surface at some angle 2 from normal which is equal to the angle of incidence.
Refraction Refraction occurs when light ray passes from one medium to another i. Refraction occurs whenever density of medium changes. The refraction can also observed at air and glass interface. When wave passes through less dense medium to denser medium, the wave is refracted bent towards the normal. The refraction bending takes place because light travels at different speed in different mediums.
The speed of light in free space is higher than in water or glass. Refractive Index The amount of refraction or bending that occurs at the interface of two materials of different densities is usually expressed as refractive index of two materials.
Refractive index is also known as index of refraction and is denoted by n. Based on material density, the refractive index is expressed as the ratio of the velocity of light in free space to the velocity of light of the dielectric material substance. The refractive index for vacuum and air os 1. Equation can be written as, This equation shows that the ratio of refractive index of two mediums is inversely proportional to the refractive and incident angles. As refractive index substituting these values in equation f.
Critical Angle When the angle of incidence 1 is progressively increased, there will be progressive increase of refractive angle 2. At some condition 1 the refractive angle 2 becomes 90o to the normal. The angle of incidence 1 at the point at which the refractive angle 1 becomes 90 degree is called the critical angle. It is denoted by c.
The critical angle is defined as the minimum angle of incidence 1 at which the ray strikes the interface of two media and causes an angle of refraction 2 equal to 90o. Fig 1. Total Internal Reflection TIR When the incident angle is increase beyond the critical angle, the light ray does not pass through the interface into the other medium. This gives the effect of mirror exist at the interface with no possibility of light escaping outside the medium.
In this condition angle of reflection 2 is equal to angle of incidence 1. TIR can be observed only in materials in which the velocity of light is less than in air. The two conditions necessary for TIR to occur are: The refractive index of first medium must be greater than the refractive index of second one.
The angle of incidence must be greater than or equal to the critical angle. Then above equation reduces to, The angle 0 is called as acceptance angle and omax defines the maximum angle in which the light ray may incident on fiber to propagate down the fiber. The Cone of acceptance is the angle within which the light is accepted into the core and is able to travel along the fiber.
The launching of light wave becomes easier for large acceptance cone. The angle is measured from the axis of the positive cone so the total angle of convergence is actually twice the stated value. Numerical Aperture NA The numerical aperture NA of a fiber is a figure of merit which represents its light gathering capability. Larger the numerical aperture, the greater the amount of light accepted by fiber. The acceptance angle also determines how much light is able to be enter the fiber and hence there is relation between the numerical aperture and the cone of acceptance.
NA is not a function of fiber dimension. Example 1. A light ray is incident from medium-1 to medium If the refractive indices of medium-1 and medium-2 are 1. Optical Fiver as Waveguide An optical fiber is a cylindrical dielectric waveguide capable of conveying electromagnetic waves at optical frequencies.
The electromagnetic energy is in the form of the light and propagates along the axis of the fiber. The structural of the fiver determines the transmission characteristics.
The propagation of light along the waveguide is decided by the modes of the waveguides, here mode means path. Each mode has distict pattern of electric and magnetic field distributions along the fiber length.
Only few modes can satisfy the homogeneous wave equation in the fiver also the boundary condition a waveguide surfaces. When there is only one path for light to follow then it is called as single mode propagation. When there is more than one path then it is called as multimode propagation. Single fiber structure A single fiber structure is shown in Fig. This cylinder is called as core of fiber. The core is surrounded by dielectric, called cladding. The index of refraction of core glass fiber is slightly greater than the index of refraction of cladding.
Modes of Fiber Fiber cables cal also be classified as per their mode. Light rays propagate as an electromagnetic wave along the fiber. The two components, the electric field and the magnetic field form patterns across the fiber. These patterns are called modes of transmission. The mode of a fiber refers to the number of paths for the light rays within the cable. According to modes optic fibers can be classified into two types.
Multimode fiber. Multimode fiber was the first fiber type to be manufactured and commercialized. The term multimode simply refers to the fact that numerous modes light rays are carried simultaneously through the waveguide. Multimode fiber has a much larger diameter, compared to single mode fiber, this allows large number of modes. Single mode fiber allows propagation to light ray by only one path. Single mode fibers are best at retaining the fidelity of each light pulse over longer distance also they do not exhibit dispersion caused by multiple modes.
Thus more information can be transmitted per unit of time. This gives single mode fiber higher bandwidth compared to multimode fiber. Some disadvantages of single mode fiber are smaller core diameter makes coupling light into the core more difficult. Precision required for single mode connectors and splices are more demanding. Fiber Profiles A fiber is characterized by its profile and by its core and cladding diameters.
One way of classifying the fiber cables is according to the index profile at fiber. The index profile is a graphical representation of value of refractive index across the core diameter. There are two basic types of index profiles. Step index fiber. Graded index fiber. Step Index SI Fiber The step index SI fiber is a cylindrical waveguide core with central or inner core has a uniform refractive index of n1 and the core is surrounded by outer cladding with uniform refractive index of n2.
The cladding refractive index n2 is less than the core refractive index n1. But there is an abrupt change in the refractive index at the core cladding interface. Refractive index profile of step indexed optical fiber is shown in Fig. The refractive index is plotted on horizontal axis and radial distance from the core is plotted on vertical axis. In the graded index GRIN fiber the refractive index is not uniform within the core, it is highest at the center and decreases smoothly and continuously with distance towards the cladding.
The refractive index profile across the core takes the parabolic nature. In graded index fiber the light waves are bent by refraction towards the core axis and they follow the curved path down the fiber length. This results because of change in refractive index as moved away from the center of the core.
A graded index fiber has lower coupling efficiency and higher bandwidth than the step index fiber. Comparison of Step Index and Graded Index Fiber Optic Fiber Configurations Depending on the refractive index profile of fiber and modes of fiber there exist three types of optical fiber configurations. These optic-fiber configurations are - Single mode step index fiber.
Multimode graded index fiber. Single mode Step index Fiber In single mode step index fiber has a central core that is sufficiently small so that there is essentially only one path for light ray through the cable. The light ray is propagated in the fiber through reflection. Single mode fiber is also known as fundamental or mono mode fiber. Single mode fiber will permit only one mode to propagate and does not suffer from mode delay differences. These are primarily developed for the nm window but they can be also be used effectively with time division multiple TDM and wavelength division multiplex WDM systems operating in nm wavelength region.
The core fiber of a single mode fiber is very narrow compared to the wavelength of light being used. Therefore, only a single path exists through the cable core through. Usually, 20 percent of the light in a single mode cable actually travels down the cladding and the effective diameter of the cable is a blend of single mode core and degree to which the cladding carries light. The disadvantage of this type of cable is that because of extremely small size interconnection of cables and interfacing with source is difficult.
Another disadvantage of single mode fibers is that as the refractive index of glass decreases with optical wavelength, the light velocity will also be wavelength dependent. Thus the light from an optical transmitter will have definite spectral width.
It is easy to manufacture. The light rays are propagated down the core in zig-zag manner. There are many paths that a light ray may follow during the propagation.
The light ray is propagated using the principle of total internal reflection TIR. Since the core index of refraction is higher than the cladding index of refraction, the light enters at less than critical angle is guided along the fiber. Light rays passing through the fiber are continuously reflected off the glass cladding towards the centre of the core at different angles and lengths, limiting overall bandwidth.
The disadvantage of multimode step index fibers is that the different optical lengths caused by various angles at which light is propagated relative to the core, causes the transmission bandwidth to be fairly small. Because of these limitations, multimode step index fiber is typically only used in applications requiring distances of less than 1 km.
The light ray is propagated through the refraction. The light ray enters the fiber at many different angles. As the light propagates across the core toward the center it is intersecting a less dense to more dense medium. Therefore the light rays are being constantly being refracted and ray is bending continuously. This cable is mostly used for long distance communication. The modes travelling in a straight line are in a higher refractive index so they travel slower than the serpentine modes.
This reduces the arrival time disparity because all modes arrive at about the same time. It is seen that light rays running close to the fiber axis with shorter path length, will have a lower velocity because they pass through a region with a high refractive index. Rays on core edges offers reduced refractive index, hence travel more faster than axial rays and cause the light components to take same amount of time to travel the length of fiber, thus minimizing dispersion losses. Typical attenuation coefficients of graded index fibers at nm are 2.
The main advantages of graded index fiber are: Reduced refractive index at the centre of core. Comparatively cheap to produce. Mode Theory for Cylindrical Waveguide To analyze the optical fiber propagation mechanism within a fiber, Maxwell equations are to solve subject to the cylindrical boundary conditions at core-cladding interface.
Hence the analysis of optical waveguide is more complex than metallic hollow waveguide analysis.
The two lowest order does are HE11 and TE Overview of Modes The order states the number of field zeros across the guide.
The electric fields are not completely confined within the core i. The low order mode confines the electric field near the axis of the fiber core and there is less penetration into the cladding. While the high order mode distribute the field towards the edge of the core fiber and penetrations into the cladding. Therefore cladding modes also appear resulting in power loss. In leaky modes the fields are confined partially in the fiber core attenuated as they propagate along the fiber length due to radiation and tunnel effect.
Cladding also improves the mechanical strength of fiber core and reduces surface contamination. Plastic cladding is commonly used.
Materials used for fabrication of optical fibers are silicon dioxide SiO2 , boric oxide-silica. If the core refractive index is 1. A step index multimode fiber with a numerical aperture of a 0. Mode Field Diameter and Spot Size The mode filed diameter is fundamental parameter of a single mode fiber. This parameter is determined from mode field distributions of fundamental LP01 mode. In step index and graded single mode fibers, the field amplitude distribution is approximated by Gaussian distribution.
In single mode fiber for fundamental mode, on field amplitude distribution the mode filed diameter is shown in fig. The material must be transparent for efficient transmission of light.
It must be possible to draw long thin fibers from the material. Fiber material must be compatible with the cladding material. Glass and plastics fulfills these requirements. Most fiber consists of silica SiO2 or silicate. Various types of high loss and low loss glass fibers are available to suit the requirements. Plastic fibers are not popular because of high attenuation they have better mechanical strength.
Glass Fibers Glass is made by fusing mixtures of metal oxides having refractive index of 1.
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The principal raw material for silica is sand and glass. The fiber composed of pure silica is called as silica glass. The desirable properties of silica glass are: Resistance to breakage from thermal shocks low thermal expansion.
Better transparency. Other types of glass fibers are Halide glass fibers. Active glass fibers Chalgenide glass fibers Plastic optical fibers Fiber Fabrication Methods The vapor-phase oxidation process is popularly used for fabricating optical fibers. In this process vapors of metal halides such as SiCl4 and Gecl4 reactive with oxygen and forms powder of SiO2 particles. The SiO2 particles are collected on surface of bulk glass and then sintered to form a glass rod called Preform.
The preforms are typically mm diameter and cm long from which fibers are drawn. A simple schematic of fiber drawing equipment The preform is feed to drawing furnace by precision feed mechanism. The preform is heated up in drawing furnace so that it becomes soft and fiber can be drawn easily. The fiber thickness monitoring decides the speed of take up spool.
The fiber is then coated with elastic material to protect it from dust and water vapor. Fig, 1. During the SiO2 deposition O2 and metal halide vapors can be controlled so the desired core-cladding diameters can be incorporated. The mandrel is removed when deposition process is completed; this preform is used for drawing thin filament of fibers in fiber drawing equipment. The rod is continuously rotated and moved upward to maintain symmetry of particle deposition.
The advantages of VAD process are - Both step and graded index fibers are possible to fabricate in multimode and single mode. The performs does not have the central hole. The performs can be fabricated in continuous length. Clean environment can be maintained.
A hollow silica tube is heated to about oC and a mixture of oxygen and metal halide gases is passed through it. The soot that develops from this deposition is consolidated by heating.
The tube is rotated while the heater is moved to and along the tube and the soot forms a thin layer of silica glass. The rotation and heater movement ensures that the layer is of constant thickness. Graded index fiber is produced by careful continuous control of the constituents.
The temperature is now increased to about oC and the tube is collapsed to form a solid rod called a preform. The preform is about 25 mm in diameter and 1 meter in length.
This will produce 25 km of fiber. The preform is placed at a height called a pulling tower and its temperature is increased to about oC. To prevent contamination, the atmosphere is kept dry and clean. Laser gauges continually monitor the thickness of the fiber and automatically adjust the pilling rate to maintain required thickness.
After sufficient cooling the primary buffer is applied and the fiber is drummed. It reduces mechanical stress on glass films.
There is no soot formation and hence sintering is not required. Non-isothermal microwave plasma at low pressure initiates the chemical reaction. Double-Crucible Method Double-crucible method is a direct melt process. In double-crucible method two different glass rods for core and Cladding are used as feedstock for two concentric crucibles.
The inner crucible is for core and outer crucible is for cladding. The fibers can be drawn from the orifices in the crucible.
Major advantage of double crucible method is that it is a continuous production process. Fiber Optic Cables The fiber optic cable is to be used under variety of situations such as underground, Outdoor poles or submerged under water.
The structure of cable depends on the situation where it is to be used, but the basic cable design principles remain same. Maximum allowable axial load on cable decides the length of the cable be reliably installed.
Also the fiber cables must be able to absorb energy from impact loads. The outer sheath must be designed to protect glass fibers from impact loads and from corrosive environmental elements. Fiber Arrangements Several arrangements of fiber cables are done to use it for different applications. The most basic form is two fiber cable designs. It is also known as basic building block of fiber cable. For providing strength to the core several coatings of different materials are applied as shown in fig 1.
Multiple fiber cable can be combined together using similar techniques. The basic fiber building blocks are used to form large cable. These units are bound on a buffer material which acts as strength element along with insulated copper conductor. The fiber building blocks are surrounded by paper tape, PVC jacket, yarn and outer sheath. To ease identification, individual fibers are colour coded Table 1.
Plastic Fiber Optic Cables Fibers can also be manufactured from transparent plastic which offers advantages of larger diameter 1 mm , increased flexibility, can be cut using a hot razor blade, ease of termination. But because of high intrinsic loss use of plastic fibers is normally restricted to only few metres. Plastic optic fiber POF offers noise immunity and low cable weight and volume and is competitive with shielded copper wire making it suitable for industrial applications.
Also, silica fiber can tolerate higher temperature than plastic fiber. On the other hand, POF is more flexible, less prove to breakage, easier to fabricate and cost is low than glass fibers.
These advantages and disadvantages are summarized in Table 1. Recommended Questions 1. State and explain the advantages and disadvantages of fiber optic communication systems? State and explain in brief the principle of light propagation? Explain the important conditions for TIR to exit in fiber.? Derive an expression for maximum acceptance angle of a fiber? Explain the acceptance come of a fiber? Define numerical aperture and state its significance also?
Explain the different types of rays in fiber optic? Explain the following —? A Step index fiber B Graded index fiber What is mean by mode of a fiber? Write short notes on following — A Single mode step index fiber B Multimode step index fiber C Multimode graded index fiber.
Explain the fiber materials used in fabrication requirements? In case of glass fibers how the refractive index can be varied? Briefly explain following techniques of fabrication?
Introduction One of the important properties of optical fiber is signal attenuation. It is also known as fiber loss or signal loss. The signal attenuation of fiber determines the maximum distance between transmitter and receiver. The attenuation also determines the number of repeaters required, maintaining repeater is a costly affair.
Another important property of optical fiber is distortion mechanism. As the signal pulse travels along the fiber length it becomes broader.
After sufficient length the broad pulses starts overlapping with adjacent pulses. This creates error in the receiver. Hence the distortion limits the information carrying capacity of fiber. Attenuation Attenuation is a measure of decay of signal strength or loss of light power that occurs as light pulses propagate through the length of the fiber. In optical fibers the attenuation is mainly caused by two physical factors absorption and scattering losses.
Absorption is because of fiber material and scattering due to structural imperfection within the fiber. Micro bending of optical fiber also contributes to the attenuation of signal. The rate at which light is absorbed is dependent on the wavelength of the light and the characteristics of particular glass. Glass is a silicon compound; by adding different additional chemicals to the basic silicon dioxide the optical properties of the glass can be changed.
The Rayleigh scattering is wavelength dependent and reduces rapidly as the wavelength of the incident radiation increases. The attenuation of fiber is governed by the materials from which it is fabricated, the manufacturing process and the refractive index profile chosen.
Let the couples optical power is p 0 i. This parameter is known as fiber loss or fiber attenuation. Attenuation is also a function of wavelength. Optical fiber wavelength as a function of Wavelength is shown in Fig. Example 2. Determine — 1 Overall signal attenuation in dB. Each splice introducing attenuation of 1 dB. A continuous 12 km long optical fiber link has a loss of 1.
Given data: Absorption loss results in dissipation of some optical power as hear in the fiber cable. Although glass fibers are extremely pure, some impurities still remain as residue after purification. The amount of absorption by these impurities depends on their concentration and light wavelength.
Absorption is caused by three different mechanisms. Absorption by Atomic Defects Atomic defects are imperfections in the atomic structure of the fiber materials such as missing molecules, high density clusters of atom groups.
These absorption losses are negligible compared with intrinsic and extrinsic losses. The radiation dames the internal structure of fiber. The damages are proportional to the intensity of ionizing particles. This results in increasing attenuation due to atomic defects and absorbing optical energy. The total dose a material receives is expressed in rad Si , this is the unit for measuring radiation absorbed in bulk silicon. Extrinsic Absorption Extrinsic absorption occurs due to electronic transitions between the energy level and because of charge transitions from one ion to another.
A major source of attenuation is from transition of metal impurity ions such as iron, chromium, cobalt and copper. The effect of metallic impurities can be reduced by glass refining techniques.
Another major extrinsic loss is caused by absorption due to OH Hydroxil ions impurities dissolved in glass. Vibrations occur at wavelengths between 2. The absorption peaks occurs at , and nm. These are first, second and third overtones respectively. Between these absorption peaks there are regions of low attenuation. Thus intrinsic absorption sets the fundamental lower limit on absorption for any particular material.
Intrinsic absorption results from electronic absorption bands in UV region and from atomic vibration bands in the near infrared region. The electronic absorption bands are associated with the band gaps of amorphous glass materials.
Absorption occurs when a photon interacts with an electron in the valene band and excites it to a higher energy level. In the IR infrared region above 1.
The inherent IR absorption is due to interaction between the vibrating band and the electromagnetic field of optical signal this results in transfer of energy from field to the band, thereby giving rise to absorption, this absorption is strong because of many bonds present in the fiber.
Attenuation spectra for the intrinsic loss mechanism in pure Ge is shown in Fig. The loss in infrared IR region above 1. The expression is derived for GeO2-SiO2 glass fiber.
Rayleigh Scattering Losses Scattering losses exists in optical fibers because of microscopic variations in the material density and composition. As glass is composed by randomly connected network of molecules and several oxides e. These two effects results to variation in refractive index and Rayleigh type scattering of light. Rayleigh scattering of light is due to small localized changes in the refractive index of the core and cladding material.
There are two causes during the manufacturing of fiber. The first is due to slight fluctuation in mixing of ingredients. The random changes because of this are impossible to eliminate completely. The other cause is slight change in density as the silica cools and solidifies. When light ray strikes such zones it gets scattered in all directions. The overall losses in this fiber are more as compared to single mode fibers. The material dispersion parameter for a glass fiber is 20 ps nm-I km-1 at a wavelength of 1.
Estimate the pulse broadening due to material dispersion within the fiber when light is launched from an injection laser source with a peak wavelength of 1. A multimode step index fiber has a numerical aperture of 0. Estimate the bandwidth-length product for the fiber assuming only intermodal dispersion and a return to zero code when: a there is no mode coupling between the guided modes; b mode coupling between the guided modes give a characteristic length equivalent to 0.
A multimode, optimum near parabolic profile graded index fiber has a material dispersion parameter of 30 ps nm-I km-1 when used with a good LED source of rms spectral width 25 nm. Estimate the total rms pulse broadening per kilometer within the fiber assuming waveguide dispersion to be negligible. Hence estimate the bandwidth-length product for the fiber.
The difference in the effective refractive indices nx - ny for the two orthogonally polarized modes in conventional single mode fibers are in the range 9.In single mode fiber for fundamental mode, on field amplitude distribution the mode filed diameter is shown in fig. Other sources of photo detector noise are from dark current and leakage current.
The recombination in these semiconductors is quite slow i. An optical amplifier amplify the optical bit stream without converting it into electrical form. There are two basic types of index profiles. Debye 'Electromagnetic waves along long cylinders of dieiectric', Annal. While the high order mode distribute the field towards the edge of the core fiber and penetrations into the cladding. Explain various types of misalignments in fiber cables.
More recently this wavelength range has been extended to include the 1.
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