Conventional methods of long distance communication use radio waves (106 Hz) and micro waves (1010 Hz) as carrier waves. A light beamacting as carrier waves is capable of carrying far more informationsince optical frequencies are extremely large (1015 Hz). Soon after the discovery of laser, some preliminary experiments inpropagation of information carrying light waves through the openatmosphere wave carried out, but it was realized that the unwantedelements such as rain, fog etc. leads to adverse effects. Thus inorder to have an efficient and dependable communication system one
would require a guiding medium in which the information carrying lightwaves could be transmitted. This resulted in the development ofoptical fibre which is an efficient guiding medium for laser light.Basic principle-total internal reflectionThe basic principle of optical fibre is multiple total internalreflection. When a ray of light travel from denser to a rarer medium,at an angle of incidence greater than critical angle c, the ray isnot reflected but it is reflected into the same denser medium. Thisproperty is called total internal reflection. Light signals aretransmitted through optic fibres by multiple total internalreflection.Fibre construction and fibre dimensionAn optical fibre consists of a vary thin transparent cylindrical corehaving refractive index n1 surrounded by a cylindrical shell calledcladding of slightly lower refractive index n2. The core claddingsystem is surrounded by plastic jackets.Typical value of core diameter is 50µm.Typical value of outer diameterof cladding is 125µm. With jacket, totally optical fibre has an outerdiameter 150µm.Propagation mechanism in optical fibresConsider an optical fibre with n1 and n2 as the refractive indices ofcore and cladding material respectively. Consider a ray of lightentering through one end making an angle with the axis.By Snell's law, n1 = sin i / sinOr sin = sin i / n1 ------ (1)At core-cladding interface, refractive index µ = n1/n2Total internal reflection takes place at core-cladding interface ifthe angle of incidence at core-cladding interface is equal to orgreater than critical anglec.We have µ=n1/n2=1/sincsinc = n2/n1 -----(2)For total internal reflection to take place, > cor, sin > sinci.e., sin > n2/n1 [from eqn(2)]
But from the ABC, sin = coscos > n2/n1i.e., (1-sin2)1/2 > n2/n11-sin2 > (n2/n1)2sin2 < 1-(n2/n1)2sin2 < (n12-n22)/n12sin < [(n12-n22)/n12]1/2sin < (n12-n22)1/2/n1But from equation (1), sin = sini/n1sini /n1 < (n12-n22)1/2/n1sini < (n12-n22)1/2i < sin-1(n12-n22)1/2Thus if the angle of incidence is greater than sin-1(n12-n22)1/2,total internal reflection and transmission of light will not takeplace.Acceptance angleThe maximum angle of incidence at which light may enter the fibre inorder to be propagated is, im= sin-1(n12-n22)1/2This angle is called acceptance angle for the fibre.Numerical apertureNumerical aperture of an optical fibre is a measure of its gatheringcapacity and it is denoted as the sine of acceptance angle.i.e., NA = sinim= (n12-n22)1/2Fractional index change ()Fractional index change () is the ratio of the refractive indexdifference between the core and cladding to the refractive index ofthe core of an optical fibre. = (n1– n2) / n1Relation between NA and We have = (n1– n2) / n1(n1– n2) = n1 --------- (1)NA = (n12-n22)1/2 = [(n1+ n2) (n1-n2)] ½ = [(n1+ n2) n1] ½ {Applying equation (1)}Since, n1 n2, n1+ n2 = 2n1NA = [2 n12]1/2Types of optical fibresOn the basis of materialsOn the basis of materials used for core and cladding, optical fibresare classified into three categories:1. Glass fibres (glass core with glass cladding)2. Plastic fibres (plastic core with plastic cladding)3. PCS fibres (polymer cladding with silica core)Plastic fibres have the advantage of more flexibility than glassfibres but attenuation is greater in plastic fibres, comparing withglass fibres.
On the basis of refractive index profile- Step index and graded index fibreIf the optical fibre has a core of uniform constant refractive indexn1 and a cladding of slightly lower refractive index n2, it is calleda step index fibre. The cross sectional refractive index profile is asshown:If the core of optic fibre has a non-uniform refractive index thatdecreases gradually from the centre towards the core-claddingboundary, it is called a graded index (GRIN) fibre. The claddingsurrounding the core has a uniform refractive index, slightly lowerthan the refractive index of the core. The cross sectional refractiveindex profile is as shown:Modes of propagationWhen the light rays are guided through the fibres, it propagates indifferent modes. Modes can be visualized as the possible number ofallowed paths of light in an optical fibre. On the basis of the modesof propagation, optical fibres are classified into two:1. Single mode fibre that supports only one mode of propagation. Forthis a step index fibre with small core diameter (around 10m) isused.2. Multimode fibre that support a number of modes. For this, a stepindex fibre with large core diameter (around 50m) or a graded index(GRIN) is used.V-number of a multimode fibreThe number of modes supported for propagation by a multimode fibre isdetermined by a parameter called V-number (denoted as V). If thesurrounding medium is air, then the V-number is given by,where'd' is the core diameter, '' is the wavelength of lightpropagating in the fibre. The number of modes supported by the fibreis given by,Number of modes = V2/2AttenuationAttenuation is the loss of power suffered by the optical signal as itpropagates through the fibre. It is also referred as fibre loss.Attenuation co-efficient or attenuation,where Pi is the optical power launched at the input and Po the output
power after traveling a distance L km.Different mechanisms of attenuation1. Absorption: The optical fibre material and the impurities presentin the material absorb light leading to fibre loss.2. Rayleigh scattering: This occurs due to the local variations inrefractive index. The Rayleigh scattering loss depends on thewavelength. It varies as 1/4 and becomes significant at lowerwavelengths. Below 0.8m, the scattering loss is very high.3. Radiation losses: This occurs due to the bending of the fibre.There are two types of bends.(a) Macroscopic: During transportation and installation, it happensthat the optical fibre cable is bent over large radius. Such bendslead to loss of light.(b) Microscopic: Due to winding of optical fibre cable, microscopicbends occur. Light rays get scattered at the small bends and escapeinto the cladding. This can be minimized by selecting a suitablejacket that can withstand the stresses.Dispersion losses due to various modes of propagation1) Waveguide dispersionIn single mode fibres, part of the light ray will be refracted intothe cladding. The loss due to this is referred as waveguidedispersion. Waveguide dispersion is negligible in multimode fibres.2) Intermodal dispersionThe intermodal dispersion occurs in multimode fibres where raysassociate with various modes travel different distances through thefibre. As a result, the signal broadens and the output signal is nolonger identical with the input signal. Signal broadening is less ingraded index and step index single mode fibres.3) Material dispersion or chromatic dispersionIf we use white light, all the colours of the input radiation are notreaching the other end at the same time since they travel withdifferent velocities. Signal distortion of this kind is calledmaterial dispersion or chromatic dispersion.Applications of optical fibres
1. In fibre optic communication2. In fibre optic sensors3. For industrial automation4. In security alarm systems5. In local area network (LAN) of computers6. For high speed data transmission in computers7. Medical applications- for diagnosis and surgical applications (endoscopy)8. Military applications (fibre guided missiles)Point to point light wave communication using optic fibreA simple block diagram of fibre optic communication system is shown below:Optical transmitterA light emitting diode (LED) or a semiconductor laser can be used asoptical source. Modulation modulates the input signal and opticalsignal and then transmitted through optical fibre cables to thereceiver.Optical receiverA photodiode can be used as optical detector. The detected wave isdemodulated to extract the signal.Advantages of fibre optic communication1) Wide band width.2) Low attenuation and other transmission losses.3) Small size and weight.4) Safe from electrical interference caused by lightning, electricmotors, fluorescent tube and other electrical noise sources.5) Lack of cross talk between parallel fibres.6) Easy installation and easy maintenance.7) Flexible.8) Temperature resistance.9) Highly economical.10) High degree of signal security.11) Longer life span.Disadvantages of fibre optic communication1) Highly skilled man power required for splicing.2) Optic connectors which are used for splicing are highly expensive.
would require a guiding medium in which the information carrying lightwaves could be transmitted. This resulted in the development ofoptical fibre which is an efficient guiding medium for laser light.Basic principle-total internal reflectionThe basic principle of optical fibre is multiple total internalreflection. When a ray of light travel from denser to a rarer medium,at an angle of incidence greater than critical angle c, the ray isnot reflected but it is reflected into the same denser medium. Thisproperty is called total internal reflection. Light signals aretransmitted through optic fibres by multiple total internalreflection.Fibre construction and fibre dimensionAn optical fibre consists of a vary thin transparent cylindrical corehaving refractive index n1 surrounded by a cylindrical shell calledcladding of slightly lower refractive index n2. The core claddingsystem is surrounded by plastic jackets.Typical value of core diameter is 50µm.Typical value of outer diameterof cladding is 125µm. With jacket, totally optical fibre has an outerdiameter 150µm.Propagation mechanism in optical fibresConsider an optical fibre with n1 and n2 as the refractive indices ofcore and cladding material respectively. Consider a ray of lightentering through one end making an angle with the axis.By Snell's law, n1 = sin i / sinOr sin = sin i / n1 ------ (1)At core-cladding interface, refractive index µ = n1/n2Total internal reflection takes place at core-cladding interface ifthe angle of incidence at core-cladding interface is equal to orgreater than critical anglec.We have µ=n1/n2=1/sincsinc = n2/n1 -----(2)For total internal reflection to take place, > cor, sin > sinci.e., sin > n2/n1 [from eqn(2)]
But from the ABC, sin = coscos > n2/n1i.e., (1-sin2)1/2 > n2/n11-sin2 > (n2/n1)2sin2 < 1-(n2/n1)2sin2 < (n12-n22)/n12sin < [(n12-n22)/n12]1/2sin < (n12-n22)1/2/n1But from equation (1), sin = sini/n1sini /n1 < (n12-n22)1/2/n1sini < (n12-n22)1/2i < sin-1(n12-n22)1/2Thus if the angle of incidence is greater than sin-1(n12-n22)1/2,total internal reflection and transmission of light will not takeplace.Acceptance angleThe maximum angle of incidence at which light may enter the fibre inorder to be propagated is, im= sin-1(n12-n22)1/2This angle is called acceptance angle for the fibre.Numerical apertureNumerical aperture of an optical fibre is a measure of its gatheringcapacity and it is denoted as the sine of acceptance angle.i.e., NA = sinim= (n12-n22)1/2Fractional index change ()Fractional index change () is the ratio of the refractive indexdifference between the core and cladding to the refractive index ofthe core of an optical fibre. = (n1– n2) / n1Relation between NA and We have = (n1– n2) / n1(n1– n2) = n1 --------- (1)NA = (n12-n22)1/2 = [(n1+ n2) (n1-n2)] ½ = [(n1+ n2) n1] ½ {Applying equation (1)}Since, n1 n2, n1+ n2 = 2n1NA = [2 n12]1/2Types of optical fibresOn the basis of materialsOn the basis of materials used for core and cladding, optical fibresare classified into three categories:1. Glass fibres (glass core with glass cladding)2. Plastic fibres (plastic core with plastic cladding)3. PCS fibres (polymer cladding with silica core)Plastic fibres have the advantage of more flexibility than glassfibres but attenuation is greater in plastic fibres, comparing withglass fibres.
On the basis of refractive index profile- Step index and graded index fibreIf the optical fibre has a core of uniform constant refractive indexn1 and a cladding of slightly lower refractive index n2, it is calleda step index fibre. The cross sectional refractive index profile is asshown:If the core of optic fibre has a non-uniform refractive index thatdecreases gradually from the centre towards the core-claddingboundary, it is called a graded index (GRIN) fibre. The claddingsurrounding the core has a uniform refractive index, slightly lowerthan the refractive index of the core. The cross sectional refractiveindex profile is as shown:Modes of propagationWhen the light rays are guided through the fibres, it propagates indifferent modes. Modes can be visualized as the possible number ofallowed paths of light in an optical fibre. On the basis of the modesof propagation, optical fibres are classified into two:1. Single mode fibre that supports only one mode of propagation. Forthis a step index fibre with small core diameter (around 10m) isused.2. Multimode fibre that support a number of modes. For this, a stepindex fibre with large core diameter (around 50m) or a graded index(GRIN) is used.V-number of a multimode fibreThe number of modes supported for propagation by a multimode fibre isdetermined by a parameter called V-number (denoted as V). If thesurrounding medium is air, then the V-number is given by,where'd' is the core diameter, '' is the wavelength of lightpropagating in the fibre. The number of modes supported by the fibreis given by,Number of modes = V2/2AttenuationAttenuation is the loss of power suffered by the optical signal as itpropagates through the fibre. It is also referred as fibre loss.Attenuation co-efficient or attenuation,where Pi is the optical power launched at the input and Po the output
power after traveling a distance L km.Different mechanisms of attenuation1. Absorption: The optical fibre material and the impurities presentin the material absorb light leading to fibre loss.2. Rayleigh scattering: This occurs due to the local variations inrefractive index. The Rayleigh scattering loss depends on thewavelength. It varies as 1/4 and becomes significant at lowerwavelengths. Below 0.8m, the scattering loss is very high.3. Radiation losses: This occurs due to the bending of the fibre.There are two types of bends.(a) Macroscopic: During transportation and installation, it happensthat the optical fibre cable is bent over large radius. Such bendslead to loss of light.(b) Microscopic: Due to winding of optical fibre cable, microscopicbends occur. Light rays get scattered at the small bends and escapeinto the cladding. This can be minimized by selecting a suitablejacket that can withstand the stresses.Dispersion losses due to various modes of propagation1) Waveguide dispersionIn single mode fibres, part of the light ray will be refracted intothe cladding. The loss due to this is referred as waveguidedispersion. Waveguide dispersion is negligible in multimode fibres.2) Intermodal dispersionThe intermodal dispersion occurs in multimode fibres where raysassociate with various modes travel different distances through thefibre. As a result, the signal broadens and the output signal is nolonger identical with the input signal. Signal broadening is less ingraded index and step index single mode fibres.3) Material dispersion or chromatic dispersionIf we use white light, all the colours of the input radiation are notreaching the other end at the same time since they travel withdifferent velocities. Signal distortion of this kind is calledmaterial dispersion or chromatic dispersion.Applications of optical fibres
1. In fibre optic communication2. In fibre optic sensors3. For industrial automation4. In security alarm systems5. In local area network (LAN) of computers6. For high speed data transmission in computers7. Medical applications- for diagnosis and surgical applications (endoscopy)8. Military applications (fibre guided missiles)Point to point light wave communication using optic fibreA simple block diagram of fibre optic communication system is shown below:Optical transmitterA light emitting diode (LED) or a semiconductor laser can be used asoptical source. Modulation modulates the input signal and opticalsignal and then transmitted through optical fibre cables to thereceiver.Optical receiverA photodiode can be used as optical detector. The detected wave isdemodulated to extract the signal.Advantages of fibre optic communication1) Wide band width.2) Low attenuation and other transmission losses.3) Small size and weight.4) Safe from electrical interference caused by lightning, electricmotors, fluorescent tube and other electrical noise sources.5) Lack of cross talk between parallel fibres.6) Easy installation and easy maintenance.7) Flexible.8) Temperature resistance.9) Highly economical.10) High degree of signal security.11) Longer life span.Disadvantages of fibre optic communication1) Highly skilled man power required for splicing.2) Optic connectors which are used for splicing are highly expensive.
