INTRODUCTIONTransmission of light via a dielectric wave guide was proposed and investigated in the beginning of 20th centur!" transparent dielectric rod of silica glass with RI around #!$ cannot be used alone!" clad dielectric rod is used along with core in the mid #%$0sRa theor transmissionRefractive index: n&c'vc&speed of light wave in free space()*#0+m's,!v&velocit of light in non conducting medium!n!00 for air - n!)) for water-#!.$ for silica glass-2!.$ for diamond!Re/ection and Refraction 0hen a light ra encounters a boundar separating two di1erent media - part ofra is re/ected bac2 in to 3rst medium and the remainder is bent (refracted, as it enters the second material!"t interface b 4nell5s lawn# sin6# &n2 sin62

6#&angle between the incident ra and the normal to the surface is 2nown as angle of incidence!62&angle between the refracted ra and the normal to the surface is 2nown as angle of refraction!"s the angle of incidence becomes larger -refractedangle 62approaches 7'2! beond this point no refraction is possible and the light ras become totall internall re/ected! Critical angle 6c 8light ra in air is parallel to the surface ! 0hen the light ra in air is parallel to glass surface- then 62&%0 o so that sin 62& #critical angle is sin 6c & n2'n#Tpical structure of optical 3berClassi3cation4tep inde*9:refractive inde* of core is uniform and undergoes an abrupt change at the cladding boundar! ;raded inde*9:core refractive inde* varies as a function of radial distance from the center of the 3ber!4tep and graded inde* again divided in to single mode and multimode! Tpes of optical 3bers4tep inde* 3bersCore diameter :$0ed into two maIor tpes9 Raleigh and =ie scattering! Coth result from the non ideal phsical properties of the 3ber! Raleigh 4cattering Raleigh scattering is the dominant intrinsic loss mechanism in the low absorption window between the ultraviolet and infrared absorption tails!It results from inhomogeneities of a random nature occurring on a small scale compared with the wavelength of the light! These inhomogeneities manifest themselves as refractive inde* /uctuations and arise from densit and compositional variations which are fro>en into the glass lattice on cooling! The compositional variations ma be reduced b improved fabrication- but the inde* /uctuations caused b the free>ing:in of densit inhomogeneities are fundamental and cannot be avoided! The subse?uent scattering due to the densit /uctuations- which is in almost all directions! produces an attenuation proportional to #'L. following the Raleigh scattering formula=ie 4catteringAinear scattering ma also occur at inhomogeneities which are comparable in si>e to the guided wavelength!These result from the nonperfect clindrical structure of the waveguide and ma be caused b 3ber imperfections such as irregularities in the core:cladding interface- coreMcladding refractive inde* di1erences along the 3ber length- diameter /uctuations- strains and bubbles!The scattering created b such inhomogeneities is mainl in the forward direction and is called =ie scattering!The inhomogeneities ma be reduced b9(a, removing imperfections due to the glass manufacturing process!(b, carefull controlled e*trusion and coating of the 3ber!(c, increasing the 3ber guidance b increasing the relative refractive inde* di1erence!.,NONAINH"R 4C"TTHRIN; AO44H4Optical waveguides do not alwas behave as completel linear channels whose increase in output optical power is directl proportional to the input optical power!This non: linear scattering causes the optical power from one mode to be transferred in either the forward or bac2ward direction to the same- or other modes- at a di1erent fre?uenc! It depends criticall upon the optical power densit within the 3ber and hence onl becomes signi3cant above threshold power levels!4timulated Crillouin 4cattering Crillouin scattering ma be regarded as the modulation of light through thermal molecular vibrations within the 3ber! The scattered light appears as upper and lower sidebands which are separated from the incident light b the modulation fre?uenc! The incident photon in this scattering process produces a phonon of acoustic fre?uenc as well as a scattered photon! This produces an optical fre?uenc shift which varies with the scattering angle because the fre?uenc of the sound wave varies with acoustic wavelength! The fre?uenc shift is a ma*imum in the bac2ward direction reducing to >ero in the forward direction ma2ing Crillouin scattering a mainl bac2ward process!4timulated Raman 4cattering4timulated Raman scattering is similar to stimulated Crillouin scattering e*cept that a high fre?uenc optical photon rather than an acoustic phonon is generated in the scattering process! "lso Raman scattering occurs in the forward direction and ma have an optical power threshold of up to three orders of magnitude higher than the Crillouin threshold in a particular 3ber!$,Niber bend lossesOptical 3bers su1er radiation losses at bends or curves on their paths!The part of the mode which is on the outside of the bend is re?uired to travel faster than that on the inside so that a wave front perpendicular to the direction of propagation is maintained! Jence part of the mode in the cladding needs to travel faster than the velocit of light in that medium! "s this is not possible- the energ associated with this part of the mode is lost through radiation!The loss can generall be represented b a radiation attenuation coeOcient which has the formwhere R is the radius of curvature of the 3ber bend and c# and c2 are constants which are independent of R!losses ma be reduced b9(a, designing 3bers with large relative refractive inde* di1erencesP(b, operating at the shortest wavelength possible!DI4BHR4IONdispersion mechanisms within the 3ber cause broadening of the transmitted light pulses as the travel along the channel!it ma be observed that each pulse broadens and overlaps with its neighbours eventuall becoming indistinguishable at the receiver input! The e1ect is 2nown as intersmbol interference (I4I,!Nor no overlapping of light pulses down on an optical 3ber lin2 the digital bit rate CT must be less than the reciprocal of the broadened (through dispersion, pulse duration (2Q,! Jence9"nother more accurate estimate of the ma*imum bit rate for an optical channel with dispersion ma be obtained b considering the light pulses at the output to have a ;aussian shape with an rms width of R!The ma*imum bit rate is given appro*imatel b It ma be observed that the multimode step inde* 3ber e*hibits the greatest dispersion of a transmitted light pulse multimode graded inde* 3ber gives a considerabl improved performance! single mode 3ber gives the minimum pulse broadening the information:carring capacit of the 3ber is restricted b the amount of pulse dispersion per unit length!capacit of an optical 3ber which is 2nown as the bandwidthMlength product ie! (Copt * A,20 =JS 2m- # ;h> 2m and #00 ;J> 2m for multimode step inde* multimode graded inde* and single mode step inde* 3bers respectivel!Intramodal dispersionResults from 3nite spectral line width of optical source!4ince optical sources donot emit Iust a single fre?uenc but a band of fre?uencies- then there ma be propagation dela di1erences between spectral components of optical signals!This causes broadening of each mode!The dela di1erence ma be caused bdispersive properties of wave guide material(material dispersion, and also guidance e1ect with in the 3ber structure(wave guide dispersion,!Intermodal dispersionPulse broadeningdue tomodal dispersionresults from the propagation dela di1erences between modes with in a multimode 3ber!"s the di1erent mode which constitute a pulse with in a multimode 3ber travel along the channel at di1erent group velocities!Overall dispersionOptical 3ber fabricationThe basic steps area, Breform fabrication=etal halides react with o*gen to form white powder of 4iO2b,Niber drawing Di1erent methods for preform fabrication are #,outside vapor phase o*idation(O@BO,2,vapor a*ial deposition(@"D,),modi3ed chemical vapor deposition(=C@D,.,plasma activated chemical vapor deposition(BC@D,#,outside vapor phase o*idation(O@BO,2,vapor a*ial deposition(@"D,2,vapor a*ial deposition(@"D,SiO2 particles are formed in same as OVPOAs these particles emerges from silica torches, they are deposited to the end surface of silica glass rod hich acts as seed!Perform is fabricated continuously"hen preform is long enough it ,goes directly to draing toer"dvantages #! Breform has no central hole as in O@BO!2!Brefom can be fabricated in continuous lengths that can e1ect process costs)!Deposition chamber and >one melting ring are connected tightl to each other produces a clean environment!),=odi3ed chemical vapor deposition(=C@D,),=odi3ed chemical vapor deposition(=C@D,.,plasma activated chemical vapor deposition(BC@D,Niber drawingNiber drawingprocess begins b feeding the glass preform into the drawing furnace! The drawing furnace softens the end of the preform to the melting point! =anufacturers then pull the softened preform into a thin glass 3lament (glass 3ber,! To protect the bare 3ber from contaminants- manufacturers add an acrlate coating in the draw process! The coating protects the bare 3ber from contaminants such as atmospheric dust and water vapor!Double crucible methodDouble crucible methodThe double:crucible method is the most common direct:melt process! The double:crucible method combines the molten rods into a single preform using two concentric crucibles! Optical 3bers are drawn from this molten glass using a similar 3ber drawing process as in vapor phase o*idation!Rod:in:tube methodRod:in:tube method;lass rod with higher refractive inde* is placed in glass tube with a lower refractive inde* of thermall compatible material and unit is fed in to furnace!;lass tube fuses and softens with rod and tapers to thin 3ne 3ber fabricated!