OPTICAL FIBER COMMUNICATION RAMESH BHARTI Major elements of an optical fiber link The Nature of Light Quantum Theory Light consists of small particles (photons) Wave Theory Light travels as a transverse electromagnetic wave Ray Theory Light travels along a straight line and obeys laws of geometrical optics. Ray theory is valid when the objects are much larger than the wavelength (multimode fibers) Refraction and reflection Step Index Fiber The step index optical fiber. The central region, the core, has greater refractiv index than the outer region, the cladding. The fiber has cylindrical symmetry use the coordinates r, , z to represent any point in the fiber. Cladding is normally much thicker than shown. Meridian Ray Representation Total Internal Reflection Cladding Graded Index Fiber Single Mode Step Index Fiber Fiber Key Parameters Comparison of fiber structures Fiber Key Parameters Step and Graded Index Fibers Total Internal Reflection Skew Rays Skew rays Major Dispersions in Fiber Modal Dispersion: Different modes travel at different velocities, exist only in multimodal conditions Waveguide Dispersion: Signal in the cladding travel with a different velocity than the signal in the core, significant in single mode conditions Material Dispersion: Refractive index n is a function of wavelength, exists in all fibers, function of the source line width Effects of Dispersion and Attenuation Dispersion for Digital Signals Modal Dispersion The Nature of Light Quantum Theory Light consists of small particles (photons) Wave Theory Light travels as a transverse electromagnetic wave Ray Theory Light travels along a straight line and obeys laws of geometrical optics. Ray theory is valid when the objects are much larger than the wavelength (multimode fibers) Refraction and reflection Snells Law: n1 Sin 1 =n2 Sin 2 Critical Angle: Sin c=n2/n1 Step Index Fiber Core and Cladding are glass with appropriate optical properties while buffer is plastic for mechanical protection n1 n2 n1>n2 Step Index Fiber nyn2n1CladdingCorezyr|Fiber axisThe step index optical fiber. The central region, the core, has greater refractiveindex than the outer region, the cladding. The fiber has cylindrical symmetry. Weuse the coordinates r, |, z to represent any point in the fiber. Cladding isnormally much thicker than shown. 1999S.O.Kasap, Optoelectronics (PrenticeHall)Single Mode Step Index Fiber Protective polymerinc coatingBuffer tube: d = 1mmCladding: d = 125 - 150 mCore: d =8 - 10 mnrThe cross section of a typical single-mode fiber with a tight buffertube. (d = diameter)n1n2 1999 S.O. Kasap,Optoelectronics (Prentice Hall)Meridian Ray Representation 1221222112 nnnn n ~= ATotal Internal Reflection CladdingCoreo < omaxABu < ucABu > uco > omaxn0n1n2LostPropagatesMaximum acceptance angleomax is that which just givestotal internal reflection at thecore-cladding interface, i.e.wheno = omax thenu = uc.Rays witho > omax (e.g. rayB) become refracted andpenetrate the cladding and areeventually lost.Fiber axis 1999S.O.Kasap,Optoelectronics (PrenticeHall)Comparison of fiber structures Graded Index Fiber nbncO O'Ray 1AB'BuAuBuB'Ray 2MuB'c/nbc/na12B''naabcWe can visualize a graded indexfiber by imagining a stratifiedmedium with the layers of refractiveindicesna > nb > nc ... Consider twoclose rays 1 and 2 launched from Oat the same time but with slightlydifferent launching angles. Ray 1just suffers total internal reflection.Ray 2 becomes refracted atB andreflected at B'. 1999 S.O. Kasap,Optoelectronics (Prentice Hall)n1n2213nOn1213nn2OO' O''n2(a) Multimode stepindex fiber. Ray pathsare different so thatrays arrive at differenttimes.(b) Graded index fiber.Ray paths are differentbut so are the velocitiesalong the paths so thatall the rays arrive at thesame time.23 1999 S.O. Kasap,Optoelectronics (Prentice Hall)Step and Graded Index Fibers n decreases step by step from one layerto next upper layer; very thin layers.Continuous decrease inn gives a raypath changing continuously.TIR TIR(a) A ray in thinly stratifed medium becomes refracted as it passes from onelayer to the next upper layer with lowern and eventually its angle satisfies TIR.(b) In a medium wheren decreases continuously the path of the ray bendscontinuously.(a) (b) 1999 S.O. Kasap,Optoelectronics (Prentice Hall)Total Internal Reflection Fiber axis12345Skew ray13245Fiber axis123Meridional ray1, 32(a) A meridionalray alwayscrosses the fiberaxis.(b) A skew raydoes not haveto cross thefiber axis. Itzigzags aroundthe fiber axis.Illustration of the difference between a meridional ray and a skew ray.Numbers represent reflections of the ray.Along the fiberRay path projectedon to a plane normalto fiber axisRay path along the fiber 1999S.O.Kasap, Optoelectronics (PrenticeHall)Skew Rays Skew rays Skew rays circulate around the core and increase the dispersion Fiber Key Parameters Fiber Key Parameters Polarizations of fundamental mode Two polarization states exist in the fundamental mode in a single mode fiber Polarization Mode Dispersion (PMD) Each polarization state has a different velocity PMD Major Dispersions in Fiber Modal Dispersion: Different modes travel at different velocities, exist only in multimodal conditions Waveguide Dispersion: Signal in the cladding travel with a different velocity than the signal in the core, significant in single mode conditions Material Dispersion: Refractive index n is a function of wavelength, exists in all fibers, function of the source line width Effects of Dispersion and Attenuation Dispersion for Digital Signalst0EmitterVery shortlight pulsesInput OutputFiberPhotodetectorDigital signalInformationInformationt0~2 t1/2TtOutput IntensityInput Intensity t1/2An optical fiber link for transmitting digital information and the effect ofdispersion in the fiber on the output pulses. 1999S.O.Kasap, Optoelectronics (PrenticeHall)Low order modeHigh order modeCladdingCoreLight pulset0tSpread, AtBroadenedlight pulseIntensityIntensityAxialSchematic illustration of light propagation in a slab dielectric waveguide. Light pulseentering the waveguide breaks up into various modes which then propagate at differentgroup velocities down the guide. At the end of the guide, the modes combine toconstitute the output light pulse which is broader than the input light pulse. 1999 S.O. Kasap,Optoelectronics (Prentice Hall)Modal Dispersion n2Lightn2n1yE(y)E(y,z,t ) = E(y)cos(et |0z)m = 0Field of evanescentwave(exponent ial decay)Fi el d of gui ded waveThe electric field pattern of the lowest mode traveling wave along theguide. This mode hasm = 0 and the lowestu. It is often referred to as theglazing incidence ray. It has the highest phase velocity along the guide. 1999S.O.Kasap, Optoelectronics (PrenticeHall)Field Distribution in the Fiber Higher order modes Larger MFD yE(y)m = 0m = 1m = 2CladdingCladdingCore2an1n2n2The electric field patterns of the first three modes ( m = 0, 1, 2)traveling wave along the guide. Notice different extents of fieldpenetration into the cladding. 1999S.O.Kasap, Optoelectronics (PrenticeHall)Mode-fieldDiameter (2W0) In a Single Mode Fiber, ) / exp( ) (2020w r E r E =At r = wo, E(Wo)=Eo/e Typically Wo > a Power in the cladding Lower order modes have higher power in the cladding. yE(y)CladdingCladdingCore2 > 11 > ce2 < e1e1 < ecut-offvg1yvg2 > vg1The electric field of TE0 mode extends more into thecladding as the wavelength increases. As more of the fieldis carried by the cladding, the group velocity increases. 1999S.O.Kasap, Optoelectronics (PrenticeHall)Higher the Wavelength More the Evanescent Field