Optic Fiber Waveguides Types of Fibers - Contact Information

Optic Fiber WaveguidesSavera TanwirTypes of Fibers‣Step-Index Fibers‣Graded-Index FibersFiber Optic Communication Systems 2

Step-Index Fiber‣ SI fiber consists of a central core with refractive index n 1 ,surrounded by a cladding with refractive index n 2‣ The critical angle for SI fiber is given bysinθ c = n 2 /n 1‣ Fractional Refractive Index Change∆ = (n 1 – n 2 )/n 1 n1 is typically between1.44 and 1.46 ∆ is always positive since n1is larger than n2 for a critical angle to existcladdingcoreθn 2n 1claddingcoreFiber Optic Communication Systems 3Step-Index Fiber‣Why do we need cladding?– Air satisfies the condition n 1 > n 2– Fiber can easily bend or get scratched,causing losses– Cladding protects the core from contaminationand helps preserve its physical integrityFiber Optic Communication Systems 4

‣ FormsStep-Index Fiber– A glass core cladded with glass– A glass core cladded with plastic– A plastic core cladded with plastic– The refractive index step is smallest for all-glassfibers and largest for all-plastic fibersThis is because of the small range of refractive indicesavailable in glasses and a large range in plasticsNA = n o sin α o = sqrt ( n 12 –n 22 )= n 1 sqrt (2∆)Fiber Optic Communication Systems 5Properties of SI Fibers– The NA and pulse spread are smallest for all-glassfibers and highest for all-plastic fibers– Fibers with little pulse spread have large rate-lengthproducts. The NA of these fibers is small making itdifficult to couple light in them– All-glass fibers have the lowest losses and smallestpulse spreadingUseful at moderately high information rates and for fairly longlengths. Coupling from light source is difficult due to small NAFiber Optic Communication Systems 6

Properties of SI Fibers‣ PCS (Plastic cladded silica) fibers have higher lossesand large pulse spreads they are used for shorter links– Their higher numerical apertures increase the source couplingefficiency, but this advantage is lost in long fibers due toincreased absorption‣ All plastic fibers are limited to very short paths by theirhigh propagation losses.– Path lengths are usually less than a few tens of meters– Large acceptance angles improve coupling efficiencyFiber Optic Communication Systems 7Properties of SI Fibers‣ Light can also be trapped in cladding if its refractiveindex is greater than the material surrounding it‣ It happens because some light rays are introduced intothe fiber at angles beyond the acceptance angle‣ Also at discontinuities like splices and connectors, wherelight may be detected beyond the core node anglesn on 2n 1claddingcoreFiber Optic Communication Systems 8

Properties of SI Fibers‣ The light traveling in the cladding modeattenuates more quickly than light in core– Outer boundary of the cladding is in contact with alossy medium– The power attenuates so rapidly that the claddingmodes are in significant at the end of a long fiberFiber Optic Communication Systems 9Example‣Suppose the all-glass fiber is surroundedby air. Compute the critical angle at thecladding-air boundaryFiber Optic Communication Systems 10

Graded-Index Fiber‣ A GRIN fiber has a core material whose refractive indexdecreases continuously with distance from the fiber axisn(r) = n 1 .sqrt(1 – 2(r/a) α ∆)n(r) = n 1 .sqrt(1 –2∆) = n 2r ≤ ar > aWheren 1= refractive index along the fiber axisn 2= refractive index of the claddinga = core radiusα = refractive index profile variation∆ = parameter determining the scale of profile changeFiber Optic Communication Systems 11Graded-Index Fiber‣ Changing RI continually redirects the rays towards thefiber axis‣ This can be modeled using series of small steps (changein RI)Fiber Optic Communication Systems 12

Graded-Index Fiber‣The most common refractive index profilefor a graded index fiber is very nearlyparabolic‣The parabolic profile results in continualrefocusing of the rays in the coreFiber Optic Communication Systems 13GRIN Acceptance AngleEscaping Ray2α 0Trapped Ray2α 0NA = n 1 (2∆) ½ (1 – (r/a) 2 ) ½Fiber Optic Communication Systems 14

Advantages of SI‣ The advantages of a step-index multimode fiber arerelated to the relatively large core area and highnumerical apertures. Both of these properties allow lightto be easily coupled into the fiber.‣ In turn, this allows the use of inexpensive terminationtechniques, low cost diodes, and high power handlingcapability.‣ These fibers are therefore widely short distance datacommunications, and fiber sensors.Fiber Optic Communication Systems 15Disadvantages of SI‣ A disadvantage to step-index multimode is bandwidth.‣ The path the light takes down a step-index multimodefiber will be longer or shorter depending on the angle ofpropagation. This difference in path length causes thepulse of light to spread out during its journey down thefiber.‣ This is known as modal dispersionFiber Optic Communication Systems 16

Advantages of GRIN‣ The graded profile and smaller core increases bandwidthover step-index fibers, but the core sizes are still largeenough for convenient termination and use of lower costdiodes.‣ These are used in medium to long distancecommunications‣ Low pulse distortionFiber Optic Communication Systems 17Attenuation‣ Attenuation is the reduction in amplitude andintensity of a signal‣ The attenuation of an optical fiber measures theamount of light lost between input and output‣ Receivers require some minimum power level,to accurately detect the transmitted signalFiber Optic Communication Systems 18

Attenuation‣ Losses occur at the channel coupler, splices,connectors and within the fiber itself‣ These transmission losses limit path length‣ Requirements for fiber material–Low loss– Ability to formed into hairlike long strands– Capable of slight variations to achieve different RIs– Continuous change in RI for Graded Index FiberFiber Optic Communication Systems 19Glass - Attenuation‣ Glass fibers have low absorption than plasticfibers‣ Glass fibers are preferred for long haulcommunications‣ Glass in formed by fusing molecules of silica(silicon dioxide SiO 2 ) and other material(titanium, thallium, germanium, boron etc.)Fiber Optic Communication Systems 20

Glass - Attenuation‣Objective: manufacture low-loss fiber‣Attenuation in Glass fiber– Absorption– Scattering– Geometric EffectsFiber Optic Communication Systems 21‣Intrinsic Absorption:Absorption– Glass absorbs heavily within specific wavelengthregions– A natural property of glass– It is very strong in the short-wavelength ultravioletportion– Peak lossUltraviolet region butunimportantFiber Optic Communication Systems 22

Absorption‣ Intrinsic absorption peak also occur between 7 and 12µm in the infrared‣ Infrared loss is associated with vibration of chemicalbonds‣ Intrinsic losses are most significant in wide region wherefiber systems can operate– Intrinsic losses restrict the extension of fiber systems toward theultraviolet as well as toward longer wavelengths– Fiber Optic Comm operates in a range of wavelengths from 0.5to 1.6 µmFiber Optic Communication Systems 23Impurities‣Impurities are major source of loss inpractical fiber‣Types of impurities– Transition-metal ions– OH ionsFiber Optic Communication Systems 24