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OPTICAL FIBERS

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OPTICAL FIBERS
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  OPTICAL FIBER AND ITS APPLICATIONS  NTRODUCT ON: An optical fiber (or fibre) is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applie science an engineering  concerne !ith the esign an application of optical fibers. Optical fibers are !iel #se in fiber$optic co%%#nications& !hich per%its trans%ission over longer istances an at higher ban!iths than other for%s of co%%#nications. Fibers are #se instea of %etal !ires beca#se signals travel along the% !ith less loss& an the are also i%%#ne to electro%agnetic interference. Fibers are also #se for ill#%ination& an are !rappe in b#nles so the can be #se to carr i%ages& th#s allo!ing vie!ing in tight spaces. 'peciall esigne fibers are #se for a variet of other applications& incl#ing sensors an fiber lasers.ight is ept in the core of the optical fiber b total internal reflection. This ca#ses the fiber to act as a !aveg#ie. Fibers !hich s#pport %an propagation paths or transverse %oes are calle %#lti$%oe fibers (**F)& !hile those !hich can onl s#pport a single %oe are calle single$%oe fibers ('*F). GEETHAANJALI ALL INDIA SENIOR SECONDARY SCHOOLPage | 0  OPTICAL FIBER AND ITS APPLICATIONS + 'TOR, OF F -R O/T C': Daniel Collaon first escribe this 0light fo#ntain0 or 0light pipe0 in an 1234 article entitle On the reflections of a ra of light insie a parabolic li5#i strea%  . This partic#lar ill#stration co%es fro% a later article b Collaon& in 1223.Fiber optics& tho#gh #se e6tensivel in the %oern !orl& is a fairl si%ple an ol technolog . 7#iing of light b refraction& the principle that %aes fiber optics possible& !as first e%onstrate b Daniel Collaon an  8ac5#es -abinet in /aris in the earl 1239s.  8ohn T nall incl#e a e%onstration of  it in his p#blic lect#res in onon a oen ears later. T nall also !rote abo#t the propert of total internal reflection inian intro#ctor boo abo#tthe nat#re of light in 12;9: 0<hen the light passes fro% air into !ater& the refracte ra is bent to!ars   the perpenic#lar... <hen the ra passes fro% !ater to air it is bent fro%   the perpenic#lar... If the angle !hich the ra in !ater encloses !ith the perpenic#lar to the s#rface be greater than 32 egrees& the ra !ill not 5#it the !ater at all: it !ill be totall reflecte   at the s#rface.... The angle !hich %ars the li%it !here total reflection begins is calle the li%iting angle of the %ei#%. For !ater this angle is 32=4;>& for flint glass it is ?2=31>& !hile for ia%on it is 4?=34>.0 Atten#ations in %oern optical cables are far less than those in electrical copper cables& leaing to long$ha#l fiber connections !ith repeater istances of ;9@19 ilo%etres (3?@B? %i). The erbi#%$ope fiber a%plifier& !hich re#ce the cost of long$istance fiber s ste%s b re#cing or even in %an cases eli%inating the nee for optical$electrical$optical repeaters& !as co$evelope b tea%s le b Davi N. /a ne of the Universit of 'o#tha%pton& an %%an#el Des#rvire at -ell abs in 1B2.  The %ore rob#st optical fiber co%%onl #se toa #tilies glass for both core an sheath an is therefore less prone to aging processes. It !as invente b 7erhar -ernsee in 1B;? of 'chott 7lass in 7er%an . GEETHAANJALI ALL INDIA SENIOR SECONDARY SCHOOLPage | 1  OPTICAL FIBER AND ITS APPLICATIONS /rinciple of operation: An optical fiber is a c linrical ielectric !aveg#ie (non con#cting  !aveg#ie) that trans%its light along its a6is& b the process of total internal reflection. The fiber consists of a core s#rro#ne b a claing la er& both of !hich are %ae of ielectric %aterials. To confine the optical signal in the core& the refractive ine6 of the core %#st be greater than that of the claing. The bo#nar bet!een the core an claing %a either be abr#pt& in step$ine6 fiber& or gra#al& in grae$ine6 fiber   ne6 of refraction  The ine6 of refraction is a !a of %eas#ring the spee of light in a %aterial. ight travels fastest in a vac##%& s#ch as o#ter space. The act#al spee of light in a vac##% is abo#t ?99 %illion %eters (12 tho#san %iles) per secon. Ine6 of refraction is calc#late b iviing the spee of light in a vac##% b the spee of light in so%e other %ei#%. The ine6 of refraction of a vac##% is therefore 1& b efinition. The t pical val#e for the claing of an optical fiber is 1.3. The core val#e is t picall 1.32. The larger the ine6 of refraction& the slo!er light travels in that %ei#%. Fro% this infor%ation& a goo r#le of th#%b is that signal #sing optical fiber for co%%#nication !ill travel at aro#n 499 %illion %eters per secon. Or to p#t it another !a & to travel 1999 ilo%eters in fiber& the signal !ill tae  %illisecons to propagate. Th#s a phone call carrie b fiber bet!een ' ne an Ne! ,or&a 14999 ilo%eter istance& %eans that there is an absol#te %ini%#% ela of 9 %illisecons (or aro#n 11th of a secon) bet!een !hen one caller speas to !hen the other hears. (Of co#rse the fiber in this case !ill probabl travel a longer ro#te& an there !ill be aitional ela s #e to co%%#nication e5#ip%ent s!itching an the process of encoing an ecoing the voice onto the fiber).  Total internal reflection <hen light traveling in a ense %ei#% hits a bo#nar at a steep angle (larger than the 0critical angle0 for the bo#nar )& the light !ill be co%pletel reflecte. This effect is #se in optical fibers to confine light in the core. ight travels along the fiber bo#ncing bac an forth off of the bo#nar . -eca#se the light %#st strie the bo#nar !ith an angle greater than the GEETHAANJALI ALL INDIA SENIOR SECONDARY SCHOOLPage | 2  OPTICAL FIBER AND ITS APPLICATIONS critical angle& onl light that enters the fiber !ithin a certain range of angles can travel o!n the fiber !itho#t leaing o#t. This range of angles is calle the acceptance cone of the fiber. The sie of this acceptance cone is a f#nction of the refractive ine6 ifference bet!een the fiber>s core an claing. O/T CA F -R T,/':  In grae$ine6 fiber& the ine6 of refraction in the core ecreases contin#o#sl bet!een the a6is an the claing. This ca#ses light ra s to ben s%oothl as the approach the claing& rather than reflecting abr#ptl fro% the core$claing bo#nar . The res#lting c#rve paths re#ce %#lti$path ispersion beca#se high angle ra s pass %ore thro#gh the lo!er$ine6 peripher of the core& rather than the high$ine6 center. The ine6 profile is chosen to %ini%ie the ifference in a6ial propagation spees of the vario#s ra s in the fiber. This ieal ine6 profile is ver close to a parabolic relationship bet!een the ine6 an the istance fro% the a6is *#lti$%oe fiber Fiber !ith large core ia%eter %a be anal e b geo%etrical optics. '#ch fiber is calle %#lti$%oe fiber& fro% the electro%agnetic anal sis (see belo!). In a step$ine6 %#lti$%oe fiber& ra s of light are g#ie along the fiber core b total internal reflection. Ra s that %eet the core$claing bo#nar at a high angle (%eas#re relative to a line nor%al to the bo#nar )& greater than the critical angle for this bo#nar & are co%pletel reflecte. The critical angle (%ini%#% angle for total internal reflection) is eter%ine b the ifference in ine6 of refraction bet!een the core an claing %aterials. Ra s that %eet the bo#nar at a lo! angle are refracte fro% the core into the claing& an o not conve light an hence infor%ation along the fiber. The critical angle eter%ines the acceptance angle of the fiber& often reporte as a n#%erical apert#re. A high n#%erical apert#re allo!s light to propagate o!n the fiber in ra s both close to the a6is an at vario#s angles& allo!ing efficient co#pling of light into the fiber. GEETHAANJALI ALL INDIA SENIOR SECONDARY SCHOOLPage | 3
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