Fundamentals of Optical Fiber Systems

Transcription

1 Fudametals of Optical Fiber Systems Compiled by Mohammad Towhidul Islam, Lecturer, North South Uiversity, for the ETE 3 studets of the Departmet of Computer Sciece ad Egieerig Light plays a vital role i our daily lives. It is used i compact disc (CD) players, i which a laser reflectig off a CD trasforms the returig sigal ito music. It is used i grocery store checkout lies, where laser beams read bar codes for prices. It is used by laser priters to record images o paper. It is used i digital cameras that capture our world ad allow pictures to be displayed o the Iteret. It is the basis of the techology that allows computers ad telephoes to be coected to oe aother over fiber-optic cables. Ad light is used i medicie, to produce images used i hospitals ad i lasers that perform eye surgery. BASIC FIBER OPTIC COMMUNICATION SYSTEM Fiber optics is a medium for carryig iformatio from oe poit to aother i the form of light. Ulike the copper form of trasmissio, fiber optics is ot electrical i ature. A basic fiber optic system cosists of a trasmittig device that coverts a electrical sigal ito a light sigal, a optical fiber cable that carries the light, ad a receiver that accepts the light sigal ad coverts it back ito a electrical sigal. The complexity of a fiber optic system ca rage from very simple (i.e., local area etwork) to extremely sophisticated ad expesive (i.e., logdistace telephoe or cable televisio trukig). For example, the system show i Figure could be built very iexpesively usig a visible LED, plastic fiber, a silico photodetector, ad some simple electroic circuitry. The overall cost could be less tha $0. O the other had, a typical system used for log-distace, high-badwidth telecommuicatio could cost tes or eve hudreds of thousads of dollars. Figure : Basic fiber optic commuicatio system

2 The basic questio is, "How much iformatio is to be set ad how far does it have to go?" With this i mid we will examie the various compoets that make up a fiber optic commuicatio system ad the cosideratios that must be take ito accout i the desig of such systems. Figure shows the typical fiber optic cable. Figure : Typical Fiber Optic Cable Advatage of Optical Fiber Commuicatio: Eormous potetial badwidth: The optical carrier frequecy has a far greater potetial trasmissio BW tha metallic cable systems. Small size ad weight: Optical fiber has small diameters. Hece, eve whe such fibers are covered with protective coatig they are far smaller ad lighter tha correspodig copper cables. Electrical Isolatio: Optical fibers which are fabricated from glass or sometimes a plastic polymer are electrical isulators ad ulike their metallic couterpart, they do ot exhibit earth loop or iterface problems. This property makes optical fiber trasmissio ideally suited for commuicatio i electrically hazardous eviromets as fiber created o arcig or spark hazard at abrasio or short circuits.

3 Sigal security: The light from optical fiber does ot radiate sigificatly ad therefore they provide a high degree of sigal security. This feature is attractive for military, bakig ad geeral data trasmissio i.e. computer etworks applicatio. Low trasmissio loss: The techological developmets i optical fiber over last twety years has resulted i optical cables which exhibits very low atteuatio or trasmissio loss i compariso with best copper coductors. Potetial low cost: The glass which provides the optical fiber trasmissio medium is made from sad. So, i compariso to copper coductors, optical fiber offers the potetial for low cost lie commuicatio. Disadvatage of Optical Fiber Commuicatio: It requires a higher iitial cost i istallatio Although the fiber cost is low, the coector ad iterfacig betwee the fiber optic costs a lot. Fiber optic requires specialized ad sophisticated tools for maiteace ad repairig. Basic Law of Optics At the heart of a optical commuicatio system is the optical fiber that acts as the trasmissio chael carryig the light beam loaded with iformatio? As metioed earlier, the guidace of the light beam (through the optical fiber) takes place because of the pheomeo of total iteral reflectio (TIR), which we will ow discuss. We first defie the refractive idex () of a medium: where c ( 3 X0 8 m/s) is the speed of light i free space ad v represets the velocity of light i that medium. For example, for light waves, =.5 for glass ad =.33 for water.

4 \ a) b) Figure : (a) A ray of light icidet o a deser medium ( < ). b)a ray icidet o a rarer medium ( > ). (c) For <, if the agle of icidece is greater tha critical agle, it will udergo total iteral reflectio.

5 As you kow, whe a ray of light is icidet at the iterface of two media (like air ad glass), the ray udergoes partial reflectio ad partial refractio as show i Figure a. The vertical dotted lie represets the ormal to the surface. The agles θ, θ, ad θ 3 represet the agles that the icidet ray, refracted ray, ad reflected ray make with the ormal. Accordig to Sell's law ad the law of reflectio, si θ = si θ ad θ = θ 3 Further, the icidet ray, reflected ray, ad refracted ray lie i the same plae. I Figure a, sice > we must have (from Sell's law) θ < θ, i.e., the ray will bed toward the ormal. O the other had, if a ray is icidet at the iterface of a rarer medium ( < ), the ray will bed away from the ormal (see Figure b). The agle of icidece, for which the agle of refractio is 90º, is kow as the critical agle ad is deoted by θ c. Thus, whe θ = θ = si c θ = 90. Whe the agle of icidece exceeds the critical agle (i.e., whe θ > θ c), there is o refracted ray ad we have total iteral reflectio (see Figure c). ( ) Acceptace Agle: Multimode optical fiber will oly propagate light that eters the fiber withi a certai coe, kow as the acceptace coe of the fiber. The half-agle of this coe is called the acceptace agle, θmax.

6 θ c θ θ i Figure 3 I the above figure it was show that the light beam eters from air to the optical fiber, a less dese to the deser medium, with a exteral agle θi. This causes the light refracted towards the ormal at a agle θ. To propagate the light beam dow the optical fiber, the light beam at the core ad claddig iterface must take a agle less tha the critical agle θc. Calculatio of critical agle: From Sells law we ca write, siθi = siθ = si(90 θc) = cosθc cosθc siθi = However, we may keep as. Therefore, siθ = θ i cosθc = si c = siθ = = θi is thus kow as Acceptace agle or half of acceptace agel. i

7 Numerical Aperture Numerical Aperture is the measuremet of the acceptace agle of a optical fiber, which is the maximum agle at which the core of the fiber will take i light that will be cotaied withi the core. Take from the fiber core axis (ceter of core), the measuremet is the square root of the squared refractive idex of the core mius the squared refractive idex of the claddig. Types of Optical Fiber NA = si θi Optical fibers come i two mai types. Sigle-mode fiber has a small core that forces the light waves to stay i the same path, or mode. This keeps the light sigals goig farther before they eed to be beefed up, or amplified. Most log-distace, or log-haul, fiber optic telephoe lies use sigle-mode fiber. The secod type, called multimode fiber, has a much larger core tha sigle-mode fiber. This gives light waves more room to bouce aroud iside as they travel dow the path. The extra movemet evetually causes the pulses to smear, ad lose iformatio. That meas multimode fiber sigals ca t travel as far before they eed to be cleaed up ad reamplified. Multimode fibers ca carry oly a third or less the

8 iformatio-carryig capacity or badwidth tha sigle-mode fiber ad they wo't work for log distaces. Network egieers prefer multimode fiber for shorter-distace commuicatio, such as i a office buildig or a local area etwork (LAN), because the techology is less expesive. However, with the growig demad for more badwidth betwee computers ad over the Iteret, sigle-mode fiber is becomig more popular for smaller etworks, too. Therefore, we ca categorize the fiber optic commuicatio i two categories:. Step Idex a. Sigle Mode b. Multimode. Guided Idex Step Idex: These types of fibers have sharp boudaries betwee the core ad claddig, with clearly defied idices of refractio. The etire core uses sigle idex of refractio. Sigle Mode Step Idex: Sigle mode fiber has a core diameter of 8 to 9 micros, which oly allows oe light path or mode. Multimode Step-Idex Fiber: Multimode fiber has a core diameter of 50 or 6.5 micros (sometimes eve larger). It allows several light paths or modes. This causes modal dispersio some modes take loger to pass through the fiber tha others because they travel a loger distace

9 Multimode Graded-Idex Fiber Graded-idex refers to the fact that the refractive idex of the core gradually decreases farther from the ceter of the core. The icreased refractio i the ceter of the core slows the speed of some light rays, allowig all the light rays to reach the receivig ed at approximately the same time, reducig dispersio. Idex of refractio As the above figure shows, the light rays o loger follow straight lies; they follow a serpetie path beig gradually bet back toward the ceter by the cotiuously decliig refractive idex. This reduces the arrival time disparity because all modes arrive at about the same time. The modes travelig i a straight lie are i a higher refractive idex, so they travel slower tha the serpetie modes. These travel farther but move faster i the lower refractive idex of the outer core regio. Atteuatio Atteuatio ad pulse dispersio represet the two most importat characteristics of a optical fiber that determie the iformatio-carryig capacity of a fiber optic commuicatio system. The decrease i sigal stregth alog a fiber optic waveguide caused by absorptio ad scatterig is kow as atteuatio. Atteuatio is usually expressed i db/km.

10 Due to atteuatio, the power output (Pout) at the ed of km of optical fiber drops to some fractio (k) of the iput power (Pi) i.e. Pout = k.pi (k less tha ). Clearly, after km, Pout = k.pi, ad, after L km, Pout = k L.Pi. Hece, the ratio of the power out of L km of optical fibre to the power i is give by takig the log of both sides ad multiplyig by 0 gives the power loss i db as where α (= 0 log0k)is the atteuatio coefficiet of the fiber i db/km Sice atteuatio is the loss, therefore, it is always expressed as α P out = P i 0 L 0 Dispersio Dispersio, expressed i terms of the symbol t, is defied as pulse spreadig i a optical fiber. As a pulse of light propagates through a fiber, elemets such as umerical aperture, core diameter, refractive idex profile, wavelegth, ad laser lie width cause the pulse to broade. This poses a limitatio o the overall badwidth of the fiber as demostrated i Figure 4. Figure 4 Pulse broadeig caused by dispersio

11 Dispersio is geerally divided ito two categories: modal dispersio ad chromatic dispersio. We will discuss oly the modal dispersio. Modal dispersio is defied as pulse spreadig caused by the time delay betwee lower-order modes (modes or rays propagatig straight through the fiber close to the optical axis) ad higher-order modes (modes propagatig at steeper agles). This is show i Figure 5. Modal dispersio is problematic i multimode fiber, causig badwidth limitatio, but it is ot a problem i sigle-mode fiber where oly oe mode is allowed to propagate. Figure 5: Mode propagatio i a optical fiber Itermodal Dispersio \ x Let the time takes for the zero order mode travelig alog the cetral axis is give by:

12 Where vg is the velocity of the light iside the core ad give by c v g = Path legth x depeds o the propagatio agle ad is give by L x = siθ t = 0 The time differece betwee the highest order mode ad lowest order mode i step idex multimode fiber is give by: L t SI = t t0 = c Neglectig the differece betwee ad, the above equatio ca be expressed as: L v g t SI = t t = 0 L( NA) c V Parameter: The umber of modes of multimode fiber cable depeds o the wavelegth of light, core diameter ad material compositio. This ca be determied by the Normalized frequecy parameter (V). The V is expressed as: πd πd V = = ( NA) Where d=fiber core diameter λ=wavelegth of light NA=umerical aperture λ For a sigle mode fiber, V.405 ad for multimode fiber, V 0. Mathematically, the umber of modes for a step idex, fiber is give by: V N SI = For a graded idex fiber, the umber of mode is give by: V N GI = 4 λ

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