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1、1Chapter 3 Fiber Optics and Integrated OpticsGradient- index optics the refractive index is the function of space Fiber optics Optical wave- guide, tele- communication Integrated optics miniaturized optical systemTrue or false statement:The light travels in the straight line in the air. (1) True (2)
2、 Falsen refractive index - -density of the air T temperature of the air ) 1(nTn/1) 1(2How does light travel?If n=constant; Light travels in straight line If n varying in space; Light travels in curved line! It follows the law of refraction!3.1 Gradient Refractive Index1.Atmospheric refractionThe lig
3、ht is bending towards the higher index side?Sun rising 2+4, schlieren head; 3, gradient- index field; 5, knife edge; 6, camera lens; 7, image plane.Homogeneous index: knife edge block light no image formed on image plane Gradient index: gradient deflect the light light bypass the knife real image of
4、 the gradient formed.10Other experimental methods? Ronchi grid method ? Color schlieren methodWhat is an Optical Fiber?An optical fiber is a waveguide for light consists of :coreinner part where wave propagatescladdingouter part used to keep wave in corebufferprotective coatingjacketouter protective
5、 shieldcan have a connector too3.2 Fiber Optics11?Two types of fiber: step- index; gradient fiber ?Structure : core(higher n) ; cladding(lower n) ?Total internal reflectionTypes of Fibersstep- index multimodencncnfncncnfnc ncnfstep- index singlemodeGRIN121. Step- Index fiberNA of a Fiber()maxsin =ou
6、tsidenNAThe NA defines a cone of acceptance for light that will be guided by the fibermax90- tmaxnfncmust be critical angle()maxsin =outsidenNA22 cfstepnnNA=niicfstepnnnNA22=NA in air131.40NA nf1.457,1.00,NA nf1.457,1.33,1.81.4nf1.41.51.61.71.800.20.40.60.811.21.4NA changes with nairwaterNA is sensi
7、tive to n1.40NA nf1.457,1.00,NA nf1.472,1.00,NA nf1.530,1.00,1.81.4nf1.41.51.61.71.800.20.40.60.811.21.45% change1% change14NA and Acceptance angle1.40NAi1.00,NAi1.33,900i015304560759000.20.40.60.811.21.4iairwaterTwo types of fiber with different propagation modes:single- mode fiber: only single mod
8、e is permitted small core diameter: 8.3(core) /125(cladding) m Multi- mode fiber: several modes are permitted large core diameter: 5062.5(core) /125(cladding)m 15Types of fiber endsbeam patterns can be:sphericalcylindricalbundles90 degreeAngle PreservationIn an ideal fiber, the angle of incidence wi
9、ll equal the exit angle.22 1fi nn216example: critical bend radiusRough surfaces, bending, and other real- world imperfections will case a change in the exit cone.Fiber Tapers12()()2211sinsin=ddd1d2way to change the acceptance angles of a fibersometimes used to collimate light172. Gradient- Index Fib
10、erSimplification:continuous n change discrete layers of n From Snell s refraction law: At the nth boundary, at the distance R from the axis:Therefore:With n(R) Sin I(R) I(R) Until: Sin I (R ) =1 I(R) =90, The ray return back to the center( optical axis)2211sinsinInIn=)(sin)(sin11RIRnIn=tconsRIRntan)
11、(sin)(=rLight Source2o2oEscaping RayTrapped RayGRIN CORE18Additional Fiber TypesRadiusRadiusSegmented profileW- TypeAlpha- 1 dispersion shiftedDepressed cladding dispersion flatenedSimple StepIndex ProfileField Intensity(All single mode)193. ApplicationsA. Transmission of light to conduct light out
12、of small places ? Inside heart, digestive tract, stomach, respiratory tract, lung, etc.B. Tele- communicationelectrical digital signalOptical transmitterelectrical digital signalInformation decoderMultimedia outputInformation SourceInformation encoderOptical receiverOptical fiberOptical fiberoptical
13、 signalLight sourceRelay stationRelay stationOptical fiberoptical signalSignal ModulatorMultiplexingDemodulating Demultiplexing20advantages: a.light in weight，efficient use of space in conduits b. less expensive c.Free from electrical interference，aircraft, military, security d. Flexible e.Secure to
14、 interception f.Low power lost g.enormous capacity of transmission: WDM/ DWDM(Dense Wavelength Division Multiplexing)， Higher data rates over longer distances- - more “ bandwidth”for internet traffic Problem remained: Attenuation: power lost ( minimum at 1.55m) Dispersion: modal, material (minimum a
15、t 1.31m), Types of Dispersion in Fibersmodal-time delay from path length differences -usually the biggest culprit in step- indexmaterial-n() : different times to cross fiber - (note: smallest effect 1.3 m)waveguide-changes in field distribution - (important for SM)non- linear-n can become intensity-
16、 dependentNOTE:GRIN fibers tend to have less modal dispersion because the ray paths are shorter 21Effect of Modal Dispersiontimetimetimemodal example:step index 24 ns km - 1 GRIN 122 ps km- 1initial pulsefarther downfarther still4. Bel & Decibel (dB): Comparative unitInput: 1 output: 2AttenuationBel Decibel
