Fiber Optic Light SourcesOverviewWhat is an Optic Source?LEDs – Light Emitting DiodeSLEDs – Surface Emitting LEDsSurface Emitting LEDELEDs – Edge Emitting LEDsEdge Emitting LEDLDs – Laser DiodesLaser Diode Optical CavityLasing CharacteristicsTunable LaserTunable Laser CavityTunable Laser OperationSummaryFiber Optic Light SourcesFiber Optic Light SourcesFiber Optic CommunicationsFiber Optic CommunicationsECEE – 641 Dr. KurzwegECEE – 641 Dr. KurzwegBy: Antonios BoulosBy: Antonios BoulosOverviewOverviewWhat is an Optical SourceWhat is an Optical SourceLEDsLEDsSLEDs – Surface Emitting LEDsSLEDs – Surface Emitting LEDsELEDs – Edge Emitting LEDsELEDs – Edge Emitting LEDsLDs – Laser DiodesLDs – Laser DiodesTunable LasersTunable LasersConclusionConclusionQuestionsQuestionsWhat is an Optic Source?What is an Optic Source?The heart of a fiber optical data system The heart of a fiber optical data system A Hybrid Device A Hybrid Device Converts electrical signals into optical signals Converts electrical signals into optical signals Launches these optical signals into an optical Launches these optical signals into an optical fiber for data transmission.fiber for data transmission.Device consists of an interface circuit, Device consists of an interface circuit, drive circuit, and components for optical drive circuit, and components for optical source. (LEDs, ELEDs, SLEDs, LDs, etc)source. (LEDs, ELEDs, SLEDs, LDs, etc)LEDs – Light Emitting DiodeLEDs – Light Emitting DiodeEmits incoherent light through Emits incoherent light through spontaneous emission.spontaneous emission.Used for Multimode systems w/ Used for Multimode systems w/ 100-200 Mb/s rates.100-200 Mb/s rates.Broad spectral width and wide Broad spectral width and wide output pattern.output pattern.850nm region: GaAs and 850nm region: GaAs and AlGaAsAlGaAs1300–1550nm region: InGaAsP 1300–1550nm region: InGaAsP and InPand InPTwo commonly used types: Two commonly used types: ELEDs and SLEDsELEDs and SLEDsSLEDs – Surface Emitting LEDsSLEDs – Surface Emitting LEDsPrimary active region is a small circular area Primary active region is a small circular area located below the surface of the semiconductor located below the surface of the semiconductor substrate, 20-50substrate, 20-50µm diameter and up to 2.5µm µm diameter and up to 2.5µm thick. thick. Emission is isotropic and in lambertian pattern.Emission is isotropic and in lambertian pattern.A well is etched in the substrate to allow the direct A well is etched in the substrate to allow the direct coupling of emitted light to the optical fibercoupling of emitted light to the optical fiberEmission area of substrate is perpendicular to Emission area of substrate is perpendicular to axis of optical fiberaxis of optical fiberCoupling efficiency optimized by binding fiber to Coupling efficiency optimized by binding fiber to the substrate surface by epoxy resin with the substrate surface by epoxy resin with matching refractive indexmatching refractive indexSurface Emitting LEDSurface Emitting LEDELEDs – Edge Emitting LEDsELEDs – Edge Emitting LEDsPrimary active region is a narrow strip that lies beneath Primary active region is a narrow strip that lies beneath the semiconductor substratethe semiconductor substrateSemiconductor is cut and polished so emission strip Semiconductor is cut and polished so emission strip region runs between front and back.region runs between front and back.Rear face of semiconductor is polished so it is highly Rear face of semiconductor is polished so it is highly reflective while front face is coated with anti-reflective, reflective while front face is coated with anti-reflective, light will reflect from rear and emit through front facelight will reflect from rear and emit through front faceActive Regions are usually 100-150Active Regions are usually 100-150µm long and the µm long and the sstrips are 50-70trips are 50-70µm wide which are designed to match µm wide which are designed to match typical core fibers of 50-100µm.typical core fibers of 50-100µm.Emit light at narrower angle which allows for better Emit light at narrower angle which allows for better coupling and efficiency than SLEDscoupling and efficiency than SLEDsEdge Emitting LEDEdge Emitting LEDLDs – Laser DiodesLDs – Laser DiodesEmit coherent light through Emit coherent light through stimulated emissionstimulated emissionMainly used in Single Mode Mainly used in Single Mode SystemsSystemsLight Emission range: 5 to Light Emission range: 5 to 10 degrees10 degreesRequire Higher complex Require Higher complex driver circuitry than LEDsdriver circuitry than LEDsLaser action occurs from Laser action occurs from three main processes: three main processes: photon absorption, photon absorption, spontaneous emission, and spontaneous emission, and stimulated emission.stimulated emission.Laser Diode Optical CavityLaser Diode Optical CavityOne reflecting mirror is at one end while the other end One reflecting mirror is at one end while the other end has a partially reflecting mirror for partial emissionhas a partially reflecting mirror for partial emissionRemaining power reflects through cavity for amplification Remaining power reflects through cavity for amplification of certain wavelengths, a process known as optical of certain wavelengths, a process known as optical feedback.feedback.Construction very similar to the ELEDs.Construction very similar to the ELEDs.Lasing CharacteristicsLasing CharacteristicsLasing threshold is Lasing threshold is minimum current that must minimum current that must occur for stimulated occur for stimulated emissionemissionAny current produced below Any current produced below threshold will result in threshold will result in spontaneous emission onlyspontaneous emission onlyAt currents below threshold At currents below threshold LDs operate as ELEDsLDs operate as ELEDsLDs need more current to LDs need more current to operate and more current operate and more current means more complex drive means more complex drive circuitry with higher heat circuitry with higher heat dissipationdissipationLaser diodes are much Laser diodes are much more temperature sensitive more temperature sensitive than LEDsthan LEDsTunable LaserTunable LaserTunable LaserTunable LaserEmployed in broad-band interconnections and