Fiber Optic Light SourcesFiber Optic Light Sources

Fiber Optic CommunicationsFiber Optic Communications

ECEE – 641 Dr. KurzwegECEE – 641 Dr. Kurzweg

By: Antonios BoulosBy: Antonios Boulos

OverviewOverview

What is an Optical SourceWhat is an Optical SourceLEDsLEDsSLEDs – Surface Emitting LEDsSLEDs – Surface Emitting LEDsELEDs – Edge Emitting LEDsELEDs – Edge Emitting LEDsLDs – Laser DiodesLDs – Laser DiodesTunable LasersTunable LasersConclusionConclusionQuestionsQuestions

What 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 SLEDs

SLEDs – 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 index

Surface Emitting LEDSurface Emitting LED

ELEDs – 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 SLEDs

Edge Emitting LEDEdge Emitting LED

LDs – 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 LEDs

Tunable LaserTunable Laser

Tunable LaserTunable Laser Employed in broad-band interconnections and Employed in broad-band interconnections and

broadcast networks where the need for high power, broadcast networks where the need for high power, narrow line width, and a tunable single-frequency narrow line width, and a tunable single-frequency emission is a must.emission is a must.

Laser that is able to produce controllable multiple Laser that is able to produce controllable multiple wavelengths within single cavity.wavelengths within single cavity.

Able to switch transmission of different wavelengths Able to switch transmission of different wavelengths without using multiplexer for transmission to many without using multiplexer for transmission to many different channels at by tuning the output frequency to different channels at by tuning the output frequency to its designated channel.its designated channel.

Tunable Laser CavityTunable Laser Cavity

Consists of an Active Region, and two passive regions: Consists of an Active Region, and two passive regions: Phase Control and GratingPhase Control and Grating

Active region is a double heterostructure of a low Active region is a double heterostructure of a low bandgap between two high gap low index claddings.bandgap between two high gap low index claddings.

Two passive regions made from semiconductor with Two passive regions made from semiconductor with intermediate bandgap between active and cladding.intermediate bandgap between active and cladding.

Tunable Laser OperationTunable Laser Operation

Current is injected into the Active Region causing the Current is injected into the Active Region causing the entire optical cavity to oscillate in a single longitudinal entire optical cavity to oscillate in a single longitudinal mode.mode.

A current is then injected into the grating control region A current is then injected into the grating control region causing a refractive index decrease which induces a shift causing a refractive index decrease which induces a shift of the Bragg wavelength and variation in the mode.of the Bragg wavelength and variation in the mode.

The phase region with the injected phase current allows The phase region with the injected phase current allows for recovery in Bragg wavelength in order to keep the for recovery in Bragg wavelength in order to keep the same mode in the center of the filter band.same mode in the center of the filter band.

This results in an output with variable wavelength.This results in an output with variable wavelength.

SummarySummary Optical light sources convert electrical signals into optical signals Optical light sources convert electrical signals into optical signals

and launch them.and launch them. Commonly used light sources include LEDs, ELEDs, SLEDs, and Commonly used light sources include LEDs, ELEDs, SLEDs, and

LDs.LDs. LEDs produce nonlinear incoherent light whereas a Laser Diode LEDs produce nonlinear incoherent light whereas a Laser Diode

produces linear coherent light.produces linear coherent light. Incoherent light sources used in multimode systems as where Laser Incoherent light sources used in multimode systems as where Laser

Diodes/Tunable Lasers in single mode systemsDiodes/Tunable Lasers in single mode systems Laser diodes must operate above their threshold region to produce Laser diodes must operate above their threshold region to produce

coherent light, otherwise operating as ELED.coherent light, otherwise operating as ELED. Laser diodes are much faster in switching response than LEDsLaser diodes are much faster in switching response than LEDs Tunable laser is able to produce coherent light output with controlled Tunable laser is able to produce coherent light output with controlled

variable wavelength variable wavelength Tunable laser is used in multi wavelength systems by replacing a Tunable laser is used in multi wavelength systems by replacing a

system where many sources are coupled into a multiplexing device system where many sources are coupled into a multiplexing device systemsystem