03-10-2012, 04:57 PM
Fiber Optic Light Sources
FiberOpticLightSources.ppt (Size: 280 KB / Downloads: 161)
What is an Optic Source?
The heart of a fiber optical data system
A Hybrid Device
Converts electrical signals into optical signals
Launches these optical signals into an optical fiber for data transmission.
Device consists of an interface circuit, drive circuit, and components for optical source. (LEDs, ELEDs, SLEDs, LDs, etc)
LEDs – Light Emitting Diode
Emits incoherent light through spontaneous emission.
Used for Multimode systems w/ 100-200 Mb/s rates.
Broad spectral width and wide output pattern.
850nm region: GaAs and AlGaAs
1300–1550nm region: InGaAsP and InP
Two commonly used types: ELEDs and SLEDs
SLEDs – Surface Emitting LEDs
Primary active region is a small circular area located below the surface of the semiconductor substrate, 20-50µm diameter and up to 2.5µm thick.
Emission is isotropic and in lambertian pattern.
A well is etched in the substrate to allow the direct coupling of emitted light to the optical fiber
Emission area of substrate is perpendicular to axis of optical fiber
Coupling efficiency optimized by binding fiber to the substrate surface by epoxy resin with matching refractive index
LDs – Laser Diodes
Emit coherent light through stimulated emission
Mainly used in Single Mode Systems
Light Emission range: 5 to 10 degrees
Require Higher complex driver circuitry than LEDs
Laser action occurs from three main processes: photon absorption, spontaneous emission, and stimulated emission.
Summary
Optical light sources convert electrical signals into optical signals and launch them.
Commonly used light sources include LEDs, ELEDs, SLEDs, and LDs.
LEDs produce nonlinear incoherent light whereas a Laser Diode produces linear coherent light.
Incoherent light sources used in multimode systems as where Laser Diodes/Tunable Lasers in single mode systems
Laser diodes must operate above their threshold region to produce coherent light, otherwise operating as ELED.
Laser diodes are much faster in switching response than LEDs
Tunable laser is able to produce coherent light output with controlled variable wavelength
Tunable laser is used in multi wavelength systems by replacing a system where many sources are coupled into a multiplexing device system