Fiber Optic Light Sources Fiber Optic Communications ECEE – 641 Dr. Kurzweg By: Antonios Boulos

Overview What is an Optical Source LEDs SLEDs – Surface Emitting LEDs ELEDs – Edge Emitting LEDs LDs – Laser Diodes Tunable Lasers Conclusion Questions

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

Surface Emitting LED

ELEDs – Edge Emitting LEDs Primary active region is a narrow strip that lies beneath the semiconductor substrate Semiconductor is cut and polished so emission strip region runs between front and back. Rear face of semiconductor is polished so it is highly reflective while front face is coated with anti-reflective, light will reflect from rear and emit through front face Active Regions are usually 100-150µm long and the strips are 50-70µm wide which are designed to match typical core fibers of 50-100µm. Emit light at narrower angle which allows for better coupling and efficiency than SLEDs

Edge Emitting LED

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.

Laser Diode Optical Cavity One reflecting mirror is at one end while the other end has a partially reflecting mirror for partial emission Remaining power reflects through cavity for amplification of certain wavelengths, a process known as optical feedback. Construction very similar to the ELEDs.

Lasing Characteristics Lasing threshold is minimum current that must occur for stimulated emission Any current produced below threshold will result in spontaneous emission only At currents below threshold LDs operate as ELEDs LDs need more current to operate and more current means more complex drive circuitry with higher heat dissipation Laser diodes are much more temperature sensitive than LEDs

Tunable Laser Tunable Laser Employed in broad-band interconnections and broadcast networks where the need for high power, narrow line width, and a tunable single-frequency emission is a must. Laser that is able to produce controllable multiple wavelengths within single cavity. Able to switch transmission of different wavelengths without using multiplexer for transmission to many different channels at by tuning the output frequency to its designated channel.

Tunable Laser Cavity Consists of an Active Region, and two passive regions: Phase Control and Grating Active region is a double heterostructure of a low bandgap between two high gap low index claddings. Two passive regions made from semiconductor with intermediate bandgap between active and cladding.

Tunable Laser Operation Current is injected into the Active Region causing the entire optical cavity to oscillate in a single longitudinal mode. A current is then injected into the grating control region causing a refractive index decrease which induces a shift of the Bragg wavelength and variation in the mode. The phase region with the injected phase current allows for recovery in Bragg wavelength in order to keep the same mode in the center of the filter band. This results in an output with variable wavelength.

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