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

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

3. 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)

4. LEDs – Light Emitting Diode
Emits incoherent light through spontaneous emission.
Used for Multimode systems w/ 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

5. 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

6. Surface Emitting LED

7. 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 µm long and the strips are 50-70µm wide which are designed to match typical core fibers of µm.
Emit light at narrower angle which allows for better coupling and efficiency than SLEDs

8. Edge Emitting LED

9. 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.

10. 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.

11. 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

12. 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.

13. 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.

14. 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.

15. 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