Fiber Optic Light Sources

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. 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 LEDs – Light Emitting Diode

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 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

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