Fiber-Optic Communications

1. Fiber-Optic Communications James N. Downing

2. Chapter 5 Optical Sources and Transmitters

3. 5.1 Source Considerations • Fiber must match: • Power • Size • Modal characteristics • Numerical aperture • Linewidth • Fiber window • Wavelength • Data type

4. 5.2 Electronic Considerations • Conductors • Flow of electrons • Insulators • Block current flow • Semiconductors • Require more energy than conductors but less than insulators for current to flow

5. 5.2 Electronic Considerations • The PN Junction • Two junctions—one highly doped with negative charge carriers and the other doped with positive charge carriers—are fabricated next to each other. • When an external voltage is applied in forward bias (positive terminal attached to the positively doped region), current will flow through the p-n junction. • When the external voltage is applied in reverse, no current will flow through the p-n junction.

6. 5.3 The Light-Emitting Diode (LED) • LED Operation • When a p-n junction is forward biased, electrons obtain enough energy (bandgap energy) to jump to a higher energy level where they begin to lose their energy . • When those electrons lose the energy needed to keep them in the higher level, they drop back to the valence level (recombination)..

7. 5.3 The Light-Emitting Diode (LED) • LED Operation • When the electrons recombine, a photon of light is emitted. • This is called spontaneous emission. • The light is emitted in all directions (coherent). • This light can be focused through a lens to be used for displays.

8. 5.3 The Light-Emitting Diode (LED) • Linewidth • Defined by the difference between the energy of photon and the band gap energy • Internal quantum efficiency: Efficiency of the photo producing process

9. 5.3 The Light-Emitting Diode (LED) • LED Physical Structure • Homojunction • Both p and n sides are same base material • Surface-emitting LED • Light comes out all sides • Much light is wasted

10. 5.3 The Light-Emitting Diode (LED) • LED Structure • Heterojunction • Has different base materials • Edge-emitting LED

11. 5.3 The Light-Emitting Diode (LED) • LED Performance • Voltage: 1.5 to 2.5 volts • Current: 50 to 300 mA • Couples 10 to 100 μW of power into a fiber • Fiber window: 850 to 1550 nm • Linewidth: 15 to 60 nm • Data rates: 100 Mbps • Inexpensive • Rugged • Used in LANS

12. 5.4 The Laser Diode • LASER • Light Amplification by Stimulated Emission of Radiation • Stimulated Emission • An external photon hits an excited electron forcing another photon to be emitted at the same wavelength. That created photon excites another, etc.

13. 5.4 The Laser Diode • Population Inversion • A necessary condition for laser action • The number (population) of the excited electrons or photons are much greater than those in the ground state.

14. 5.4 The Laser Diode • Positive Feedback • Turns the amplifier into an oscillator • Accomplished by fabricating mirrors at each end of the medium causing the photons to bounce back and forth from one end to the other

15. 5.4 The Laser Diode • Laser Output Mode Structure • The range of optical frequencies is finite • Mode-suppression ratio (MSR) • Measure of how the physical structure of the device can be tuned to a single mode

16. 5.4 The Laser Diode • Laser Diode Physical Structure • Similar to edge-emitting LEDs but with a thinner active region • Broad-area-semiconductor laser • No light confinement at the faces parallel to junction plane • Elliptical pattern • Unsuitable for communications

17. 5.4 The Laser Diode • Laser Diode Physical Structure • Buried heterostructure laser • Single mode output • Bandwidth and thickness of active layer control

18. 5.4 The Laser Diode • Quantum Well Lasers • Better conversion efficiency, confinement, and wavelength availability • Distributed Feedback • Selectively reflects only one wavelength due to the Bragg grating inside the structure

19. 5.4 The Laser Diode • External Cavity Lasers • Implemented by moving one mirror outside of the active region resulting in a single longitudinal mode output with a high MSR • Vertical Cavity Surface Emitting Lasers • Single-mode, narrow linewidth, circular output for easy coupling

20. 5.4 The Laser Diode • Tunable Lasers • High power • Stable • Single mode • Narrow linewidth • Long-haul and ultra-long-haul communications

21. 5.5 Transmitters • The transmitter is a device that converts an electrical communication signal into an optical one, modulates the signal, and couples the modulated signal back into a fiber. • Consists of • Source, modulator, driver, and coupling devices

22. 5.5 Transmitters • Modulator • Amplitude modulation primary method • AM produces changes in the population of the charge carriers of the LED. • The change in population will also produce a change in the refractive index of the fiber, which in turn creates a “chirp.”

23. 5.5 Transmitters • Electrical Driving Circuit • Provides appropriate current and voltage • Consists of • LED: A single transistor and a few resistors • LASER: More complex. The laser is a current driven device and requires precise current and temperature control to maintain a stable output.

24. 5.5 Transmitters • Source to Fiber Coupling • Efficiencies vary from 1% for LEDs to 80% for VCSEL transmitters • Direct Coupling • Fiber is epoxied to the source • Lens Coupling • A lens is used to optimize the process • May have tapered fiber • Efficiencies of near 100%

25. 5.5 Transmitters • Transmitter Packaging • Provides protection form environment and weather • Provides mechanical stability