Fiber-Optic Communications James N. Downing

Chapter 5 Optical Sources and Transmitters

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

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

5.2 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.

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)..

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.

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

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

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

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

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.

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.

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

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

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

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

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

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

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

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

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.”

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.

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%

5.5 Transmitters Transmitter Packaging –Provides protection form environment and weather –Provides mechanical stability