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