SUMMARY

Now that you have completed this chapter, let's review some of the new terms, concepts, and ideas that you have learned. You should have a thorough understanding of these principles before moving on to chapter 7.

A FIBER OPTIC TRANSMITTER is a hybrid electro-optic device. It converts electrical signals into optical signals and launches the optical signals into an optical fiber.

An OPTICAL SOURCE converts electrical energy (current) into optical energy (light).

The principal LIGHT SOURCES used in fiber optics are semiconductor light-emitting diodes (LEDs) and laser diodes (LDs).

SEMICONDUCTOR LD'S emit coherent light. Light waves having a fixed-phase relationship are referred to as coherent light.

SEMICONDUCTOR LED'S emit incoherent light. Light waves that lack a fixed-phase relationship are referred to as incoherent light.

The RELEVANT OPTICAL POWER is the amount of optical power coupled into the fiber. It depends on the angle over which the light is emitted, the size of the source's light-emitting area relative to the fiber core size, the alignment of the source and fiber, and the coupling characteristics of the fiber (such as the NA and the refractive index profile).

SOURCE-TO-FIBER COUPLING EFFICIENCY is a measure of the relevant optical power.

SILICON (Si) and GALLIUM

ARSENIDE (GaAs) are the two most common semiconductor materials used in electronic and electro-optic devices. In a semiconductor device, PHOTONS (LIGHT) are emitted when current flows through the active area.

SPONTANEOUS EMISSION occurs when photons are emitted in a random manner. Spontaneous emission produces incoherent light.

STIMULATED EMISSION occurs when a photon interacts with the laser material to produce additional photons.

A LIGHT-EMITTING DIODE (LED) is a semiconductor device that emits incoherent light, through spontaneous emission, when a current is passed through it. The basic LED types used for fiber optic communication systems are the SURFACE-EMITTING LED (SLED), the EDGE-EMITTING LED (ELED), and the SUPERLUMINESCENT DIODE (SLD).

In SURFACE-EMITTING LED'S (SLEDs), the size of the primary active region is limited to a small circular area of 20 m to 50 m in diameter. The active region is the portion of the LED where photons are emitted. SLEDs usually emit more total power into the air gap at the fiber interface than an ELED, but they do not launch as much power into the fiber. SLEDS also tend to emit power over a wider spectral range than ELED.

EDGE-EMITTING LED'S (ELEDs) emit light in a narrow emission angle allowing for better source-to-fiber coupling. They couple more power into small NA fibers than SLEDs. The polished or cut surfaces at each end of the ELED active stripe are called FACETS.

SUPERLUMINESCENCE occurs when the spontaneous emissions of an ELED experience gain due to higher injected currents and reflections from facets.

SUPERLUMINESCENT DIODES (SLDs) are similar in geometry to lasers but have no built-in optical feedback mechanism required by laser diodes for stimulated emission to achieve lasing. Although the output is not fully coherent, superluminescent diodes (SLDs) emit light that consists of amplified spontaneous emissions. The spectral width and beam angle of SLDs are narrower than that of conventional LEDs and wider than that of LDs.

The ADVANTAGES of SLDs over conventional LEDs include higher coupled power, narrower spectral width, and greater bandwidths. The DISADVANTAGES include nonlinear power-current characteristics, higher temperature sensitivity, and lower reliability.

A LASER is a device that produces optical radiation using stimulated emission rather than spontaneous emission. Laser is an acronym for light amplification by the stimulated emission of radiation.

The LASING THRESHOLD is the lowest drive level at which the output of the laser results primarily from stimulated emission rather than spontaneous emission.

The THRESHOLD CURRENT is the lowest current at which stimulated emission exceeds spontaneous emission.

A LASER DIODE is a semiconductor diode that emits coherent light by lasing. The LD's output has a narrow spectral width and small output beam angle.

TRANSMITTER OUTPUT INTERFACES fall into two categories: optical connectors and optical fiber pigtails.

FIBER OPTIC TRANSMITTERS using LDs require more complex circuitry than transmitters using LEDs.

Because LDs are threshold devices, LDs are supplied with a bias just below the threshold in the off state. This bias is often referred to as a prebias.

The least complex FIBER OPTIC TRANSMITTERS are typically packaged in transistor outline (TO) cans or hybrid microcircuit modules in dual inline packages (DIPs).

More complex FIBER OPTIC TRANSMITTERS typically are packaged in hybrid microcircuit modules in either DIP or butterfly lead packages, circuit cards, or complete stand-alone fiber optic converters.

FIBER OPTIC TRANSMITTERS can be classified into two categories: digital and analog.

DIGITAL TRANSMITTERS modulate the fiber optic source between two discrete optical power levels. These levels are essentially on and off with the exception that some light is emitted in the off state by some transmitters.

ANALOG TRANSMITTERS continuously vary the output optical power level as a function of the input electrical signal.

For LOW-DATA-RATE APPLICATIONS (0 to 20 Mbps), fiber optic transmitters generally use LEDs operating in either the 850-nm or 1300-nm window.

For MODERATE-DATA-RATE APPLICATIONS (50 to 200 Mbps), fiber optic transmitters generally use LEDs operating in the 1300-nm window.

For HIGH-DATA-RATE APPLICATIONS, most fiber optic transmitters use laser diodes as sources.

LASER SOURCES are almost never used in low- or moderate-frequency analog applications because LED sources require much less complex circuitry.