Handbook for Sound Engineers

Transmission Techniques: Fiber Optics 463

of fiber certification and diagnostics. The companion
PC software documents, reports, and manages all test
data. OptiFiber enables audio network owners of all
experience levels to certify fiber to industry and
customer specifications, and troubleshoot short-haul
connection links and thoroughly document their results.

15.5 Sources

Sources are transmitters of light that can be coupled into
fiber optic cables. Basically the two major sources used
in fiber optic communications are light emitting diodes
(LEDs) and laser diodes. Both are made from semicon-
ductor materials.
LEDs and laser diodes are created from layers of P-
and N-type semiconductor materials, forming a junc-
tion. Applying a small voltage across the junction
causes electrical current, consisting of electrons and
holes, to flow. Light photons are emitted from the junc-
tion when the electrons and holes combine internal to
the junction.
Although the LED provides less power and operates
at slower speeds, it is amply suited to applications
requiring speeds to several hundred megabits and trans-
mission distances of several kilometers. It is also more
reliable, less expensive, has a longer life expectancy,
and is easier to use. For higher speeds or longer trans-
mission distances, the laser diode must be considered.
Table 15-2 lists the characteristics of typical sources.

15.5.1 LEDs

LEDs are made from a variety of materials located in
the Group III to Group V of the Periodic Table of Ele-
ments. The color or emission wavelength depends upon
the material. Table 15-3 shows some common LED
materials used to generate the corresponding colors and
wavelengths.

You might have seen LEDs being used in VU or

peak-reading meter displays, or as simple status indica-

tors. LEDs used in fiber optics are designed somewhat

differently than a simple display LED. The complexities

arise from the desire to construct a source having char-

acteristics compatible with the needs of a fiber optic

system. Principal among these characteristics are the

wavelength and pattern of emission. There are special

packaging techniques for LEDs to couple maximum

light output into a fiber, Fig. 15-19.

There are three basic types of designs for fiber optic

LEDs:

• Surface emitting LED.


• Edge emitting LED.
  •Microlensed LED.

Surface Emitting LED. Surface emitting LEDs, Fig.

15-20A, are the easiest and cheapest to manufacture.

The result is a low-radiance output whose large emis-

sion pattern is not well suited for use with optical fibers.

The problem is that only a very small portion of the

light emitted can be coupled into the fiber core.

The Burrus LED, named after its inventor Charles A.

Burrus of Bell Labs, is a surface-emitting LED with a

hole etched to accommodate a light collecting fiber, Fig.

15-21. However, the Burrus LED is not frequently used

in modern systems.

Edge Emitting LED. The edge emitting LEDs, Fig.

15-20B, use an active area having stripe geometry.

Because the layers above and below the stripe have dif-

ferent refractive indices, carriers are confined by the

waveguide effect produced. (The waveguide effect is

the same phenomenon that confines and guides the light

in the core of an optical fiber.) The width of the emitting

area is controlled by etching an opening in the silicon

oxide insulating area and depositing metal in the open-

ing. Current through the active area is restricted to the

area below the metal film. The result is a high-radiance

elliptical output which couples much more light into

small fibers than surface emitting LEDs.

Microlensed LED. More recently, technology has

advanced such that it is possible, under production con-

Table 15-2. Characteristics of Typical Sources

Type Output

Power

(“W)

Peak Wave-

length

(nm)

Spectral

Width

(nm)

Rise

Time

(ns)

LED 250 820 35 12

700 820 35 6

1500 820 35 6

LASER 4000 820 4 1

6000 1300 2 1

Courtesy AMP Incorporated.

Table 15-3. Materials to Make LEDs and Laser

Diodes

Material Color Wavelength

Gallium phosphide green 560 nm

Gallium arsenic phosphide yellow-red 570–700 nm

Gallium aluminum arsenide near-infrared 800–900 nm

Indium gallium arsenic phosphide near-infrared 1300–1500 nm