Handbook for Sound Engineers

466 Chapter 15

3. Highly directional: The light is emitted in a highly
   directional pattern with little divergence. Diver-
   gence is the spreading of a light beam as it travels
   from a source.

15.5.3 Superluminescent Diodes (SLDs)

A source called the superluminescent diode (SLD) is
now available for use. The performance and cost of the
SLD fall somewhere in between the LED and the laser.
The SLD was first investigated in 1971 by the Soviet
physicist, Kurbatov. The SLD may operate like a
edge-emitting LED at low currents, while at high-injec-
tion currents, the output power increases superlinearly
and the spectral width narrows as a result of the onset of
optical gain.

15.5.4 Vertical Cavity Surface Emitting Laser
(VCSEL)

A more recent source is the vertical cavity surface emit-
ting laser (VCSEL). It is a specialized laser diode that
promises to revolutionize fiber optic communications
by improving efficiency and increasing data speed. The
acronym VCSEL is pronounced vixel. It is typically
used for the 850 nm and 1300 nm windows in fiber
optic systems.

15.5.5 LED and Laser Characteristics

15.5.5.1 Output Power

Both LEDs and laser diodes have VI voltage versus cur-
rent characteristic curves similar to regular silicon
diodes. The typical forward voltage drop across LEDs
and laser diodes is 1.7 volts.
In general, the output power of sources decreases in
the following order: laser diodes, edge emitting LEDs,
surface emitting LEDs. Fig. 15-23 shows some curves
of relative output power versus input current for LEDs,
SLDs, and laser diodes.

15.5.5.2 Output Pattern

The output or dispersion pattern of light is an important
concern in fiber optics. As light leaves the chip, it
spreads out. Only a portion of light actually couples into
the fiber. A smaller output pattern allows more light to
be coupled into the fiber. A good source should have a
small emission diameter and a small NA. The emission
diameter defines how large the area of emitted light is,

and the NA defines at what angles the light is spreading

out. If either the emitting diameter or the NA of the

source is larger than those of the receiving fiber, some

of the optical power will be lost. Fig. 15-24 shows typi-

cal emission patterns for the LED, SLD, and laser.

15.5.5.3 Wavelength

Optical fibers are sensitive to wavelength, therefore the

spectral (optical) frequency of a fiber optic source is

important. LEDs and laser diodes do not emit a single

wavelength; they emit a range of wavelengths. This

range is known as the spectral width of the source. It is

measured at 50% of the maximum amplitude of the

peak wavelength. As an example, if a source has a peak

Figure 15-23. Optical power output versus input current

for LEDs, SLDs, and laser diodes.

Figure 15-24. Emission patterns of sources.

Input current–mA

Optical power output–mW

SDL LASER

LED

LASER

SDL

LED