Learning About Options in Fiber - Cables Plus USA

SECTION 2—FIBER-OPTIC BASICS
SPECTRAL WIDTH
Earlier, we discussed material dispersion and the
fact that different wavelengths travel through a
fiber at different velocities. The dispersion
resulting from different velocities of different wavelengths
limits bandwidth.
Figure 2-16—PN Photodiode
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p
Lasers and LEDs 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 percent of the maximum amplitude
of the peak wavelength.
DETECTORS
The detector in the fiber-optic system converts the
optical signal into an electrical signal compatible
with conventional equipment and communications
networks.
A good signal detector responds well to light at
the peak intensity wavelength of the light source
and fiber combination used (800-900 nanometers,
1,000-2,000 nanometers). It also operates with low
interference, has high reliability, long operating
life, and small size.
PHOTODIODE BASICS
In moving from the conduction band to the valence
band (the energy bands in semiconductor material),
by recombining electron-hole pairs, an electron
gives up energy. In a LED, this energy is an
emitted photon of light with a wavelength determined
by the band gap separating the two bands.
Emission occurs when current from the external
circuit passes through the LED. With a photodiode,
the opposite phenomenon occurs: light falling on
the diode creates current in the external circuit.
Absorbed photons excite electrons from the
valence band to the conduction band, a process
known as intrinsic absorption. The result is the creation
of an electron-hole pair. These carriers, under
the influence of the bias voltage applied to the
diode, drift through the material and induce a
current in the external circuit. For each electron-hole
pair thus created, an electron is set flowing as
current in the external circuit. Several types of semiconductor
detectors can be used in fiber-optic
systems — the pn photodiode, the pin photodiode,
and the avalanche photodiode.
The pn Photodiode
The simplest device is the pn photodiode. (Refer
to Figure 2-16.) Two characteristics of this diode,
however, make it unsuitable for most fiber-optic
applications.
First, because the depletion area is a relatively
small portion of the diode’s total volume, many of
the absorbed photons do not result in external
current. The created hole and free electrons
recombine before they cause external current. The
received power must be fairly high to generate
appreciable current.
Second, the slow tail response from slow diffusion
makes the diode too slow for medium- and highspeed
applications. This slow response limits
operations to the kilohertz range.
Figure 2-17—PIN Photodiode
The pin Photodiode
The pin photodiode is designed to overcome the
deficiencies of its pn counterpart. While the pin
diode works like the pn diode, it has its peak sensitivity
to light signals at 1,000-2,000 nanometers
in wavelength and can be used with LED sources
and medium- to high-loss fiber.
The name of the pin diode comes from the layering
of its materials: positive, intrinsic, negative—pin.
(Refer to Figure 2-17.) Care must be exercised in
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