from differences between LED and LD sources.
Transmitter designs compensate for differences in
optical output power, response time, linearity, and
thermal behavior between LEDs and LDs to ensure
proper system operation. Fiber-optic transmitters
using LDs require more complex circuitry than
transmitters using LEDs.
Transmitter output interfaces generally fall into
two categories: optical connectors and optical fiber
pigtails (fig. 6-1). Optical pigtails are attached to the
transmitter optical source. This pigtail is generally
routed out of the transmitter package as a coated fiber
in a loose buffer tube or a single fiber cable. The pigtail
is either soldered or epoxied to the transmitter package
to provide fiber strain relief. The buffer tube or single
fiber cable also is attached to the transmitter package to
provide additional strain relief.
The transmitter output interface may consist of a
fiber-optical connector. The optical source may
couple to the output optical connector through an
intermediate optical fiber. One end of the optical fiber
is attached to the source. The other end terminates in
Figure 6-1.Pigtailed and connectorized fiber-optic devices.
the transmitter optical output connector. The optical
source also may couple to the output optical connector
without an intermediate optical fiber. The optical
source is placed within the transmitter package to
launch power directly into the fiber of the mating
optical connector. In some cases, lenses are used to
more efficiently couple light from the source into the
mating optical connector.
OPTICAL DETECTORS AND FIBER-
A fiber-optic transmitter is an electro-optic device
capable of accepting electrical signals, converting
them into optical signals, and launching the optical
signals into an optical fiber. The optical signals
propagating in the fiber become weakened and
distorted because of scattering, absorption, and
dispersion. The fiber-optic device responsible for
converting the weakened and distorted optical signal
back to an electrical signal is a fiber-optic receiver.
A fiber-optic receiver is an electro-optic device
that accepts optical signals from an optical fiber and
converts them into electrical signals. A typical fiber-
optic receiver consists of an optical detector, a low-
noise amplifier, and other circuitry used to produce the
output electrical signal (fig. 6-2). The optical detector
converts the incoming optical signal into an electrical
signal. The amplifier then amplifies the electrical
signal to a level suitable for further signal processing.
The type of other circuitry contained within the
receiver depends on what type of modulation is used
and the receivers electrical output requirements.
A transducer is a device that converts input
energy of one form into output energy of another. An
optical detector is a transducer that converts an
optical signal into an electrical signal. It does this by
generating an electrical current proportional to the
Figure 6-2.Block diagram of a typical fiber-optic receiver.