wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Media 2-9
2.3.3 transmitters and Receivers
Optical transmitters generate the signals carried by fiber-optic communication systems. Light sources
compatible to the properties of optical fibers are used in optical communication. Visible red light–
emitting diodes (LEDs) are used, that transmit wavelengths better than the near-infrared. Near-infrared
LEDs and semiconductor laser made on gallium arsenide and gallium aluminum arsenide emitting at
750–900.nm are used with glass optical fibers for relatively short links and moderate speed systems.
These light sources have been used in the first generation of optical communication. The second and
third generations of OCSs have been using the semiconductor laser. Its operating wavelength is 1300.nm
through glass fibers and has a loss of 0.35–0.5.dB/km at this wavelength. The transmission window
for the fourth generation of optical communication is 1550.nm and the attenuation is 0.15–0.5.dB/km.
Optical fibers doped with the rare earth erbium also generate light near 1550.nm, but they are used
more often to amplify an optical signal that has traveled a long distance. Erbium-doped fiber amplifier
(EDFA) has revolutionized optical communication. It has wide bandwidth (20–70.nm), high gain
(20–40 dB), high output power (>200.mW), modulation format, wavelength insensitive, low distortion,
and low noise.
Photomultipliers respond to incident light by delivering charge to the anode. Photoelectrons are
accelerated toward a series of electrodes (dynodes). These are maintained at successively higher potential
with respect to the cathode. On striking a dynode surface, each electron causes the emission of
several secondary electrons, which in turn are accelerated toward next dynode, and multiplication continues.
Silicon photodiodes are one of the most popular radiation detectors. They have small size, high
quantum efficiency, wavelength range from 0.4 to 1.μm, good linearity response, large bandwidth, simple
biasing requirements, and relatively low cost. Most fast photodiodes require internal amplification.
This useful amplification of photocurrent is achieved in the avalanche photodiode. A basic p–n junction
is operated under very high reverse bias. Carriers traversing the depletion region gain sufficient energy
and enable further carriers to be excited across energy gap by impact excitation.
Optimum performance of the OCS is achieved by the proper alignment. The generic representation of
an OCS is shown in Figure 2.9. It is composed of three major blocks: optical transmitter, fiber link, and
optical receiver. An optical source, which is generally a LASER, signal is fed to a modulator’s input
and the second input in the form of electrical signal is fed to the modulator’s other input. This input in
case of optical modulator can be used to directly modulate the optical signal up to 5.Gbps. Several kinds
of modulators are being used for communication purposes, the Mach–Zender modulator is the most
common one. The modulated signal is transmitted through the fiber link, which can span over hundreds
to thousands of kilometers. The fiber link, in addition to regenerative repeaters and optical amplifiers,
also employs splitters to distribute or drop optical signal on any particular link. The splitters can be
thought of as toll points at the main highway where the routing of the traffic is to be decided.
Other users
Fiber
transmission
Optical
source
Splitter
Modulator Detector Demodulator
Input data
signal
Output data
signal
Electronic
intelligence
(origin)
Electronic
intelligence
(destination)
FIGURE 2.9
Generic representation of an OCS.
© 2011 by Taylor and Francis Group, LLC