Introduction - Optical Signal Processing by Silicon Photonics - page 2

Fig. 1.1 Basic block diagram of communication system
transmission medium, which is used to carry the information from transmitter to
receiver (Fig. 1.1 ).
The visible optical carrier waves or light for communication has been in use fre-
quently over the decades. Simple systems which include signal fires, reflecting mir-
rors and, signaling lamps used recently are providing very successful, if restricted,
information transfer. An optical fiber communication system is the basic form of
any type of communication system. In electrical communications the electrical sig-
nal provided by the information source, is usually resulted from a non-electrical
message signal, to a transmitter comprising electrical and electronic components
which are capable of converting the signal into an appropriate form for propaga-
tion over the transmission medium. This is often achieved when a carrier, which
may be an electromagnetic wave, is modulated. The transmission medium however
can comprise a pair of wires, a coaxial cable or a radio link through free space,
which is used to transmit the signal to the receiver. It is receiver, where the signal
is transformed into the original demodulated electrical information signal before
passing to the destination point. However, the obvious thing is that in any trans-
mission medium attenuation is occurred to the signal, or it may suffer losses and
degradation occurs due to contamination caused by random signals and noise,
also distortions are possibly imposed by different mechanisms within the medium
itself. Therefore, in any communication system there is a maximum permitted dis-
tance set as threshold between the receiver and the transmitter beyond which the
system effectively restrict the intelligible communication. For effective long haul
applications these factors demand the installation of repeaters or line amplifiers
at intervals, in order to get rid of signal distortion and to boost signal level before
transmission process is sustained down the link.
The modulation of an optical carrier may be done using either an analog or
digital information signal. In the system that is modulated with analog signal, the
variation of the light emitting from the optical source in an uninterrupted manner.
The digital modulation, however, involves distinct changes in the light intensity.
Analog modulation is although easier and simpler to implement with an optical
fiber communication system is not much efficient, it also requires a signal-to-
noise ratio at the receiver end far greater than digital modulation. Also, the lin-
earity needed for analog modulation is not always provided by semiconductor
optical sources, especially at high modulation frequencies. For these reasons,