Advanced Optical Communication: Theory and Techniques

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ADVANCED OPTICAL COMMUNICATION
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INTRODUCTION
A general optical transmission is briefly described for which we assume that a digital pulse signal
is transmitted over optical fiber unless otherwise specified. The optical link consists of an optical fiber,
transmitter, receiver and amplifier, each of which is dealt with in the subsequent subsections.
Figure 1 Optical Transmission Link
Optical fiber is a dielectric medium for carrying information from one point to another in the form of
light. Unlike the copper form of transmission, the optical fiber is not electrical in nature. To be more
specific, fiber is essentially a thin filament of glass that acts as a waveguide. A waveguide is a physical
medium or path that allows the propagation of electromagnetic waves, such as light. Due to the physical
phenomenon of total internal reflection, light can propagate following the length of a fiber with little loss.
Light can travel through any transparent material, but the speed of light will be slower in the material than
in a vacuum. The ratio of the speed of light in a vacuum to that in a material is known as the material’s
refractive index (n) and is given by n = c/v, where c is the speed in a vacuum and v is the speed in the
material. When light travels from one material of a given refractive index to another material of a
different refractive index (i.e., when refraction occurs), the angle at which the light is transmitted in the
second material depends on the refractive indices of the two materials as well as the angle at which light
strikes the interface between the two materials. According to Snell’s law, we have na sin θa=nb sin θb ,
where na and nb are the refractive indices of the first substance and the second substance, respectively;
and θa and θb are the angles from the normal of the incident and refracted lights, respectively. The total
internal reflection is given as,
sin θc= ηclad
ηcore
for refractiveindices of cladding∧core respectively
THEORY
Radio over Fiber (RoF) is an optical fiber link to distribute modulated RF signals from a central
location to remote antenna units. The RoF systems are introduced to replace a central antenna with a low

power distributed antenna system (DAS). RoF system is consisting of many base stations, which are
connected to a single central station. RoF systems centralize the RF signal processing function in one
shared location, and use optical fiber link to distribute the RF signals to the RAUs or BSs. Fiber optic
communication system are based on the fiber optic cable which provides for extremely high data rates
which allow the very large masses of data to be transmitted at the speed of light. The fiber optic cable,
FOC, transmits data over very long distance. One very common example is the sea or ocean underground
cable that connect continents on the planet earth. According to the basic model, the bandwidth of the fiber
optic communication system determines the maximum data rate. A FOC link denotes the signal pathway
between two points using the cable. The pathway is the mode or channel that enables transmission of the
data from the sender to the receiver points. The links are often described in terms of their ability to send
and receive signals as part of the communication system. There are two modes of communication
generally referred to as the simplex and duplex. This is a classification based on the direction or flow of
data or information. For the simplex mode of communication, data flows in one direction only, for
instance, home radio communication or broadcasting. The duplex mode refers to the communication
where the sender and receiver can communicate to each other. The half-duplex allows one speaker at a
time while the full-duplex allows for information to flow both ways at the same time.
On the receiver end there is another light sensitive device that is known as the photocell or the
light detector that detects the light pulses. The photo detector converts the light pulses into an electrical
signal. The electrical pulses are amplified and reshaped back into digital form. The light sources at the
sender and receiver points must be capable of operating at the same data rate. The circuitry that drives the
light source and the circuitry that amplifies and processes the detected light must both have suitable high-
frequency response. The fiber is required to carry the information signals without distortions at the set
data rate. For the single mode transmission FOC, repeater units are used to restore the signal strength that
may attenuate while propagating. Special relay stations are used to pick up light beams, convert them
back to electrical pulses that are amplified and then they are retransmitted on another beam. There are
several stages of repeaters that may be needed over very long distances. The attenuation problem occurs
as a universal problem for all the transmission cables, for instance, the electrical copper cables.
Radio over Fiber (ROF) Technique
It is the integration of wireless and fiber optical communication technologies, and modulating
wireless signals over optical carrier for transporting over fiber optic cable. The digital ROF link
can maintain the dynamic range more independently than the optical fiber link distance. The

analogue radio frequency signal is digitized by using the photonic analogue-to-digital converter.
The generated digitized radio frequency data stream and RF analogue signals are transported
over optical fiber networks by using WDM technique. The photonic DAC of RF system at the
base station or the access point converts the digital photonic signals to its analogue optical
modulated signal.