Introduction
By the end of nineteenth century, a British physicist John Tyndall demonstrated to
Royal Society that the ordinary light could be guided by along curved stream of water by
total internal reflection (TIR). By keeping this as a reference, until the invention of laser
by Maiman in 1960, light could be guided with limited success through glass fibers. One
of the main reasons for utilizing laser beam was that the beam has high degree of
coherency than the ordinary light. However, till early 1960’s light propagation through
fiber was confined only to scientific studies because at that time some of the best optical
glasses had attenuations of the order of several thousand dB/km and hence the optical
fiber communication seem practically meaningless. Then after few years of research in
glass technology, with the help of pure silica it was demonstrated that the losses reduce to
few tens of dB/km. By 1970, the workers at Corning glass works produced the first fiber
with loss under 20 dB/km. Then finally, by 1979, the fiber loss was brought down to just
0.2 dB/ km near 1.55  m spectral regions.
The low loss fiber brought revolution in the field of optical fiber communication.
During the 1980’s, almost all the industrialized nations switched over to optical finer
communication. According to surveys in United States of America alone more than three
million kilometers of optical fiber cables were laid to replace electrical communication
by optical fiber communications. Let us now see the construction, manufacturing
materials used, working principle, types and applications of optical fiber in forthcoming
topics.
Construction of optical fiber
As shown in fig. 3.1.an optical fiber is cylindrical shaped cable which consist
different parts as follows:
(i)
Core which is the inner part of the optical fiber,
(ii)
Clad which surrounds the core in concentric manner,
(iii)
Buffer which encases the layers of core and clad to protect them from damage,
(iv)
Jacket, the outer most layer of the optical fiber, enclosed the buffer and is
made up of polyurethane which basically is a kind of polymer. Jacket
basically protects the fiber against the chemical reactions with surroundings
and also from abrasion and crushing.
Such fibers, each one protected by individual jackets are grouped to form a cable and
hence a cable may comprise a one to several hundred optical fibers.
Fig. 3.1 Different parts of optical fiber cable
Working Principle: Total Internal Reflection (TIR)
Total internal reflection is the working principle of the optical fiber which typically
defined as the reflection of the total amount of incident light at the boundary between two
media. Due to the change in refractive indices of media, the light rays are bending in such
a manner that beyond certain angle of incidence, say critical angle θc, they are going to
propagate in the same medium in which they originate
Acceptance angle and Numerical aperture
Consider an optical fiber having
a core of refractive index n1 and
cladding of refractive index n2. Let the
incident light makes an angle i with the
core axis as shown in figure (3). Then
the light gets refracted at an angle θ and
fall on the core-cladding interface at an
angle where,
...................... (1)
By Snell’s law at the point of entrance
of light in to the optical fiber we get,
.......................
(2)
where n0 is refractive index of medium
outside the fiber. For air n0 =1.When
light travels from core to cladding it moves from denser to rarer medium and so it may be
totally reflected back to the core medium if θ' exceeds the critical angle θ'c. The critical
angle is that angle of incidence in denser medium (n1) for which angle of refraction
become 90°. Using Snell’s laws at core cladding interface,
or
...................... (3)
Therefore, for light to be propagated within the core of optical fiber as guided
wave, the angle of incidence at core-cladding interface should be greater than θ'c. As i
increases, θ increases and so θ' decreases. Therefore, there is maximum value of angle of
incidence beyond which, it does not propagate rather it is refracted in to cladding medium
( fig: 3(b)). This maximum value of i say im is called maximum angle of acceptance and
n0 sin im is termed as the numerical aperture (NA).From equation(2),
But, from eq. (3), we have,
Therefore,
So, we have,
The significance of NA is that light entering in the cone of semi vertical angle i m
only propagate through the fiber. The higher the value of im or NA more is the light
collected for propagation in the fiber. Numerical aperture is thus considered as a light
gathering capacity of an optical fiber. Numerical Aperture is defined as the Sine of half of
the angle of fiber’s light acceptance cone. i.e. NA= Sin θa where θa, is called acceptance
cone angle.
Advantages of optical fiber communication system:
Compared to metallic coaxial cable, the optical fibers are better in following
ways:
1) High information carrying capacity:
The information carrying capacity is very large (1014 Hz) in both the digital and
analog form compared to metallic co axial cable. Because of high frequency of the
optical carrier signal and the availability of high-speed sources and detectors large
bandwidth of THz in analog communication or tera bits /second (TBPS) for digital
communication is possible and has been achieved using dense wavelength division
multiplexing (DWDM) technology.
2) Highly secured:
Optical fiber confine the signal in it as its working principle is total internal
reflection and hence the signal is highly secure. So, optical fiber offers high degree of
security and privacy.
3) Unaffected by external interferences:
Optical fibers, glass or plastic, are electrical insulators. No electric currents flow
through them, either due to the transmitted signal or due to external radiation striking
it. Therefore, fibers are unaffected by radio frequency interference (RFI) and
electromagnetic interference (EMI). RFI refers to the interference caused by the radio
and television stations, radar and other signals originating in electronic equipment.
EMI includes these sources as well as those caused by natural phenomena (such as
lightning) or caused unintentionally (such as sparking).
4) Small size and lightweight
As the optical fibers are compact and lightweight, they are much easy to carry and
transport. Comparing the metallic co axial cable with fiber cable, following surprises
are coming out: The metal cable contains 900 twisted- wire pairs and its diameter is
70 mm. Each pair carries 24 channels (1.56 Mbits/sec) so the cable capacity is 21600
calls. One fiber cable developed for telephone applications has a 12.7 mm diameter
and contains 144 fibers carries 672 channels (43 Mbits/sec). This fiber cable has a
capacity of 96768 calls.
5) Easy utilization in different environments
The signal which passed through the fiber is an optical, so there is no any issue regarding
spark as it could be in the case of electric signal. Therefore, optical fiber cables can be
easily installed in the corrosive and flammable environments
Multimode Fiber
Multimode fiber, the first to be manufactured and commercialized, simply refers to the
fact that numerous modes or light rays are carried simultaneously through the waveguide.
Modes result from the fact that light will only propagate in the fiber core at discrete
angles within the cone of acceptance. This fiber type has a much larger core diameter,
compared to single-mode fiber, allowing for the larger number of modes, and multimode
fiber is easier to couple than single-mode optical fiber. Multimode fiber may be
categorized as step-index or graded-index fiber. Multimode Step-index Fiber
Figure 2 shows how the principle of total internal reflection applies to multimode stepindex fiber. Because the core's index of refraction is higher than the cladding's index of
refraction, the light that enters at less than the critical angle is guided along the fiber.
Figure 2 - Total Internal Reflection in Multimode Step-index fiber
Three different lightwaves travel down the fiber. One mode travels straight down the
center of the core. A second mode travels at a steep angle and bounces back and forth by
total internal reflection. The third mode exceeds the critical angle and refracts into the
cladding. Intuitively, it can be seen that the second mode travels a longer distance than
the first mode, causing the two modes to arrive at separate times. This disparity between
arrival times of the different light rays is known as dispersion, and the result is a muddied
signal at the receiving end. For a more detailed discussion of dispersion, see "Dispersion
in Fiber Optic Systems" however, it is important to note that high dispersion is an
unavoidable characteristic of multimode step-index fiber. Multimode Graded-index
Fiber
Graded-index refers to the fact that the refractive index of the core gradually decreases
farther from the center of the core. The increased refraction in the center of the core
slows the speed of some light rays, allowing all the light rays to reach the receiving end at
approximately the same time, reducing dispersion.
Figure 3 - Multimode Graded-index Fiber
Figure 3 shows the principle of multimode graded-index fiber. The core's central
refractive index, nA, is greater than that of the outer core's refractive index, nB. As
discussed earlier, the core's refractive index is parabolic, being higher at the center. As
Figure 3 shows, the light rays no longer follow straight lines; they follow a serpentine
path being gradually bent back toward the center by the continuously declining refractive
index. This reduces the arrival time disparity because all modes arrive at about the same
time. The modes traveling in a straight line are in a higher refractive index, so they travel
slower than the serpentine modes. These travel farther but move faster in the lower
refractive index of the outer core region.
Single-mode Fiber
Single-mode fiber allows for a higher capacity to transmit information because it can
retain the fidelity of each light pulse over longer distances, and it exhibits no dispersion
caused by multiple modes. Single-mode fiber also enjoys lower fiber attenuation than
multimode fiber. Thus, more information can be transmitted per unit of time. Like
multimode fiber, early single-mode fiber was generally characterized as step-index fiber
meaning the refractive index of the fiber core is a step above that of the cladding rather
than graduated as it is in graded-index fiber. Modern single-mode fibers have evolved
into more complex designs such as matched clad, depressed clad and other exotic
structures.
Figure 4 -
Single-mode fiber has disadvantages. The smaller core diameter makes coupling light
into the core more difficult. The tolerances for single-mode connectors and splices are
also much more demanding. Single-mode fiber has gone through a continuing evolution
for several decades now. As a result, there are three basic classes of single-mode fiber
used in modern telecommunications systems. The oldest and most widely deployed type
is non dispersion-shifted fiber(NDSF). These fibers were initially intended for use near
1310 nm. Later, 1550 nm systems made NDSF fiber undesirable due to its very high
dispersion at the 1550 nm wavelength. To address this shortcoming, fiber manufacturers
developed, dispersion-shifted fiber(DSF), that moved the zero-dispersion point to the
1550 nm region. Years later, scientists would discover that while DSF worked extremely
well with a single 1550 nm wavelength, it exhibits serious nonlinearities when multiple,
closely-spaced wavelengths in the 1550 nm were transmitted in DWDM systems.
Recently, to address the problem of nonlinearities, a new class of fibers were introduced.
These are classified as non zero-dispersion-shifted fibers (NZ-DSF). The fiber is
available in both positive and negative dispersion varieties and is rapidly becoming the
fiber of choice in new fiber deployment. For more information on this loss mechanism,
see the article "Fiber Dispersion."
Figure 6 - Dispersion for Alternating 20 km Lengths of (+D) NZ-DSF and (-D) NZ-DSF
Fiber
Figure 7 -
One additional important variety of single-mode fiber is polarization-maintaining (PM)
fiber. All other single-mode fibers discussed so far have been capable of carrying
randomly polarized light. PM fiber is designed to propagate only one polarization of the
input light. This is important for components such as external modulators that require a
polarized light input. Figure 7 shows the cross-section of a type of PM fiber. This fiber
contains a feature not seen in other fiber types. Besides the core, there are two additional
circles called stress rods. As their name implies, these stress rods create stress in the core
of the fiber such that the transmission of only one polarization plane of light is favored.
Single-mode fibers experience nonlinearities that can greatly affect system performance.