Fiber Optics

Fiber Optics


A technology that uses glass (or plastic) threads (fibers) to transmit data. A
fiber optic cable consists of a bundle of glass threads, each of which is
capable of transmitting messages modulated onto light waves.

Fiber optics has several advantages over traditional metal communications
lines:

· Fiber optic cables have a much greater bandwidth than metal cables.
This means that they can carry more data.

· Fiber optic cables are less susceptible than metal cables to
interference.

· Fiber optic cables are much thinner and lighter than metal wires.

· Data can be transmitted digitally (the natural form for computer data)
rather than analogically.

The main disadvantage of fiber optics is that the cables are expensive to
install. In addition, they are more fragile than wire and are difficult to splice.
Fiber optics is a particularly popular technology for local-area networks. In
addition, telephone companies are steadily replacing traditional telephone
lines with fiber optic cables. In the future, almost all communications will
employ fiber optics.

You hear about fiber-optic cables whenever people talk about the telephone
system, the cable TV system or the Internet. Fiber-optic lines are strands of
optically pure glass as thin as a human hair that carry digital information
over long distances. Optical fibers are designed to guide light waves along
their length. An optical fiber works on the principle of total internal
reflection.

Total Internal Reflection:


Total Internal Reflection is an optical phenomenon that occurs when a ray of
light strikes a medium boundary at an angle larger than the critical angle
with respect to the normal to the surface. If the refractive index is lower on
the other side of the boundary no light can pass through, so effectively all of
the light is reflected. The critical angle is the angle of incidence above which
the total internal reflection occurs.

When light crosses a boundary between materials with different refractive
indices, the light beam will be partially refracted at the boundary surface,
and partially reflected. However, if the angle of incidence is greater (i.e. the
ray is closer to being parallel to the boundary) than the critical angle — the
angle of incidence at which light is refracted such that it travels along the
boundary — then the light will stop crossing the boundary altogether and
instead be totally reflected back internally.This can only occur where light travels from a medium with a higher
refractive index to one with a lower refractive index. For example, it will
occur when passing from glass to air, but not when passing from air to glass.A practical optical fiber has in general three coaxial regions. The inner most region is the light guiding region known as the core. It is surrounded by a coaxial region known as the cladding. The outer most region is called
buffer coating. The refractive index of the cladding is always lower than that
of the core. The purpose of cladding is to make the light to be confined to
the core. Light is launched into the core and striking the core-to-cladding
interface at greater than critical angle will be reflected back into the core.
Since the angles of incidence and reflection are equal, the light will continue
to rebound and propagate through the fiber. The buffer coating protects the
cladding and the core from the harmful influence of moistureNumerical Aperture:
The optical fiber will only propagate light that enters the fiber within a
certain cone, known as the acceptance cone of the fiber. The half-angle of
this cone is called the acceptance angle, max. For step-index multimode
fiber, the acceptance angle is determined only by the indices of refraction:
where n1 is the refractive index of the fiber core, and n2 is the refractive
index of the cladding.
When a light ray is incident from a medium of refractive index n to the core
of index n1, Snell's law at medium-core interface gives

where is the critical angle for total internal reflection, since
Substituting for sin r
in Snell's law we get:
By squaring both sides

This has the same form as the numerical aperture in other optical systems, so
it has become common to define the NA of any type of fiber to be
where n1 is the refractive index along the central axis of the fiber. Note that
when this definition is used, the connection between the NA and the
acceptance angle of the fiber becomes only an approximation. In particular,
manufacturers often quote "NA" for single-mode fiber based on this
formula, even though the acceptance angle for single-mode fiber is quite
different and cannot be determined from the indices of refraction alone
Normalised Frequency (V-Number):
In an optical fiber, the normalized frequency, V (also called the V number),
is given by
where a is the core radius, is the wavelength in vacuum, n1 is the
maximum refractive index of the core, n2 is the refractive index of the
homogeneous cladding, and applying the usual definition of the numerical
aperture NA.
In multimode operation of an optical fiber having a power-law refractive
index profile, the approximate number of bound modes (the mode volume),
is given bywhere g is the profile parameter, and V is the normalized frequency, which
must be greater than 5 for the approximation to be valid.
For a step index fiber, the mode volume is given by V
2
/2. For single-mode
operation is required that V < 2.405.

Modes in an optical fiber


The number of modes in an optical fiber distinguishes multi-mode optical
fiber from single-mode optical fiber. To determine the number of modes in a
step-index fiber, the V number needs to be determined:
where k0 is the wavenumber, a is the fiber's core radius,
and n1 and n2 are the refractive indices of the core and cladding,
respectively. Fiber with a V-parameter of less than 2.405 only supports the
fundamental mode (a hybrid mode), and is therefore a single-mode fiber
whereas fiber with a higher V-parameter has multiple modes

Dispersion in waveguides


Optical fibers, which are used in telecommunications, are among the most
abundant types of waveguides. Dispersion in these fibers is one of the
limiting factors that determine how much data can be transported on a single
fiber.
The transverse modes for waves confined laterally within a waveguide
generally have different speeds (and field patterns) depending upon their
frequency (that is, on the relative size of the wave, the wavelength)
compared to the size of the waveguide.
In general, for a waveguide mode with an angular frequency ( ) at a
propagation constant (so that the electromagnetic fields in the propagation
direction z oscillate proportional to e
i( z − t)
), the group-velocity dispersion
parameter D is defined as:where = 2 c / is the vacuum wavelength and vg = d / d is the group
velocity. This formula generalizes the one in the previous section for
homogeneous media, and includes both waveguide dispersion and material
dispersion. The reason for defining the dispersion in this way is that |D| is
the (asymptotic) temporal pulse spreading t per unit bandwidth per unit
distance traveled, commonly reported in ps / nm km for optical fibers.
A similar effect due to a somewhat different phenomenon is modal
dispersion, caused by a waveguide having multiple modes at a given
frequency, each with a different speed. A special case of this is polarization
mode dispersion (PMD), which comes from a superposition of two modes
that travel at different speeds due to random imperfections that break the
symmetry of the waveguide.

Fiber Types


Waveguides are classified, on the one hand by the index of refraction profile
of the core material, and on the other hand by the mode propagating ability.
As was previously suggested, there are therefore single mode and multimode
fibers. In classifying the index of refraction profile, we differentiate between
step index, gradient index and special profile fibers. Step index fibers have a
constant index profile over the whole cross section. Gradient index fibers
have a non-linear, rotationally symmetric index profile, which falls off from
the center of the fiber outwards .
In the case of step index, multimode fibers the index of refraction is
constant, therefore the profile parameter g = . For gradient index fibers, the
index of refraction is reduced from the middle outwards. As opposed to
traveling in a straight line, the rays travel in a spiral form around the optical
axis.Attenuation (Loss) in Fibers:
Attenuation is the reduction in amplitude and intensity of a signal. Signals
may attenuate exponentially by transmission through a medium, or by
increments calculated in electronic circuitry or set by variable controls.
Attenuation is an important property in telecommunications and ultrasound
applications because of its importance in determining signal strength as a
function of distance. Attenuation is usually measured in units of decibels per
unit length of medium (dB/cm, dB/km, etc)Attenuation is caused by several different factors, but primarily scattering and absorption.The scattering of light is caused by molecular level
irregularities in the glass structure. Further attenuation is caused by light
absorbed by residual materials, such as metals or water ions, within the fiber
core and inner cladding. Light leakage due to bending, splices, connectors,
or other outside forces are other factors resulting in attenuation. Attenuation
in fibre optics, also known as transmission loss, is the reduction in intensity
of the light beam with respect to distance travelled through a transparent
medium. Attenuation coefficients in fibre optics usually use units of dB/km
through the medium due to the great transparency of modern optical media.
The medium is usually a fibre of silica glass that confines the incident light
beam to the inside. Attenuation is an important factor limiting the
transmission of a light pulse across far distances, and as a result much
research has gone into both limiting the attenuation and maximizing the
amplification of the fibre optic light beam. Attenuation in fibre optics can be
quantified using the following equation:

Fiber-optic communication


Fiber-optic communication is a method of transmitting information from
one place to another by sending light through an optical fiber. The light
forms an electromagnetic carrier wave that is modulated to carry
information. Fiber-optic communication systems have revolutionized the
telecommunications industry and played a major role in the advent of the
Information Age. Because of its advantages over electrical transmission, the
use of optical fiber has largely replaced copper wire communications in core
networks in the developed world.

The process of communicating using fiber-optics involves the following
basic steps: Creating the optical signal using a transmitter, relaying the
signal along the fiber, ensuring that the signal does not become too distorted
or weak, and receiving the optical signal and converting it into an electrical
signal.