TOPIC:OPTICAL FIBER SUBJECT:#SOLID_STATE_PHYSICS #typesofopticalfiber

1. Optical Fiber
An optical fibre is a flexible, transparentfibre made by drawing glass(silica) or
plastic to a diameter slightly thicker than that of a human hair. Optical fibres
are used most often as a means to transmitlight between the two ends of the
fibre and find wide usage in fibre-optic communication, where they permit
transmission over longer distances and at higher bandwidths (data rates) than
electrical cables. Fibres are used instead of metal wires becausesignals travel
along them with less loss; in addition, fibres are immune to electromagnetic
interference, a problem fromwhich metal wires suffer. Fibres arealso used for
illumination and imaging and are often wrapped in bundles so they may be
used to carry light into, or images out of confined spaces, as in the caseof a
fiberscope. Specially designed fibres arealso used for a variety of other
applications, some of them being fibre optic sensors and fibrelasers.
Construction
Optical fibres typically include a coresurrounded by a transparentcladding
material with a lower index of refraction. Light is kept in the coreby the
phenomenon of total internal reflection which causes the fibreto act as a
waveguide. Fibres that supportmany propagation paths or transversemodes
are called multi-mode fibres, while those that supporta single mode are called
single-mode fibres (SMF). Multi-mode fibres generally havea wider core
diameter and are used for short-distancecommunication links and for
applications wherehigh power must be transmitted. Single-modefibres are
used for mostcommunication links longer than 1,000 meters (3,300 ft).

2. Types
Multi-mode Fibre
Fibre with large corediameter (greater than 10 micrometres) may be analysed
by geometrical optics. Such fibre is called multi-mode fibre, from the
electromagnetic analysis. In a step-index multi-mode fibre, rays of light are
guided along the fibrecore by total internal reflection.

3. The propagationof lightthrougha multi-mode optical fibre.
Single-mode Fibre
Fibre with a core diameter less than aboutten times the wavelength of the
propagating light cannotbe modelled using geometric optics. Instead, it must
be analysed as an electromagnetic waveguidestructure, by solution of
Maxwell’s equations as reduced to the electromagnetic waveequation. The
electromagnetic analysis may also be required to understand behaviours such
as speckle that occur when coherent light propagates in multi-mode fibre.
As an optical waveguide, the fibre supports oneor more confined transverse
mode by which light can propagate along the fibre. Fibre supporting only one
mode is called single-modeor mono-modefibre. The behaviour of larger-core
multi-mode fibre can also be modelled using the waveequation, which shows
that such fibresupports morethan one mode of propagation.

4. The structure of a typical single-mode fibre.
1. Core: 8 µm diameter
2. Cladding: 125 µm dia.
3. Buffer: 250 µm dia.
4. Jacket: 400 µm dia.
Attenuation
Attenuation in fibre optics, also known as transmission loss, is thereduction in
intensity of the light beam as it travels through the transmission medium.
Attenuation coefficients in fibre optics usually use units of dB/km through the
medium due to the relatively high quality of transparency of modern optical
transmission 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 digital signal across largedistances. Thus, much
research has gone into both limiting the attenuation and maximizing the
amplification of the optical signal. Empirical research has shown that
attenuation in optical fibreis caused primarily by both scattering and
absorption. Single-modeoptical fibres can be made with extremely low loss.
Applications
Communication
Optical fibre is used as a medium for telecommunication and computer
networking becauseit is flexible and can be bundled as cables. Itis especially
advantageous for long-distancecommunications, becauseinfrared propagates
through the fibrewith much lower attenuation compared to electrical cables.
This allows long distances to be spanned with few repeaters.
Sensors
Fibres have many uses in remote sensing. In someapplications, the sensor is
itself an optical fibre. In other cases, fibre is used to connect a non-fibreoptic
sensor to a measurementsystem. Depending on the application, fibre may be

5. used because of its small size, or the fact that no electrical power is needed at
the remote location.
Optical fibres can be used as sensors to measure strain, temperature, pressure,
and other quantities by modifying a fibre so that the property to measure
modulates the intensity, phase, polarization, wavelength, or transittime of
light in the fibre.
Common uses for fibre optic sensors includeadvanced intrusion detection
security systems. Thelight is transmitted along a fibreoptic sensor cable
placed on a fence, pipeline, or communication cabling, and the returned signal
is monitored and analysed for disturbances.
Power Transmission
Optical fibre can be used to transmitpower using a photovoltaic cell to convert
the light into electricity. While this method of power transmission is notas
efficient as conventional ones, it is especially usefulin situations whereit is
desirable.