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Fibre Optics
Introduction
1870
Tyndall demonstrated that light can be
guided along a curved stream of water
Why?
Total internal reflection
Electronic communication use radio
and microwaves to carry information
copper wires and co-axial cables
(Limited band width, information
carrying capacity is less)
Use of optical fibre in place of wires enhances the
number of signals that can be transmitted simultaneously
1960:
Light could be guided by a glass fibre
High Attenuation
1970:
Invention of solid state laser, made optical
communication practicable
Commercial communication systems based
on optical fibres made their appearance
1977:
Optical fibres also used in Fibroscopes,
useful in medical diagnostics
An optical fibre is a transparent conduit as thin as human
hair made of glass or clear plastic, made to guide light
waves along its length
Optical Fiber
Practical optical fibre has three co-axial regions
1.
2.
3.
Core- Light guiding region
Cladding- Co-axial middle region
Sheath- Increases mechanical strength of fibre
The refractive index of cladding is always lower than that
of the core
Why?
Purpose of cladding to confine the light to core, How?
Critical angle of propagation
All the rays having ray directions less than the critical
angle will be trapped in the fibre due to total internal
reflection
Only certain ray directions are allowed to propagate
Modes of the fibre: possible number of paths of light in
the fibre
Acceptance Angle
but
n2
sin c 
n1
n n
cos c 
n1
2
1
n12  n22
sin [ i (max)] 
n0
for air n0  1, if  i (max)   0
 0  sin 1[ n12  n22 ]
angle  0 is called the accep tan ce angle
2
2
Acceptance angle is the maximum angle that a light ray can have
relative to the axis of the fibre and can propagate down the fibre
Fractional Refractive Index Change
n1  n2

n1
Fractional refractive index change should be very less than 1
for effective guide of light; of the order of 0.01
Numerical Aperture
The light gathering ability of a fibre depends on two factors, Core Size
and the Numerical Aperture
The acceptance angle and fractional refractive index change
determine the NA of the fibre, NA does not depend on the physical
dimensions of the fibre
NA = sin 0 , i.e. The numerical aperture is defined as the sine of the
acceptance angle
sin  0  n12  n22
 NA  n  n
2
1
or NA  2n12 
2
2
Types of Optical Fibres
Single Mode Fibre:
Single mode step index fibre
Multimode Fibre:
Multimode step index fibre
Multimode Graded index fibre
SMF
Single Mode Step Index Fibre: Refractive index changes abruptly at the
core-cladding boundary. Light travels along a single path i.e. along the axis.
This fibre has low value of NA and 
Intermodal dispersion does not exist (only one mode exist). With careful
choice of material, dimension and wavelength dispersion can be made
extremely small. Low dispersion makes it suitable for use with high data
rates. Fibre is costly
Multimode Step Index Fibre: Its core has larger diameter than SMF. It
has higher dispersion, i.e. less efficient transmission. Easy to manufacture
and less costly.
Graded Index (GRIN) Fibre: Multimode fibre with a core consisting of
concentric layers of different refractive indices. It has higher value at the
centre and falls of with increasing radial distance from the axis. Numerical
aperture and acceptance angle decreases with radial distance. Number of
modes is half than the similar MMF. Less dispersion, manufacturing is
more complex
GRIN
Normalized Frequency: V-number
Pulse Dispersion: Pulse-broadening effect by fibres
The pulse that appears at the output of the fibre is wider than the
input pulse. Dispersion is measured in units of time, typically
nanoseconds and picoseconds
Intermodal Dispersion
It is dispersion between the modes, caused by the
difference in propagation time for the different modes.
Numerous modes traveling in a fibre travel with different
velocities with respect to the fibre axis, leading to a
spread of the input pulse.
Intramodal Dispersion: Light in a fibre consists of a
group of wavelength. Light of different wavelength
travels at different speeds in a medium. A narrow pulse
tend to broaden as they travel down the fiber
Waveguide Dispersion: It arises due to guiding
property of fibre. The refractive index of any mode
changes with wavelength, causes pulse spreading
Large NA- More modes, more dispersion
Attenuation
Signal attenuation is defined as
ratio of the optical output power from a fibre of length L
to the input optical power, in case of an ideal fibre the
attenuation would be zero
1. Absorption by material
2. Scattering
3. Waveguide and microbend loss
Applications
1. Illumination and Image Transmission: Endoscopes
2. Optical Communications: light signals replace the traditional
electric signals. Increased bandwidth is achieved
3. Optical Fibre Sensors: The variation of refractive index of the
optical fibre under the influence of external forses is utilized in
fabrication of optical fibre sensors
Thermometer: LED, Coil of fibre optic and photo-detector
Smoke and Pollution Detector
Liquid Level Sensor: Useful in filing of petrol tanks
4. Medical Applications: Endoscopes, in Ophthalmology, in
Cardiology, treatment of cancer
5. Military Applications: An aircraft, a ship or a tank requires
tons of copper wire for communication, that can be reduced by
optical fibre. Fibre guided missiles are used in recent wars
Fibre Optics Communication System
Very much similar to a traditional communications system
Transmitter: Converts electrical signal to light signals
Optical fibre: Transmits the signals
Receiver: Captures the signals at the other end of the fibre and
converts them to electrical signals
Advantages
1.
Cheaper
2.
Smaller in size, lighter in weight, flexible yet strong
3.
Not hazardous
4.
Immune to EMI and RFI
5.
No cross talk
6.
Wider bandwidth
7.
Low loss per unit length