Optic losses and optical sensors Amala Prathiba Janet SXCCE

1. Engineering Physics
Unit – II
Attenuation & Sensors

2. Attenuation (Losses in Optical Fibers)
Types of Attenuation
Dispersion
Types of Dispersion
Fibre Optic Sensors
Displacement Sensor
Pressure or Temperature Sensor
Overview

3. Losses in Optical fiber (or)
Attenuation
When the light propagates through an
optical fiber, the power of the light from the
output end is always less than the power
launched at the input end. This loss depends on
the fiber material, wavelength of light and length
of the fiber. This loss of optical power is known as
attenuation.
The attenuation loss is generally measured
in decibel.

4. Attenuation
Definition:
It is defined as the ratio of optical power output
(Pout) from a fiber to the power input(Pin).
dB = -10 log
𝑃𝑜𝑢𝑡
𝑃𝑖𝑛
Generally,
𝑑𝐵
𝑘𝑚
= -
10
𝐿
log
𝑃𝑜𝑢𝑡
𝑃𝑖𝑛
Where,
Pout →Power coming out of the fiber
Pin → Power launched into the fiber
L → Length of the fiber
The loss per kilometer is standard unit for describing
attenuation loss in all fiber designs.

5. Attenuation Types
The losses occurring in optical fiber may be mainly
attributed to three mechanisms, namely,
• Absorption losses
• Scattering losses
• Radiative (bending) losses

6. Absorption losses
Absorption of light occurs due to imperfections of
the atomic structures, which absorbs light. It mainly
depends on the wavelength of the light used. The
absorption of light is caused by three different
mechanisms as follows:
• Absorption by atomic defects in the glass
composition
• Extrinsic absorption by impurity atoms in the glass
material
• Intrinsic absorption by the basic constituent atoms
of the fiber

7. Scattering losses
Scattering is the loss which occur inside the
fiber and is depend on wavelength. It is because of
disordered structure of glass which makes change in
refractive index. Hence the light passing through the
fiber gets scattered. This type of scattering is also called as
Rayleigh’s scattering.

8. Radiative losses
Whenever the optical fiber has the bend then there
will be radiative losses. There are two types of bending
losses.
• Macroscopic bending losses
• Microscopic bending losses

9. Macroscopic bending losses
These losses occur when the radius of curvature of
bend is greater than the fibers diameter at the corner. At
this corner, the light radiation will not satisfy the
condition for total internal reflection hence the light gets
escaped from the fiber.

10. Microscopic bending losses
It occurs due to non–uniform pressure
created during the cabling of the fiber, even during
manufacturing itself.

11. Dispersion
When an optical signal is sent into the fiber, the pulse
spreads/broaden as it propagates through the fiber. This
phenomenon is called dispersion.

12. Types of dispersion
The most common types of dispersion in
optical fiber are
1. Intra modal dispersion
• Chromatic dispersion
• Waveguide dispersion
2. Inter model dispersion

13. Chromatic dispersion
This dispersion arises due to the variation of refractive
index with the wavelength or frequency of light. The light
waves of different wavelength travels with different velocity.
When these pulse are passed through the fiber they get
broadened and is called material dispersion or chromatic
dispersion.
In material dispersion, the pulses at different wavelengths
travel with different velocities.

14. Waveguide dispersion
It arises due to the guiding property of the fiber and
due to their different angles at which they incident at the
core cladding interface of the fiber.

15. Inter modal dispersion
When an optical or light pulse is launched into a
multimode fiber, the optical power in the pulse is distributed
over all of the modes of the fiber.
Each of the modes that can propagate in a multimode fiber
travels at a slightly different velocity.
This means that the modes in a given optical pulse arrive at
the fiber end at slightly different times, thus causing the
pulse to spread out in time as it travels along the fiber. This
effect is known as intermodal dispersion.

16. Fiber Optic Sensors
One of the most exciting applications of optical fibers
is fiber optic sensors.
A sensor is a transducer which converts one form of
energy (physical variable) into another. The sensors find
wide applications in sensing and measuring acoustic fields,
magnetic fields, current, rotation, acceleration, strain,
pressure, temperature and so on.
Fibre optical sensor is a transducer which converts any form
of signal into optical signal in the measurable form. Here,
optical fibers are used as a guiding media and hence called
wave guides.

17. Types of sensors
There are two types of sensors:
• Intrinsic sensors (or) Active sensors
• Extrinsic sensors (or) Passive sensors

18. Types of sensors
Intrinsic sensors (or) Active sensors:
In intrinsic sensors (or) active sensors, the
physical parameter to be sensed directly acts on the
fibre itself to produce the changes in the
transmission characteristics.
Example: Pressure / Temperature sensor
Extrinsic sensors (or) Passive sensors:
In extrinsic sensors (or) passive sensors,
separate sensing element will be used and the fibre
will act as a guiding media to the sensors.
Example: Displacement sensor

19. Displacement sensor
Definition:
It is a sensor which is used to sense and measure the
displacement of an object.
Construction
Displacement sensor consists of two fibers namely,
transmitting fiber and receiving fiber. One end of
the transmitting fiber is connected to a laser source
and the other end is facing the object. One end of the
receiving fiber is connected to a photo detector and
end is facing the object.

20. Working:
When the laser source is switched on, laser light is launched
into the transmitting fiber. Now, the intensity of light is measured. This
light is transmitted through the transmitting fiber and is made to fall
on the target. Then this light rays are reflected by the target. The
reflected light from the target passed through the receiving fiber and
is detected by the detector. Now, the intensity of light is measured.
Difference in intensity gives the displacement of the target. If the
difference is high, then the object is moving away from the sensor and
the displacement of the object is high. If the difference is small then
the object move towards the sensor and the displacement is small. By
using this technique, displacement of moving objects can be
determined.

21. Pressure Sensor
Definition:
It is a sensor which is used to sense and measure the
pressure of an object based on the interference
pattern.
Construction:
A laser source
A beam splitter
Transparent glass plate is inclined to an angle of 45°
Reference fiber
Test fiber and
Separate lens systems

22. Working:
The light emitted from the laser source is divided into two beams
(i) main beam and (ii) splitted beam by beam splitter kept at 45°. The
main beam passes through the reference fibre which is isolated from the
environment to be sensed. The splitted beam passes through the test
fibre kept in the environment to be sensed. The light from these two
fibres are superimposed and therefore a path difference is produced
between the two beams, causing the interference pattern.

23. Working:
The intensity of the fringe depends on the phase relation
between the two waves. If the waves are in phase, then the intensity is
maximum, this happens when the sensing fibre is not disturbed. The
intensity is minimum if the waves are out of phase due to λ/2 change in
length of sensing fibre. The intensity of interference fringes can be
measured with a photo-detector and pressure changes can be measured.