INTRODUCTION: Fibre optical sensors offer number of distinct advantages which makes them unique for many applications where conventional sensors are difficult or impossible to deploy or can not provide the same wealth of information. They are completely passive, hence can be used in explosive environment. Immunity to electromagnetic interference makes it ideal for microwave environment. They are resistant to high temperatures and chemically reactive environment, ideal for harsh and hostile environment. Small size makes it ideal for embedding and surface mounting. Has high degree of biocompatibility, non-intrusive nature and electromagnetic immunity, ideal for medical applications like intra-aortic balloon pumping. They can monitor a wide range of physical and chemical parameters. It has potential for very high sensitivity, range and resolution. Complete electrical insulation from high electrostatic potential and Remote operation over several km lengths without any lead sensitivity makes it ideal for deployment in boreholes or measurements in hazardous environment. Unique multiplexed and distributed sensors provide measurements at large number of points along single optical cable, ideal for minimising cable deployment and cable weight, monitoring extended structures like pipelines, dams. Various types of sensors are Point sensors, Integrated Sensors, Quasidistributed multiplexed sensors, Distributed sensors. Examples of such sensors are Fabry-Perot sensors, Single Fibre Bragg Grating sensors, Integrated strain sensor, Intruder Pressure sensor, Strain/Force sensor, Position sensor, Temperature sensor, Deformation sensor etc.

1. Fibre Optic Sensors
In Fibre Security System
TOPICS COVERED
• Advantages of Optical Fibre
• Advantages of Optical Fibre sensors
• Types of sensors
• Point Sensors(Fabry-Perot sensor)
• Intruder Pressure sensor
• Strain/Force sensor
• Position sensor
• Temperature sensor
• Point Sensors(Single Fiber Braggs Grating sensor)
• Integrated Sensors
• Deformation sensor
• Quasi-distributed multiplexed sensors
• Distributed sensors

2. Advantages of Optical Fibre
• Low loss
• No Electromagnetic or RF interference
• No interference from high voltages
• Light weight
• Stable within a wide temperature range
• Long service life
• Secure
• Extremely high bandwidth

3. Advantages of Optical Fibre Sensors
• Completely passive:
▫ can be used in explosive environment.
• Immune to electromagnetic and electrostatic
interference:
▫ ideal for microwave environment.
• Resistant to high temperatures and chemically reactive
environment:
▫ ideal for harsh and hostile environment.
• Small size:
▫ ideal for embedding and surface mounting.
• High degree of biocompatibility and non-intrusive nature:
▫ ideal for medical applications like intra-aortic balloon
pumping.
• Can monitor a wide range of physical and chemical
parameters.

4. Advantages of Optical Fibre Sensors
• High sensitivity, range and resolution.
• Single ended remote operation over several km:
▫ ideal for deployment in boreholes or hazardous environment.
• Multiplexing and distributed sensing at multi-points along
single optical cable:
▫ minimises cable deployment and cable weight
▫ monitors extended structures like pipelines, dams.

5. Types of Sensors-
• Point Sensors
• Measurement carried out at a single point in space.
• Multiple channels for addressing multiple points.
• Ex-Fabry-Perot sensors, Single Fibre Bragg Grating sensors.
• Integrated sensors:
• Measurement averages physical parameter over a spatial section.
• Provides a single value.
• Ex -Deformation sensor measuring strain over along base length.
• Quasi-distributed or multiplexed sensors:
• Measurand is determined at number of fixed, discrete points
along a single fibre optical cable.
• Ex -Multiplexed FBG's.
• Distributed sensor:
• Parameter measured at any point along a single optical cable.
• Ex -Systems based on Rayleigh, Raman and Brillouin scattering.

6. Types of Sensors- POINT SENSORS
• FABRY-PEROT CAVITY SENSOR- Intruder Pressure sensor

7. FABRY-PEROT CAVITY SENSOR- Intruder Pressure sensor
• Pair of parallel mirrors separated by air gap Ls.
• Called Fabry-Perot(FP) cavity or sensing interferometer.
• Semi-reflective Mirror1 -dielectric layer deposited at end
of optical fibre.
• Mirror2 - diaphragm mounted in front of optical fibre.
• Pressure p to be measured changes gap Ls.
• Incident light in optical fibre towards FP cavity is partially
reflected at first mirror.
• Remaining light is transmitted further and reflected by
second mirror.
• Second pulse delayed with respect to first by t = 2Ls / c.
• Any pressure due to intruder will reduce Ls and hence gap
between two pulses t.

8. FABRY-PEROT CAVITY SENSOR- Intruder Pressure sensor
• Pulses are fed to interferometer to see modulation and
interference.
• Interference and signal containing information about
Ls only occurs if two pulses generated from same
original pulse overlap again.
• Modulation and interference between two pulses is
maximum at maximum pressure p.
• Based on same basic principle, transducers for measuring
temperature, displacement, strain, force etc. can be
constructed.

9. POINT SENSORS
• FABRY-PEROT CAVITY SENSOR- Strain/Force sensor

10. POINT SENSORS
• FABRY-PEROT CAVITY SENSOR- Position sensor
• This explanation needs a few basics about reflection
polarization and Birefringence…

11. FABRY-PEROT CAVITY SENSOR- Position sensor
• Reflection Polarization- Reflectivity is different for light
▫ polarized in plane of incidence (p-polarized)
▫ polarized perpendicular to plane of incidence (s-polarized).
• Brewster's angle is incidence angle when a particular
polarization is perfectly transmitted through a
transparent dielectric surface, without reflection.
• EX-At Brewster’s angle, only s-polarized is reflected from
the surface.

12. FABRY-PEROT CAVITY SENSOR- Position sensor
• Birefringence- property of a material having refractive
index depending on polarization.
• Incident light splits as per polarization into two paths.
• Separation depends on how long light stays in material.
• Here birefringence wedge is used only to refract and total
reflect a single polarized light beam.

13. POINT SENSORS
• FABRY-PEROT CAVITY SENSOR- Position sensor
• Pressure on shaft will move wedge and hence the
polarization extent.

14. FABRY-PEROT CAVITY SENSOR- Position sensor
• Reflection polarizer reflects s-polarized portion of incident
light towards wedge.
• Wedge deflects the light through refraction, reflects and
again refracts it upwards away from slide surface.
• Spatial gap between downward and upward pulses depends
on wedge height, decided by transducer shaft.
• Shaft completely pressed will have no wedge and no gap.
• It will result in maximum modulation and interference.

15. POINT SENSORS
• FABRY-PEROT CAVITY SENSOR- Temperature sensor
• Temperature changes refractive index and hence
birefringence of crystal.
• Hence path length difference and gap between pulses
depends on temperature.

16. POINT SENSORS-
• SINGLE FIBRE BRAGG GRATING SENSORS
• One of the most commonly used and broadly deployed
optical sensor.
• FBG sensor reflects a wavelength of light that shifts in
response to variations in temperature and/or strain.
• Constructed by using holographic interference or phase
mask to expose a short length of photosensitive fiber to a
periodic distribution of light intensity.
• Refractive index of fiber is permanently altered according
to intensity of light it is exposed to.
• The resulting periodic variation in the refractive index is
called a fiber Bragg grating.
• Broad-spectrum light beam sent to FBG, reflects from each
segment of alternating refractive index,
▫ interferes constructively only for a specific wavelength of
light, called the Bragg wavelength.

17. SINGLE FIBRE BRAGG GRATING SENSORS
• FBG reflects a specific frequency of light
• Transmitting all others.
• λb is Bragg wavelength,
• n is the effective refractive index of fiber core,
• Λ is the spacing between gratings, Grating period.

18. SINGLE FIBRE BRAGG GRATING SENSORS
• Bragg wavelength is a function of spacing between
gratings.
• Changes in strain and temperature affect both effective
refractive index n and grating period Λ of FBG.
• This results in shift in reflected wavelength.

19. SINGLE FIBRE BRAGG GRATING SENSORS
• Change in wavelength with temperature ΔT and strain can
be approximately described by above -- where
▫ Δλ is wavelength shift,
▫ λo is initial wavelength,
▫ pe is strain-optic coefficient,
▫ ε is strain experienced by the grating,
▫ αΛ is thermal expansion coefficient
▫ αn is thermo-optic coefficient.
• αn describes change in refractive index , αΛ describes
expansion of grating, both due to temperature.

20. SINGLE FIBRE BRAGG GRATING SENSORS
• FBG’s response to both strain and temperature needs to be
distinguished.
• Temperature- FBG must remain unstrained.
• FBG inside the package should not be coupled to any
bending, tension, compression, or torsion forces.
• Expansion coefficient αΛ of glass is practically negligible.
• Changes in reflected wavelength due to temperature
primarily described by changes in refractive index αn of
fiber.

21. SINGLE FIBRE BRAGG GRATING SENSORS
• Strain-
• FBG strain sensors are more complex
▫ as both temperature and strain influence sensor’s
reflected wavelength.
• Must compensate for temperature effects on FBG.
• By installing FBG temperature sensor in close thermal
contact with FBG strain sensor.
• Subtraction of FBG temperature sensor wavelength shift
from FBG strain sensor wavelength shift yields
temperature compensated strain value.

22. Types of Sensors- INTEGRATED SENSOR
• DEFORMATION SENSOR-

23. DEFORMATION SENSOR
• Sensor consists of a pair of single-mode fibers installed in
the structure to be monitored.
• Measurement fiber is in mechanical contact with the host
structure.
• Reference fiber is placed loose near the measurement
fiber.
• Deformations of structure will result in change of length
difference between two fibers.
• Mach-Zehnder interferometer is used in tandem in control
room to replicate the test site.
• Useful if test site is unapproachable from measurement
room.
• Used in bridges, dams etc.

24. DEFORMATION SENSOR
• Miniature mirrors attached at end of each fiber in test
structure and reference interferometer.
• Fibre optical coupler feeds a light pulse into two fibres of
different length in test structure.
• A deformation of the structure leads to a change in path
difference 2nLs.
• Two return pulses are separated in time by t = 2nLs / c,
• n -refractive index of the glass fibre,
• Ls - length difference of fibres
• c - speed of light.
• The test situation is replicated at control room with
scanning mobile mirror (adjustable).

25. DEFORMATION SENSOR
• Maximum interference signal only occurs if path difference
of sensing interferometer 2nLs exactly matches that of
receiving interferometer 2nLr (adjustable).
• The sensor is temperature independent –
▫ change in temperature has same effect on both fibres
▫ leaves path difference effectively unchanged.
• Distance between anchoring points at which the fibre is
attached to the structure is called base-length.
• Michelson interferometer can also be used to make
measurement unbalance.

26. Types of Sensors- QUASI-DISTRIBUTED or
MULTIPLEXED SENSOR-
• BRAGG GRATING SENSORS-
• Benefit - number of FBGs each with different Bragg
wavelength l1, l2, …lN can be deployed along the fibre.
• This provides N measurement points within a single cable.

27. BRAGG GRATING MULTIPLEXED SENSORS
• The FBGs are able to write unique Bragg wavelengths.
• Well suited for wavelength division multiplexing.
• WDM provides each FBG sensor its unique wavelength
range within the light spectrum through a single fiber.
• Sensor measurements accurate even with losses due to
bending or transmission.
• The number of sensors depends on wavelength range of
each sensor and total available wavelength range.
• Wavelength shifts due to strain are typically more
pronounced than temperature.
• FBG strain sensors are ~5 nm range, while FBG
temperature sensors require ~1 nm.

28. BRAGG GRATING MULTIPLEXED SENSORS
• Typical interrogators provide measurement range of 60 to
80 nm.
• Each fiber array of sensors usually incorporate one to 80
sensors
• – as long as reflected wavelengths do not overlap in
the optical spectrum.
• Ensure that each sensor operates within a unique spectral
range.

29. BRAGG GRATING MULTIPLEXED SENSORS
• Multiplexing using CCD and wavelength-position
conversion.
• A broadband source illuminates FBGs.
• Reflected light wavelength as per different parameters,
from different FBGs coupled.
• Dispersive element disperses various wavelengths to
different locations on linear CCD sensor.

30. Types of Sensors- DISTRIBUTED SENSOR
• Parameter of interest is measured with certain spatial
resolution at any point along a single optical cable.
• Basic physical processes are provided by various scattering
processes.
• Laser light propagating along optical fibre, continuously
scatters back in small amounts at each location along the
fibre.
• Rayleigh scattering due to reflections at random
inhomogeneities of refractive index frozen in during
manufacture of the fibre.
• Raman scattering due to interaction with molecular
vibrations and rotations in the glass.
• Brillouin scattering due to interaction with
inhomogeneities created by sound waves in the fibre
(acoustic phonons).

31. DISTRIBUTED SENSOR
• Analysing backscattered light in wavelength domain:-
• Rayleigh scattering component is of same wavelength λ0,
as the incident light.
• Two Raman components, shifted by same amount above λ0
(Stokes component) and below λ0 (Anti-Stokes
component).
• Brillouin backscatter has two components shifted below
and above λ0.

32. DISTRIBUTED SENSOR
• Property of backscattered light depends on strain and
temperature in the fibre.
• Raman Scattering:
▫ Intensity of Raman Anti-Stokes component increases
with increasing temperature T
▫ Stokes component can be regarded as temperature
independent.
▫ By taking ratio between them, possible causes of
intensity variations, common to both- like fibre bending
losses, can be excluded.
▫ Temperature can be determined unambiguously.

33. DISTRIBUTED SENSOR
• Brillouin scattering:-
▫ Wavelength shift of scattered components, with respect
to Rayleigh wavelength, changes with both temperature
T and strain e .
▫ By extracting this wavelength shift from backscattered
light, a sensor for strain and temperature can be
realised.
• Additional measures taken to separate strain and
temperature dependence.
• Installation of a reference cable not rigidly bound to
the structure measures temperature only.
• OTDR helps to extract temperature and/or strain
profile in space.

34. DISTRIBUTED SENSOR

35. Reference
• http://fibersensys.com/security-solutions
• http://discountlowvoltage.blogspot.in/2012/09/per
imeter-fence-security-system-using.html
• http://www.cablinginstall.com/articles/print/volu
me-19/issue-3/features/the-use-of-fiber-optics-in-
security-and-surveillance-systems.html
• https://www.rp-
photonics.com/fiber_optic_sensors.html
• http://www.sensorland.com/HowPage072.html