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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

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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

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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.

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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.

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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.

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Types of Sensors- POINT SENSORS• FABRY-PEROT CAVITY SENSOR- Intruder

Pressure sensor

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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.

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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 sameoriginal 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.

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POINT SENSORS• FABRY-PEROT CAVITY SENSOR- Strain/Force

sensor

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POINT SENSORS• FABRY-PEROT CAVITY SENSOR- Position sensor• This explanation needs a few basics about reflection

polarization and Birefringence…

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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.

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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.

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

polarization extent.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Types of Sensors- INTEGRATED SENSOR• DEFORMATION SENSOR-

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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DISTRIBUTED SENSOR

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Reference• http://fibersensys.com/security-solutions• http://discountlowvoltage.blogspot.in/2012/09/

perimeter-fence-security-system-using.html• http://www.cablinginstall.com/articles/print/vol

ume-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