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Fiber optic sensors


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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.

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Fiber optic sensors

  1. 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. 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. 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. 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. 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. 6. Types of Sensors- POINT SENSORS • FABRY-PEROT CAVITY SENSOR- Intruder Pressure sensor
  7. 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. 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.
  10. 10. POINT SENSORS • FABRY-PEROT CAVITY SENSOR- Position sensor • This explanation needs a few basics about reflection polarization and Birefringence…
  11. 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. 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. 13. POINT SENSORS • FABRY-PEROT CAVITY SENSOR- Position sensor • Pressure on shaft will move wedge and hence the polarization extent.
  14. 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. 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. 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. 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. 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. 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. 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. 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.
  23. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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.
  35. 35. Reference • • imeter-fence-security-system-using.html • me-19/issue-3/features/the-use-of-fiber-optics-in- security-and-surveillance-systems.html • https://www.rp- •