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Optical sensors FP7 Project SENS-ERA “Strengthening sensor research links between the Georgian Technical University and the European Research Area” Dr Hercules Simos, Lecturer Dept of Electronics Engineering, TEI of Piraeus Overview Introduction Principles of optical sensing Classification of optical sensors Fiber optic sensors Integrated optic sensors Introduction Electrical sensors have fundamental limitations - transmission loss - electromagnetic interference noise Optical sensors use light instead of electricity and an optical fiber instead of a copper wire Optical sensors exhibit many advantages over electrical sensors: - electrical isolation - electromagnetic immunity - possibility of single-point or distributed sensing - use of multiplexing - wide dynamic range - high sensitivity, large bandwidth - low power consumption - compact, small size The development of optical sensors followed the revolution of several optical-related technologies: - optical fibers - semiconductor lasers - integrated optical devices Classification of optical sensors Optical sensors can be used to detect a variety of things Mechanical - pressure, strain, vibration, impact - velocity, rotation, acceleration, displacement - flow Environmental - temperature - humidity, ice Chemical - chemical species - pH - gas, liquid Health - blood oxygen And - radiation, electric/magnetic fields - acoustic fields Introduction Optical fiber is the basic element of the optical sensing technology Optical fibers consist of: - the core - the cladding - the coating Light transmission - Light is totally reflected from the cladding back into the core - This is achieved with a higher refractive index in the core - Transmission with minimal loss. - The outer buffer coating protects the fiber from external conditions and physical damage. Principles of optical sensors General structure of an optical sensing system Basic elements - sensing element - interrogator - optical source - optical detector - electronic signal processing tools Light characteristics change by the measured phenomena - intensity, phase, polarization, wavelength, spectral profile Fiber optic sensors Fiber optic sensors make use of an optical fiber as sensing element Main categories of optical fiber sensors - intrinsic or hybrid (the sensing region lies within the fiber) - extrinsic (sensing takes place outside the fiber) Categorized by the principle of operation (modulated property) - Intensity modulated - Phase modulated - Wavelength/spectrum modulated Categorized by configuration - Distributed optical sensors - Interferometric sensors - Fiber Bragg-grating sensors Fiber optic sensors: intensity modulated In Intensity modulated fiber sensors light intensity changes due to various mechanisms/environmental effects - micro-bending loss - breakage - fiber-to-fiber coupling - modified cladding - Reflectance - Absorption - Attenuation - Molecular scattering - Molecular effects - Evanescent fields Properties of intensity modulated sensors - Versatile, compact - Simple design and easy signal interpretation - Usually suffer from intensity fluctuations and low sensitivity Fiber optic sensors: intensity modulated Types of intensity modulated fiber sensors - Reflection type: broadband source, Pout ~ L, detects distance or pressure - Transmission type: similar to a movable reflector, detects strain or distance - Micro-bending sensor: Pout ~ bending, detects pressure - Polarization based: Pout ~ polarization, detects force Fiber optic sensors: phase modulated In phase modulated fiber sensors the optical phase of the light transmitted through the fiber is modulated by an external phenomena The phase change due to change in - optical length - refractive index - wavelength - etc.. is transformed to intensity modulation through interferometric configurations L, n, I Phase-based optical sensors exhibit higher sensitivity than intensitybased sensors Fiber optic sensors: phase modulated Phase modulated fiber sensors in interferometric configurations - Mach–Zehnder and Michelson interferometers - Fabry-Perot interferometers - Ring resonators ™ Low coherence interferometers - use of low coherence light source - high sensitivity - large dynamic range - noise resistance Fiber optic sensors: wavelength modulated Wavelength modulated fiber sensors - Based on Bragg gratings in optical fibers (FBGs) - FBG: a periodic change of the refractive index in the core of the optical fiber An FBG–based sensor is based on the changes in the transmission and reflection spectrum caused by change in the length or the index of the grating due to: - temperature, - tension, - bending, - compression - impact FBG-based sensor advantages - Versatile: many interrogation techniques - Possibility of multi-sensor access with a single system employing multiplexing - Long distance sensing with low loss Fiber optic sensors: distributed fiber sensors In distributed optical sensors an external physical parameter is measured as a function of position along the fiber - Simultaneous monitoring of parameters at different points along the fiber which behaves as the sensor itself - measurements at long distances (tens of km) - temperature or strain sensing Principle phenomena - Rayleigh scattering - Raman and Brillouin scattering Fiber optic sensors: distributed fiber sensors Rayleigh scatter distributed optical sensors - Use of changes in Rayleigh scatter along the length of a fiber - Such changes can be caused either externally through induced microbend loss or through measurand induced changes in cladding loss - Mechanical changes can be induced through modifications to the local chemical environment Optical time-domain reflectometry sensor based on Rayleigh scattering Example 1: Microbend loss Micro-bend loss distributed sensor using chemically sensitive polymers responding to selected liquids Example 2: Cladding loss Chemically sensitive cladding system responding selectively to gases (wavelength dependent loss) Fiber optic sensors: distributed fiber sensors Raman and stimulated Brillouin scatter distributed optical sensors - Modification of the spectral content of the light propagating through the fiber in response to an external measurand - The measurand is determined by evaluating the spectral content by nonlinear interactions - Raman and Brillouin scatter are deployed to evaluate the changes in the spectral content Raman scatter - Light absorbed by the fiber is reemitted as photons with a different energy distribution determined by the Raman spectrum of the material - Measuring the intensities of the Raman signal at equal energy differences in the upshifted and down-shifted directions produces a ratio which is uniquely related to temperature - This relationship has been used extensively in distributed temperature probes Fiber optic sensors: distributed fiber sensors Raman and stimulated Brillouin scatter distributed optical sensors - Modification of the spectral content of the light propagating through the fiber in response to an external measurand - The measurand is determined by evaluating the spectral content by nonlinear interactions - Raman and Brillouin scatter are deployed to evaluate the changes in the spectral content Brillouin scatter - The energy differentials concerned reflect the acoustic phonon spectrum rather than the optical phonon spectrum. - In stimulated Brillouin, backscattered radiation couples exactly to an acoustic wave with frequency half that of the incoming light. - Acoustic velocity is induced along the core of the fiber. - Stimulated Brillouin scatter can be used to detect varying strain fields given sufficient background knowledge of any temperature variations