Technological Education Institute (TEI) of Piraeus
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Transcript Technological Education Institute (TEI) of Piraeus
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