Download 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