Foreword Preface Symbols and abbreviations Fiber optic fluorescence thermometry: An introduction Temperature dependences of Cr3+ fluorescence lifetimes Phase-locked detection of fluorescence lifetime Cr3+ fluorescence thermometers and applications of PLD Alexandrite based fluorescence thermometry Cr:LiSAF based thermometer for biomedical applications High temperature rare-earth thermometry and pyrometry Self-referencing intensity-based fluorescence thermometry Rare-earth doped fluorescent and luminescent optical fibers Assessment and cross-comparisons in fiber optic fluorescence thermometry Index

Fiber optic fluorescence thermometry

A sensor embedded in the composite laminate can act as a temperature transducer during the composite cure mechanism. Once the composite is cured, the same sensor can be used to provide the information about the mechanical changes that influence the performance of the material. Fiber Bragg Grating (FBG) sensor is one such sensor which one can use for the composite cure monitoring. We present here the results obtained with an associated FBG sensor system for the cure monitoring of smart composites. The performance of the embedded FBG sensor smart composite specimens under 3- and 4-point bending conditions are also being investigated. Finally, the performance analysis has been extended to cantilever specimens.

Cure monitoring of smart composites using Fiber Bragg Grating based embedded sensors

In this paper we review the current state of development of fibre optic sensors, including those based on both monomode and multimode technology. For monomode techniques, a general formalism describing their optical characteristics is developed; electronic processing techniques and noise performance are also considered. Examples of their application in the measurement of temperature, pressure, strain, flow, rotation and magnetic field are described. Extrinsic devices such as velocimeters and vibrometers, and applications in holography, are also discussed. Multimode techniques based on intensity and wavelength modulation and quantum effects are considered, and their application to a wide range of measurands is reviewed.

Fibre optic sensors

Vibration monitoring of machinery is reducing the overall operating costs of industrial plants. Conventional vibration sensors, based on capacitive or piezoelectric principles, are limited in application due to the problem of electrical isolation. Fibre-optic based instrumentation is thus an attractive alternative method of vibration measurement in the vicinity of electrical substation. This paper discusses several techniques of vibration sensing using optical fibre technology and assesses their potential for use on electromechanical equipment. Firstly an overview of sensor based on In-Fibre Bragg Gratings is presented, and its potential for the measurement of strain and vibration is assessed. Secondly, vibration sensing using fibre-optic intensity-based measurement is presented and then Fabry-Perot Interferometer (FPI) for vibration sensing is critically reviewed. Of these, the FPI is the most attractive since it can easily be configured within a reflective fibre-optic probe. However, reported FPI sensors have been highly sensitive to measurement errors caused by mechanical vibration, temperature, and acoustic waves. This paper reviews technological developments of FPI to vibration sensing in extreme electromechanical environments and also for non-contact measurement. Finally this paper presents an overview of dual-wavelength technique for assessment of vibration signature.

Prospects for Fibre Bragg Gratings and Fabry-Perot Interferometers in fibre-optic vibration sensing

A manufacturing method was developed for integrating a network of distributed piezoceramic actuators/sensors onto laminated carbon/epoxy composite structures. The network of built-in actuators/sensors is used to monitor the health of the host composite structure by acquiring information about the condition of the structure throughout its service life. The manufacturing method applies the printed circuit technique to fabricate a thin flexible layer with a network of piezoceramics. It is used as an extra ply that is either inserted into or bonded onto the surface of a composite laminate to give it actuating and sensing capabilities. This layer that provides the added functionality is referred to as the “SMART Layer” (Stanford Multi-Actuator-Receiver Transduction Layer). Using the developed method, several SMART Layer prototypes have been fabricated and embedded inside carbon fiber composite laminates successfully. Tests were conducted on composite panels integrated with the SMART Layer to validate the design and the manufacturing procedure.

Manufacturing of composite structures with a built-in network of piezoceramics

This paper describes a comparative study of in situ cure monitoring by three methods: (i) evanescent wave spectroscopy; (ii) refractive index change; and (iii) near-infrared spectroscopy. The cure characteristics of an epoxy/amine reaction were followed in real-time during the crosslinking reaction via the above-mentioned techniques. The evanescent wave spectroscopy technique was based on monitoring the characteristic infrared absorption bands of the resin system to compute the concentration of the amine hardener as a function of cure time. Good correlation was obtained between the evanescent wave spectroscopy data and a conventional method of studying cure reactions, i.e. infrared spectroscopy. During the cure reaction, the refractive index of the resin system increases as a function of the crosslink density. This increase in the refractive index was monitored using two optical fibre techniques. In the first case, a declad region of the optical fibre was immersed in the resin system and in the second method an optical fibre reflectometer was used to track the changes in the refractive index. Once again, good correlation was obtained between the optical fibre techniques and infrared spectroscopy cure data. The results obtained from the optical fibre sensor experiments were used to model the cure kinetics of the resin system. The cure kinetic models were found to predict the cure reaction up to approximately 60% of the reaction.

https://iopscience.iop.org/article/10.1088/0964-1726/5/4/005/pdf

In situ cure monitoring of epoxy resins using optical fibre sensors

This paper reports a comparative study undertaken for different types of optical fibre sensor developed to monitor the cure of an epoxy resin system. The optical fibre sensors used to monitor the cure process were based on transmission spectroscopy, evanescent wave spectroscopy and refractive index monitoring. The transmission sensor was prepared by aligning two optical fibres within a specially prepared sleeve with a gap between the optical fibre end faces. During cure, resin from the specimen flowed into the gap between the optical fibres allowing transmission spectra of the resin to be obtained. The evanescent wave sensor was prepared by stripping the cladding from a high refractive index core optical fibre. The prepared sensor was embedded in the sample and attenuated total reflectance spectra recorded from the resin/core boundary. Refractive index monitoring was undertaken using a high refractive index core optical fibre which had a small portion of its cladding removed. The prepared sensor was embedded in the resin specimen and light from a single wavelength source was launched into the fibre. Changes in the guiding characteristics of the sensor due to refractive index changes at the resin/core boundary were used to monitor the progress of the cure reaction. The transmission and evanescent wave spectroscopy sensors were used to follow changes in characteristic near-infrared absorption bands of the resin during the cure reaction over the range 1450-1700 nm for the evanescent sensor and 2100-2250 nm for the transmission sensor. Consequently, these techniques required tuneable wavelength sources covering specific wavelength ranges. However, the refractive index based sensor used a single wavelength source. Therefore the equipment costs for this type of sensor were considerably less. Additionally, the refractive index sensor did not require a single wavelength source at any particular wavelength and could be applied to any spectral region in which the optical fibre would transmit light. The advantages and disadvantages of these three methods are discussed.

https://iopscience.iop.org/article/10.1088/0964-1726/7/4/017/pdf

In-situ cure monitoring using optical fibre sensors - a comparative study

This paper reports on a comparative study undertaken for different types of optical fiber sensor developed to monitor the cure of an epoxy resin system. The optical fiber sensors used to monitor the cure process were based on transmission spectroscopy, evanescent wave spectroscopy and refractive index monitoring. The transmission sensor was prepared by aligning two optical fibers within a specially prepared sleeve with a gap between the optical fiber end-faces. During cure, resin from the specimen flowed into the gap between the optical fibers allowing transmission spectra of the resin to be obtained. The evanescent wave sensor was prepared by stripping the cladding from a high refractive index core optical fiber. The prepared sensor was embedded in the sample and attenuated total reflectance spectra recorded from the resin/core boundary. Refractive index monitoring was undertaken using a high refractive index core optical fiber which had a small portion of its cladding removed. The prepared sensor was embedded in the resin specimen and light from a single wavelength source was launched into the fiber. Changes in the guiding characteristics of the sensor due to refractive index changes at the resin/core boundary were used to monitor the progress of the cure reaction. The transmission and evanescent wave spectroscopy sensors were used to follow changes in characteristic near-infrared absorption bands of the resin over the range 1450 - 1700 nm during the cure reaction. Consequently these techniques required tunable wavelength sources covering specific wavelength ranges. However, the refractive index based sensor used a single wavelength source. Therefore the equipment costs for this type of sensor were considerably less. Additionally, the refractive index sensor did not require a single wavelength source at any particular wavelength and could be applied to any spectral region in which the optical fiber would transmit light. The advantages and disadvantages of these three methods are discussed.

Comparative study of optical fiber cure-monitoring methods

An optical fibre pressure sensor utilizes displacement of a holographic grating to intensity modulate a signal wavelength; a second wavelength passes through the grating unmodulated and its transmittance is used to reference the signal wavelength intensity.

An Optical Fibre Pressure Sensor Using A Holographic Shutter Modulator With Two-Wavelength Intensity Referencing

We describe three novel optical fiber probes, all based on the focused-beam reflective principle, to measure displacement, form, and surface topography, respectively. Each depends on deriving twin
displacement/optical output characteristics that may be resolved by difference/sum referencing. The displacement sensor adopts twowavelength operation using a zone plate to give opposing displacement/output characteristics. The device is noncontacting, and a resolution of better than 0.1% of span is anticipated. A 3-D optical touchprobe has been built for use with coordinate measuring machines. A mathematical model has been generated that relates output to stylus movement, and the model has been verified experimentally. A resolution of 0.1 μm in the x-y plane and 1.0 μm in the z direction is achievable. A further noncontacting probe has been developed for the measurement of surface topography whose output shows low dependency on surface reflectance. A mathematical model has shown good correspondence against a wide range of surface compositions and textures; a resolution of better than 1 μm is foreseen. Proposals are made for industrial implementation of all three probes.

R. C. Spooncer

Papers

In situ cure monitoring of epoxy resins using optical fibre sensors

In-situ cure monitoring using optical fibre sensors - a comparative study

Comparative study of optical fiber cure-monitoring methods

An Optical Fibre Pressure Sensor Using A Holographic Shutter Modulator With Two-Wavelength Intensity Referencing

Optical fiber displacement sensors for process and manufacturing applications