A novel polyvinylidene fluoride (PVDF) coated fiber-optic Fabry–Perot interferometric (FFPI) sensor for temperature measurement is proposed and experimentally validated. The sensor head consists of a PVDF thin film formed by the phase inversion process via immersion precipitation at the freshly cleaved endface of a single mode telecommunications optical fibre. The resin polyvinyl butryl (butvar) was used as a cross-linker to promote good adhesion between the optical fibre glass and PVDF in order to reduce operational coupling loss. Sensor operation is demonstrated by shifts of the FFPI reflection spectra and its performance was analysed through measurement of optical thickness or path difference with temperature variation. Experimental data shows that the thermal expansion effect of the PVDF coating is more dominant than its thermo-optic effect. The proposed sensor exhibits an exponential response over 21 to 160 °C inside a tube furnace with a sensitivity that spans between 1.6 nm/°C and 7.5 nm/°C.
An intensity ratio technique has been used for characterizing fluorescence spectra from novel coumarin dyes for pH sensing, in the range of 0.5 – 6, providing results that are independent of possible fluctuations in the intensity of the excitation source, deterioration of the indicator and changes in optical coupling. The arrangement was determined to have a sensitivity of 25% per unit pH change (at a pH of 4).
High spatial resolution information of the rare-earth dopant distribution in optical fibers enriches our understanding of the fiber manufacture processes and enables improvement in the design of active photonic devices including optical fiber lasers and amplifiers. Here, data from an investigation of the backscattered fluorescence signal off the end-face of an erbium (Er3+) doped silica optical fiber obtained with a near-field scanning optical microscope (NSOM) are presented. It has been recently confirmed via a theoretical model that information about the relative Er3+ ion distribution in fibers can be inferred by simply monitoring the fluorescence signal originating from the de-excitation of specific energy levels in the investigated samples. A comparison of the Er3+ ion distribution profiles extracted from the fluorescence measurements acquired through the NSOM system with those obtained from the application of a powerful analytical ion probe is also presented.
Fiber Bragg gratings written using a phase-mask exhibit a complex refractive index pattern in the fiber core,
as observed in differential interference contrast (DIC) microscopy. This results in dual periodicities along the
fiber core and gratings at both the Bragg wavelength, twice the Bragg wavelength, and the harmonics of these.
Features in grating spectra arise from one or both of these periodicities and spectra for twice the Bragg
wavelength (or its odd harmonics) are very sensitive to the details of how the UV complex diffraction field of
the phase mask becomes imprinted as refractive index changes in the fiber core.
Focused ion beam (FIB) lithography was used to inscribe a periodic array of nanoholes directly on gold-coated optical
fiber end-faces. The excitation of the surface plasmon polaritons of the nanohole arrays on the optical fiber end-faces
provided the basis of a refractive index sensor for liquids. This optical fiber based surface plasmon resonance sensor is
compact and has the potential to be used in biomedical applications. A sensitivity of approximately 294 nm per refractive
index unit (RIU) has been demonstrated for this sensor.
The effect of pressures up to 200 kPa on pi-phase-shifted Bragg gratings was investigated in several types of
fibers placed in a pressure chamber. Standard and alternative types of pi-phase shifted gratings were studied,
and changes in reflection spectra caused by pressure changes were monitored. The alternative pi-phase shifted
grating was fabricated using a phase mask with 536-nm pitch exhibiting features at twice the Bragg
wavelength near 1552 nm due to the interleaved refractive index modulations along the fiber core with the
periodicity of the phase mask. The three gratings studied exhibited quite different variation in Bragg
wavelength with increased pressure for the different fiber types, and the alternative pi-phase-shifted grating in
Corning HI 1060 FLEX fiber also suffered a significant change in the shape of the peaks.
The effect of bending on the twin peaks of an alternative type of pi-phase shifted fibre Bragg grating (FBG) (embedded
in a resin) at twice the Bragg wavelength, has been investigated. Variation of the direction of bending resulted in
corresponding changes in the relative intensity of these peaks. Temperature independent bend measurements were
realised by monitoring the change of ratio of the reflected intensities of these peaks.
The effect of transverse strain on a fiber Bragg grating fabricated using a phase mask with 536-nm pitch has
been investigated through the study of its reflection and transmission features at twice the Bragg wavelength
near 1552 nm, that are due to reflection/transmission from FBG periodicities associated with the phase mask
periodicity. The occurrence of two peaks in the reflection spectrum is due to the interleaved refractive index
modulations along the fiber core, with the periodicity of the phase mask that produces a type of π-phaseshifted
grating. The response of these features to transverse strain is similar to that observed previously for
features at 2/3 of the Bragg wavelength that also arise from the same complex refractive index structure.
Silica-based thulium-doped fibers sensitized by ytterbium are being developed for applications in fiber amplifiers and
lasers at various wavelengths (around 800 nm, 1470 nm and 2 µm). Several studies have been performed to design and
optimize thulium- and ytterbium-doped fiber (TYDF) amplifiers and lasers at the above mentioned wavelengths.
Although some papers dealing with modeling of such a system exist, the parameters used in the simulations, like energy
transfer coefficients, have not been experimentally determined to date. In this paper we present an estimation of the
energy transfer coefficients by comparison of the measured emission of three TYDF samples with numerical simulations
of the respective emission using a spectrally and spatially resolved model of TYDF. We found that the energy transfer
coefficients are higher than those reported in Tm/Yb-doped fluoride based crystals. This fact together with the possibility
of increasing the energy transfer efficiency, by improvement of excited level lifetime of thulium by high alumina
codoping, makes thulium/ytterbium co-doped silica fibers promising for applications in fiber lasers and amplifiers.
A sensor based on a Bragg grating (FBG) fabricated in Hi-Bi optical fiber allowed measurement of transverse and
longitudinal strains. A novel interrogation technique that enables information about transverse and longitudinal strain to
be deduced from changes in FBG reflectance spectra is proposed. The scheme enables translation of spectral information
into optical power received at a pair of sensor heads.
A fiber Bragg grating sensor fabricated by a phase mask with 536 nm uniform pitch is presented. Two peaks/dips occur,
at 785 and 1552 nm, due to reflection/transmission at the Bragg wavelength and at double the Bragg wavelength, and
arising from FBG periodicities associated with half the phase mask periodicity and the phase mask periodicity,
respectively. It provides simultaneous measurement of temperature and longitudinal strain, with similar intensities in
both wavelengths making it better suited for long-distance operation and multiplexing compared with similar schemes.
By using a simple technique of UV laser irradiation at various regions along a chirped phase mask, the responses in the
region of 2/3 of the Bragg wavelength (i.e. ~1030 nm) and the Bragg wavelength (~1535 nm) are investigated
experimentally and compared. The variation of the wavelength of both transmission dips (in the region of the 2/3 of the
Bragg wavelength and the Bragg wavelength) were proportional to the increase in periodicities of phase mask. The ratios
of these wavelengths, for the irradiation of each phase mask section, showed less than 0.7% variation compared with the
value of 2/3, confirming that features at ~1030 nm are the 3rd harmonic of a grating having the phase mask periodicity.
A chirped fiber Bragg grating was used to measure the non-uniform strain profile of a notched aluminum specimen used
to simulate a cracked structure. The specimen was subjected to tensile tests that produced regions of non-uniform strain
near the notches. Analysis of power reflectance spectra from the grating, through the use of an integration method,
enabled the strain profile near the notches to be determined. Unlike other intragrating sensing methods, this method did
not require a disturbance hypothesis to be postulated. The strain profile results from this intragrating sensor were in
reasonable agreement with predictions from modeling conducted using the finite element method.
This paper focuses on optical fiber laser-based wide range temperature measurement applications, under circumstances
where different strain values were applied to the chirped fiber Bragg grating at a fixed temperature and the uniform type IIA fiber Bragg grating was used temperature-tuned. This grating forms the end reflector in the laser cavity feedback configuration. Erbium doped fiber was used as the laser gain medium and was pumped by using light from a 1480 nm laser diode. The response of the sensor was achieved over the tunable-temperature range from 70°C to 500°C when a fixed strain value of 1000 με was applied to the chirped Bragg grating, with a root mean square error of 10°C, using linear fitting over the above measurement range.
The strain and temperature responses of transmission dips at 2/3 of the Bragg wavelength in type I and IIA fiber Bragg gratings are presented. The existence of such dips is related to complex features that have been observed in microscope images of fiber Bragg gratings. The temperature and strain responses are as expected and demonstrate the suitability of using signals from both the Bragg wavelength and 2/3 of the Bragg wavelength for the simlutaneous and co-located measurement of strain and temperature, using a single light source.
KEYWORDS: Fiber Bragg gratings, Reflectivity, Temperature metrology, Optical fibers, Phase measurement, Calibration, Connectors, Sensors, Aluminum, Signal to noise ratio
A method of determining the temperature profile along a short length of optical fiber containing a chirped grating is presented. This provides an alternative to the known method of integrating differences between power-only reflectance spectra to recover a distributed measurement of temperature or strain within a chirped grating without the additional equipment requirements of a phase spectrum measurement. In contrast to other methods of processing the spectra of chirped gratings, the windowed cross-correlation method monitors local wavelength shifts directly, and offers immunity to spurious temperature or strain gradients arising from amplitude measurement errors.
The use of reflected power spectra arising from a chirped fiber Bragg grating (CFBG) to extract a nonuniform temperature distribution along the grating has been investigated. The technique uses a discrete Fourier transform (FFT) in which the measured spectrum of the CFBG due to a localised temperature change was simulated using the FFT grating design model. The model operated on the reference spectrum and hypothesis temperature distributions, T(z), to generate a spectrum representative of a localised temperature disturbance. The simulated spectrum was fitted to the measured spectrum using a three-parameter disturbance function operating on position, width and amplitude of temperature change. The rms deviation of the applied value for position of a localised temperature change was 0.14 mm.
A fiber Bragg grating was written, using the phase mask technique, in a short length of Er3+:Yb3+-codoped fibre and its temperature and strain characteristics were investigated. Two of the three parameters of interest are associated with Bragg grating properties, namely the usual dip at the Bragg wavelength, λB, and an unexpected dip at ~2/3 λB that is the third harmonic of features within the grating having double the assumed periodicity. The third parameter is the fluorescence intensity ratio arising from a pair of transitions in Yb3+ ions. This arrangement consisting of three co-located sensors offers interesting possibilities for the simultaneous measurement of three parameters.
Characterization of two simultaneous and co-located temperature-strain sensors based on FBGs and Er:Yb-codoped fiber has been performed. The sensors have demonstrated reasonable accuracies in the order of 0.4°C and 10 με over temperature and strain ranges of 20 - 150°C and 350 - 2500 με respectively.
Non-destructive 3-dimensional images of the effect of refractive index change within a fiber Bragg grating have been recorded using the differential interference contrast imaging technique. The refractive index change indicated by the images is highly non-uniform.
Measurement of temperature using optical methods has been successfully employed in industrial environments where conventional techniques become unreliable. The sensor shows sufficient sensitivity to thermal variations whilst being substantially insensitive to strain. The most promising rare-earth elements for further development have been identified.
The effect of strain on the fluorescence intensity ratio technique has been investigated using Nd3-doped fibre as the sensing material. Measurements of the fluorescence intensity ratio for applied strain values up to approximately 2000 show a relatively small sensitivity of (1.5 1.4) x lO %/is while the temperature sensitivity was found to be 1.56 %/°C. This implies a strain-temperature cross-sensitivity of(1.O 0.9) x 1O °C/p., ie. an applied strain of 1000 would induce errors ofonly 0.10 0.09 °C.
Use of rare-earth doped fiber as the sensing element for temperature sensing is reviewed. The advantages of employing a fluorescence intensity ratio technique is discussed and examples of practical applications of the technique are given.
Rare-earth materials doped into low-loss silica fibers are of interest for temperature sensing applications since the absorption properties of such are temperature dependant. Such absorption properties have been studied in ytterbium doped silica fiber using relative absorption and cut-back techniques at temperatures between 77 and 1163 K in the wavelength range 800 to 1150 nm. The wavelength dependant thermal sensitivity varies from -0.012 to 0.033 dB/K for a standardized one meter long 1000 ppm-doped fiber. These significant changes in absorption with temperature have been attributed to homogeneous line broadening. The fibers investigated show excellent potential for use as the sensing element in intrinsic fiber-optic high-temperture sensors.
We propose the principle of a high-dynamic, quasi-distributed temperature sensor. We previously have demonstrated the possibility of realizing a high-dynamic point sensor based on the behavior of the green emission in erbium doped silica fibers. We present here the study of the 1.13 micrometers and 1.24 micrometers emission lines, coming from the same levels. Those lines present the same temperature dynamic as the green ones. As the lower level of these transitions is the 4I11/2 level and not the fundamental one, the signal is absorption free. The signal wavelength also corresponds to a transparence region of the intermediate fibers. These arguments permit developing an efficient quasi-distributed configuration. In addition, the intensity ratio of the emission lines is only temperature dependant, so the measurement is self-calibrated. We also demonstrate that the emitting levels can be excited around 800 nm, by the excited state absorption process, which allow using a standard laser diode as pumping source.
The advantages of optical fibre temperature sensors over traditional technologies in hostile environments are now well accepted with commercial application of such sensors established1. However, optical fibre temperature sensing systems with improved dynamic range, system reliability, accuracy and unit cost are desirable.
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