We present the inscription and characterization of fiber Bragg gratings (FBGs) in polypropylene coreless cylindrical fibers. Polypropylene material offers several advantages, such as strong chemical resistance, biocompatibility, and high tensibility. Therefore, polypropylene FBGs can be useful for sensing in chemically aggressive environments and in biomedical applications. The coreless, cylindrical polypropylene waveguides used in these experiments had a diameter of 150 μm, typical length up to 20 cm, and a refractive index of 1.49. The inscription was performed in the 1550 nm transparency window by using a phase mask technique and 193 nm excimer laser radiation. Inscribed FBGs demonstrated complex multi-peak reflection spectra due to highly multi-mode nature of the polypropylene waveguides. Due to a high attenuation of the polypropylene, the maximum waveguide FBG interrogation length -in reflection- was 6 cm. Gratings characterization demonstrated a strain sensitivity of 0.9 pm/με, a temperature sensitivity of -60.4 pm/°C and humidity-insensitive behavior.
A resin cavity at the end face of a single mode optical fiber, monitors the presence of methanol, ethanol and isopropanol vapors with distinct wavelength shift and with ppb sensitivity.
We describe an optical fiber sensor for detection of ammonia vapors employing a fluorinated graphene-like overlayer on a tilted Bragg grating. Exploiting the laser-mediated explosive synthesis and transfer (LEST) of graphene (Gr) flakes a thin film of few-layer turbostratic graphene flakes doped with F atoms (~ 3.3 at. %) are deposited on the fiber at the location of the grating. The response of the sensor was investigated for NH4OH vapors while for reference, the effect of H2O was also monitored at identical conditions. Under increasing vapor pressure of NH4OH, wavelength shift is recorded not only in the cladding modes but also for the fundamental Bragg mode, indicating that the effect is not solely due to changes in the optical parameters of the overlayer. The monitored wavelength shift is initially negative turning to positive when vapor saturation is reached. Furthermore, there is a distinct difference in the magnitude of the monitored shifts with the higher order mode exhibiting 2.5x higher values compared to the Bragg mode. The study is ongoing and will also include overlayers of pure LEST Gr and LEST Gr decorated with Six nanoparticles.
We report on the development of two optical fiber sensors for agricultural applications. Specifically, a Fabry Perot optical fiber sensor with a hydroscopic photo- resin cavity is developed for monitoring oak barrel stave moisture evolution towards the development of a sensor for assessment of oak barrels used in wine aging. Furthermore, an optical fiber long period grating of extended length (~9 cm) is used as line sensor for tracing sprayed copper chlorophyllin water droplet distribution aiming to detect spraying drift during drone chemical pesticide operations.
Chemical pesticides drone spraying is becoming increasingly available due to its advantages such as autonomy and fast operation. A major consideration that currently limits widespan application of the technique is the undesirable drift of the spraying cloud in neighboring areas. Herein we propose the use of optical fiber long period gratings (LPGs) of extended length (~9cm) as line sensors for tracing spraying droplet distribution. Preliminary results indicate a linear trend between particle density and LPG wavelength shift and extinction ratio change. Indicatively, for a coverage of 3.9 droplets/mm2 the corresponding LPG strength and wavelength changes are 1.3dB and 65pm, respectively.
There are several techniques available for fabrication of phase-shifted gratings in single mode fibers. Yet, very few studies have examined inscription of such gratings in photonic crystal fibers (PCFs). In this paper, we report what we believe to be the first demonstration of the phase-shifted grating inscription in PCFs using a phase mask and a beam stop. The grating inscription is demonstrated for three hexagonal lattice PCFs with different air-filling fractions. The transmission spectra of the fabricated gratings reveal phase shift resonance peaks with a -3 dB bandwidth between 40 and 80 pm which is up to 4 times narrower than of the resonances of uniform Bragg gratings inscribed in the same fibers.
We report results on the development of an optical fiber long period grating sensor for organic vapor monitoring. A silk fibroin ovelayer with a thickness of the order of 400 nm is drop casted on the fiber cladding surface to enable sensing of volatile organic compound vapors. For increasing methanol vapor pressure up to 100 mbar the wavelength of the long period grating attenuation band exhibits a negative shift up to a maximum value of 4 nm. The strength of the grating attenuation band is also altered reaching a 0.27 dB strength decrease under the same maximum methanol vapor pressure. Furthermore, the recovery behavior of the sensor is presented and preliminary results of the response of the sensor to isopropanol and ethanol vapors are briefly discussed.
The sensing properties of zinc oxide (ZnO) combined with tilted fiber Bragg gratings have been studied in this work for the development of organic vapor optical fiber sensors. ZnO has been deposited onto tilted optical fiber Bragg gratings by drop casting technique to obtain coatings with thickness below 100nm. For a title angle of 4°, cladding modes up to 4 dB peak to peak amplitude were obtained and, as a consequence, the fundamental mode strength dropped below 0.5dB. The sensing features were evaluated by exposing the sensor to saturated atmospheres of three different alcohols and acetone. A negligible change was registered for methanol but in the case of ethanol and isopropanol a 24pm red shift was observed; in the case of acetone, the shift was 96pm which is of the order of spectral width of certain cladding modes. For all cases, the shifts were reversed once the organic vapors were removed. Ongoing studies are focused on the optimization of the sensing layer as well as the tilted angle to enhance the spectral response of the sensor.
The design, fabrication and characterization of phosphate based bioresorbable optical fibers is reported. Applications in diffuse optics, pH sensing and temperature sensing have been demonstrated paving the way to the use for a new generation of implantable and degradable devices for theranostics.
We review results on the growth of glassy and crystalline materials inside photonics crystal fibres (PCFs), seeking specific sensing and light confining properties. Initial results refer to the infiltration of silver metaphosphate (AgPO3) glass inside PCFs, and the subsequent thermal poling of those composite PCFs for attaining plasmon resonance characteristics. Accordingly, the wet chemistry growth and characterization of crystalline ZnO layers inside PCFs is shown also. Finally, the use of laser etching technique for the inscription of relief periodic Bragg gratings on the capillary walls of a solid core, all silica glass PCF that exhibited demarcation temperatures near 1200 °C is presented. Ongoing work refers to the exploitation of the aforementioned PCFs for developing sensing, bio-sensing and actuating optofluidic devices.
We describe a novel sensing approach based on a functionalized microstructured optical fiber-Bragg grating for specific DNA target sequences detection. The inner surface of a microstructured fiber, where a Bragg grating was previously inscribed, has been functionalized by covalent linking of a peptide nucleic acid probe targeting a DNA sequence bearing a single point mutation implicated in cystic fibrosis (CF) disease. A solution of an oligonucleotide (ON) corresponding to a tract of the CF gene containing the mutated DNA has been infiltrated inside the fiber capillaries and allowed to hybridize to the fiber surface according to the Watson-Crick pairing. In order to achieve signal amplification, ON-functionalized gold nanoparticles were then infiltrated and used in a sandwich-like assay. Experimental measurements show a clear shift of the reflected high order mode of a Bragg grating for a 100 nM DNA solution, and fluorescence measurements have confirmed the successful hybridization. Several experiments have been carried out on the same fiber using the identical concentration, showing the same modulation trend, suggesting the possibility of the reuse of the sensor. Measurements have also been made using a 100 nM mismatched DNA solution, containing a single nucleotide mutation and corresponding to the wild-type gene, and the results demonstrate the high selectivity of the sensor.
A novel DNA sensing platform based on a Peptide Nucleic Acid - functionalized Microstructured Optical Fibers gratings has been demonstrated. The inner surface of different MOFs has been functionalized using PNA probes, OligoNucleotides mimic that are well suited for specific DNA target sequences detection. The hybrid sensing systems were tested for optical DNA detection of targets of relevance in biomedical application, using the cystic fibrosis gene mutation, and food-analysis, using the genomic DNA from genetic modified organism soy flour. After the solutions of DNA molecules has been infiltrated inside the fibers capillaries and hybridization has occurred, oligonucleotidefunctionalized gold nanoparticles were infiltrated and used to form a sandwich-like system to achieve signal amplification. Spectral measurements of the reflected signal reveal a clear wavelength shift of the reflected modes when the infiltrated complementary DNA matches with the PNA probes placed on the inner fiber surface. Measurements have also been made using the mismatched DNA solution for the c, containing a single nucleotide polymorphism, showing no significant changes in the reflected spectrum. Several experiments have been carried out demonstrating the reproducibility of the results and the high selectivity of the sensors, showing the simplicity and the potential of this approach.
In this work the inner surface of a microstructured optical fiber, where a Bragg grating was previously inscribed, has
been functionalized using peptide nucleic acid probe targeting a DNA sequence of the cystic fibrosis disease. The
solution of DNA molecules, matched with the PNA probes, has been infiltrated inside the fiber capillaries and
hybridization has been realized according to the Watson - Crick Model. In order to achieve signal amplification,
oligonucleotide-functionalized gold nanoparticles were then infiltrated and used to form a sandwich-like system.
Experimental measurements show a clear wavelength shift of the reflected high order mode for a 100 nM DNA solution.
Several experiments have been carried out on the same fiber using the identical concentration, showing the same
modulation and proving a good reproducibility of the results, suggesting the possibility of the reuse of the sensor.
Measurements have been also made using a 100 nM mis-matched DNA solution, containing a single nucleotide
polymorphism, demonstrating the high selectivity of the sensor.
We have recently introduced a new approach in the utilisation and actuation of liquid matrices inside microstructured
optical fibers, by infiltrating in their capillaries magnetically active fluids, namely, ferrofluids. The specific optofluidic
approach provides the possibility of actuation of the infiltrated liquid by applying an external magnetic field, thus,
exhibiting magnetofluidic capabilities. We apply this infiltration protocol in microstructured optical fiber Bragg gratings
for developing magnetic field tunable/sensitive photonic devices and sensing probes. The material and implementation
considerations related to this infiltration approach of viscous and opaque ferrofluids inside microstructured optical fibers,
and the corresponding effects on the guiding and scattering behavior of the microstructured optical fiber Bragg gratings
are presented and discussed. An updated review on this infiltrated microstructured optical fiber devices will be presented,
focusing on the demonstration of simple magnetofluidic configurations such as "on-off" Bragg grating trimmers, "infiber"
magnetometers, ferrofluidic defected Bragg reflectors and external magnetic field modulators. The design
principles of such "in-fiber" magnetofluidic photonic devices will be analysed, along with their particular functionalities
and application prospects; while in addition, the infiltration and fiber capillary functionalisation processes will be
presented.
The feasibility of a biosensor for DNA label-free detection, based on long period fiber gratings, has been investigated.
The surface of the grating has been functionalized with Peptide Nucleic Acid (PNA) probes. DNA
strands, matched with the PNA probes, have been immobilized on the surface itself. The possibility of a resonant
wavelength shift in the transmission spectrum due to the DNA capture will be discussed. The problem of reusing
the sensor for multiple measurements will also be addressed.
The inscription of Bragg reflectors in commercial, all-silica microstructured optical fibres using picosecond and
femtosecond 248nm laser radiation, will be presented. The research results reported herein aim to the investigation of the
optimum photosensitivity regime and index engineering route, and its dependence upon the wavelength and intensity of
the laser source, for the high yield inscription of Bragg reflectors in all-silica microstructured optical fibres. The
ultraviolet laser source used was a 248nm, 5ps/500fs hybrid dye/excimer; while hydrogenated Blazephotonics ESM-12
and Crystal Fiber LMA-10 microstructured optical fibres were exposed. Refractive index evolution curves for both
average and modulated index changes, as well as, thermal annealing results are presented and discussed. Refractive
index changes of the order of 10-4 were obtained for relatively low accumulated energy density doses (<18KJ/cm2). For
the case of the 248nm femtosecond radiation, the underlying photosensitivity process was speculated to be that of two-photon
absorption, however, significant contribution from single-photon processes related with hydrogen generated and
oxygen pre-existing defects is also possible. The index engineering and thermal annealing results presented for the case
of 248nm ps and fs radiation, are compared to Bragg grating inscriptions using 193nm, 10ns excimer laser radiation.
Further, issues related to the spatial distribution of the ultraviolet laser energy density inside the fibre core for side-illumination
are presented and discussed in conjunction with the refractive index growth data.
We describe a fiber optic humidity sensor comprising a moisture - sensitive overlay on a long period fiber grating (LPFG), tuned at the 1.5 microns band. The hygrosensitive material overlaid was poly(ethylene oxide)/CoCl2 hybrid containing 10 % wt CoCl2 in the form of micro- and nano-particles synthesized in situ in the polymer of 50000 molecular weight. A thin overlay of the material is deposited from an aqueous solution on the unclad region of the LPFG and upon exposure to different ambient humidity levels its spectral properties are modified. The experimental results obtained, show changes of the spectral resonance notch and the transmission strength of the LPFG. The material parameters associated with the sensing mechanism may include those of refractive index, absorption and morphological alterations of the overlaid material. Relative humidity (RH) variations in the range from 50% to 95% can been detected with a resolution better than 0.2% RH. The response time constant is found of the order of few hundred milliseconds.
Conventional aircraft repair techniques employ bolted or riveted metallic reinforcements, which frequently introduce additional stress concentrations leading to further cracking and creating areas difficult or impossible to inspect. Bonded composite repairs (“patches”) result in the elimination of stress concentrations caused by additional fastener holes, improved strength to weight ratio and present a sealed interface. This reduces even further the danger of corrosion and fretting under the repair, gives greater flexibility in design and lessens application time while lengthening fatigue life.
Embedding optical fibres and sensors into the patch, and combining this with advanced data collection and processing systems, creating a so-called “smart patch”, will enable the real-time assessment of aircraft structural integrity resulting in reliable prediction of maintenance requirements for repaired structures. This paper describes the current state of the art in smart patch technology, and includes a detailed description of the measurement problem and of the work being undertaken to solve it, at both the component and system level. An analysis of typical crack behaviour, based on FE modelling is presented and this demonstrates the need for optical strain sensors having a very short gauge length. The paper discusses the advantages and limitations of very short Fibre Bragg Gratings (FBGs) in this context and also provides early experimental data from 1mm and 2mm gratings which have been fabricated for this purpose. The paper also describes the impact of the measurement and environmental constraints on the design of the FBG interrogation system and presents the results of initial trials. The work is being undertaken in the framework of a collaborative project (ACIDS) which is co-funded by the European Commission.
Ice accretion on flying surfaces affects the aerodynamic performance and handling qualities of aircraft, and may require different pilot corrective action, dependent upon the surface that ice is accreting onto. The current methodology for ice detection usually relies on an indirect method, normally based on ambient air temperature, and liquid water content. When a pre-set threshold level is reached, the ice protection system is activated, whether or not ice is accreting on
critical surfaces. This method is not cost effective or efficient for an ice protection system. Air Conformal Ice Detection System (ACIDS) obviates these problems by using a 'direct’ method of detection and measurement the presence and thickness of ice. This paper outlines some of the preliminary experimental work done on the optical properties of ice grown in an icing tunnel on the leading edge of an aerofoil leading to the development of a Fibre Optic Direct Ice
Detector sensor (DID) with emphasis. The result of this studies have shown that with suitable processing it is possible to use fibre optic sensors to determine the thickness of ice and texture of the ice accreted in the vicinity of the sensor. In the latter part of this paper basic fibre optic architecture is discussed and together with some preliminary results for representative icing runs.
Experimental results are presented related to the nonlinear optical response of silicon nanocomposities embedded in various media such as water, glycerol, polymer and covered by silver in glycerol. As shown, the free silicon nanocomposites exhibit saturable absorption and negative nonlinear refraction and one is covered by silver exhibit reverse saturation absorption.
The linear and non-linear optical properties of gold-coated nano-crystals embedded in glycerol have been investigated. The optical absorption spectra and the non-linear absorption of these nanostructures have been determined under various excitation conditions. The peak of the plasmon resonance like curve has been observed to be blue shifted compared to the plasmon peak of colloidal gold particles. The resonance behavior exhibited by the gold coated silicon nano-crystals is interpreted in terms of the third order non-linear susceptibility which is expressed as a saturation absorption behavior of the samples. The effect of the charged interface gold film, which covers the nanocrystals, on the electron band structure and on the linear and non-linear optical properties are discussed.
Numerous optical fibre sensing techniques have been evaluated to monitor composite material structures. Most of these sensor systems aim to measure responses to static loads or vibrational spectra. An alternative approach is to monitor the ultrasonic signature over structure and evaluate changes in this signature in terms of modifications to structural properties. This paper describes the principles and implementation of such a system. A simple line integrating interferometric optical fibre sensor is used to detect the propagation of Lamb waves at typically 250kHz within the composite material. The wavelength of these Lamb waves (typically 2cm) exceeds the dimensions of structural defects of interest to the optical fibre sensor is, in effect, detecting changes in the ultrasonic scattering signature of the test structure and relating these changes to the evolution of faults or damage within the structure. The paper will present preliminary results which explore the basic sensitivity mechanisms within the optical fibre sensor and examine the modifications to the ultrasonic signature with the introduction of predetermined damage and deterioration. The eventual aim is a fully integrated system which will enable “plug in” structural testing. These preliminary results indicate that such systems concepts are a medium term prospect.
The measurement of quasi-static strain field using optical fibers presents a considerable challenge due to the inherent sensitivity of optical fibers to temperature. This paper summarizes recent work we have carried out on two approaches to this problem. Dual mode polarimetric measurements were investigated as a means of implementing distributed temperature measurements and radio frequency subcarrier sensors have been used to perform the same measurement on an integrated basis. These techniques are contrasted and assessed against other technologies such as Bragg gratings and dispersive Fourier transform spectroscopy on the basis of measurement capability, ease of implementation and technological maturity.
Optical fiber sensors for monitoring of structures (OSMOS) is a European collaborative research project which has, over the past three years, embraced a number of technological issues related to the problem of structural monitoring in the civil engineering and aerospace industries. A key technical objective of the program was the measurement of temperature and strain using a single sensor length. A laboratory prototype using the differential sensitivities of polarimeters based on the fundamental, LP01 mode and the first higher order LP11 mode of polarization maintaining fiber demonstrated parameter recovery to within 2 C and 5 (mu) (epsilon) . A receiver enabling quasi-distributed measurements to be made with a linear spatial resolution of 70 cm using white light polarimetry was assembled. White light polarimetry was also used in conjunction with pressure sensitive fiber to detect impact damage on a composite radome structure. Impacts of 5 Joules in magnitude were detected with a spatial resolution of around 1 cm. Microwave radio frequency subcarrier measurement techniques were used to develop the engineering processes necessary to integrate optical sensors into civil engineering structures for simulated applications trials. This enabled issues such as stress transfer, mechanical bonding and sensor protection to be addressed. For the aerospace industry, embedding of optical fiber sensors remains an important issue. Here we developed techniques for embedding connectorized fibers such that the component could be machine finished after curing, an important feature of the manufacturing process.
The simple observation that the temperature coefficient of most structural materials in which strain may need to be measured differs from that of optical fiber measuring systems indicates immediately that both temperature and strain fields should be mapped with comparable accuracy in order to arrive at a reliable indication of mechanical strain. This paper initially defines the criteria for adequate compatibility between temperature and strain field measuement and then compares the techniques which have emerged in the past five years to address this problem. All techniques require the measurement of two optical parameters which are typically differential delays e.g. in interferometers and grating sensors or combined dellay and dispersion characteristics. The basic features of these measurement techniques will be addressed and comparisons made between the applicability of the various techniques. As an example, our own work measured temperature and strain to within +/- 2 C and 10 (mu) (epsilon) . Other work, e.g. with Bragg gratings achieves +/- a few tens of (mu) (epsilon) and a few degrees centrigrade.
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