Optical low coherence reflectometry (OLCR) was used for the measurement of the local fiber Bragg grating (FBG) birefringence. The birefringence with resolution of 4μ10-6 was determined directly from oscillations in the OLCR amplitude with a spatial resolution on the millimeter scale. Using OLCR phase in combination with inverse scattering algorithm the birefringence was retrieved with a high spatial resolution of 25 microns. The technique using OLCR amplitude was applied for measuring birefringence of a FBG under diametric loading.
Long-period gratings were written in a pure-silica-core fiber using the electric arc technique. To assess the influence of stress relaxation on grating formation, tomographic stress measurements were performed inside the grating. No stress annealing could be observed within the accuracy of the stress measurement, suggesting a negligible photo-elastic contribution to the arc-induced perturbation of the index profile. In contrast, stress relaxation was found inside a splice of pure-silica-core fiber where discharge conditions are different, indicating the general potential to alter the fibers stress and thus the index profile using an electric arc.
An optical fiber sensor to simultaneously measure strain and temperature was designed and embedded into an adaptive composite laminate which exhibits a shape change upon thermal activation. The sensor is formed by two fiber Bragg gratings, which are written in optical fibers with different core dopants. The two gratings were spliced close to each other and a sensing element resulted with Bragg gratings of similar strain sensitivity but different response to temperature. This is due to the dependence of the fiber thermo-optic coefficient on core dopants and relative concentrations. The sensor was tested on an adaptive composite laminate made of unidirectional Kevlar-epoxy pre-preg plies. Several 150μm diameter pre-strained NiTiCu shape memory alloy wires were embedded in the composite laminate together with one fiber sensor. Simultaneous monitoring of strain and temperature during the curing process and activation in an oven was demonstrated.
The study of the effect of high optical power on FBGs among different laboratories whitin the COST270 action required to verify the consistency and convergence of Bragg grating measurements obtained by the laboratories involved in COST270. This papers reports and discusses the results obtained during a round-robin for fibre Bragg grating metrology initiated within the COST270 action. This action was undertaken in order to minimize measurement scattering and to reduce the uncertainty in the interpretation of following COST270 round-robin measurements on the effect of high optical power on FBGs. This exercise dealed with the spectral characterization of FBGs written in standard telecommunication optical fibre (CORNING SMF-28) provided by the coordinating laboratory (Ecole Polytechnique Federale de Lausane, Institut d'Optique Appliquee).
Fiber laser systems have the potential to represent an interesting alternative to more conventional laser sources emitting in the visible spectral range. In this contribution we present new results based on fluoride Pr/Yb-doped fiber systems. This Pr/Yb-doped fiber system with emission wavelengths in the red, orange, green and blue spectral range can be operated as a tunable laser source with a tuning range (around 605 nm) of almost 8 nm and as a mode-locked laser system with a pulse duration below 100 ps. The laser system was realized using Pr/Yb-doped fluoride fibers as an active media and fiber Bragg gratings in silica fibers as the basic element for the realization of the reflective and tuning components.
Experimental, analytical and finite-element-simulation approaches are presented for the characterization of fibre Bragg grating sensors written in conventional monomode and polarization-maintaining fibers subjected to transverse loading. Firstly, a diametrical-load configuration is considered. Numerical simulations show the behavior to be nonlinear as a function of the applied load when an appropriate analytical model for the opto-mechanical response is employed. Secondly, experiments are carried out with the sensors embedded in epoxy specimens, when the latter are subjected to transversal biaxial loading. The response is monitored as a function of the vertical/horizontal load ratio. A finite-element model of the specimen with the embedded fibre and the previous analytical procedure are used to calculate the strain distributions in the fibre core resulting from loading, and predict the corresponding Bragg wavelength shifts. Experimental results are then compared to numerical predictions.
Basic components and devices for optical fiber sensor and telecommunication networks should exhibit survival times of 20 to 40 years. Fiber gratings are important basic devices for such networks. This demands good thermal and mechanical reliability of the grating itself. However, as it will be shown in this paper, both reliability aspects depend strongly on factors like grating fabrication conditions, the fiber used, a possible photosensitization process, etc. The thermal stability of fiber gratings is strongly linked to the fiber, its co-dopants and possible photosensitization by hydrogen loading. Determining the electron distribution of the involved defects can assess the reliability. To a first approximation the decay is determined by the decay frequency v0. Good thermal stability over 25 years at 40 °C can be predicted for germanosilicate fibers if the gratings are annealed for 110 hours at T≈120 °C. Higher operating temperatures demand higher annealing temperatures for reasonable annealing times and do need therefore special protections like polyimide coatings. The mechanical breaking strength of fiber gratings is related to the grating fabrication process and fiber preparation. For telecommunication applications, gratings are in general fabricated off-line. In this case, careful chemical fiber stripping and handling is needed. When gratings are written using pulsed KrF excimer laser irradiation a considerable mechanical degradation is observed, which depends on the irradiation conditions. In contrary, frequency doubled CW-Ar+ laser irradiation almost preserves the median breaking stress. Gratings fabricated by one single laser shot or written through the fiber coating show almost no degradation. However, due to their low reflectivity the use of single shot gratings is mostly restricted to sensor applications. The fabrication of grating devices by irradiation through the coating is still limited to index modulations of 10-4 due to coating degradation.
Different fiber Bragg grating reliability issues are currently under investigation in Europe. Recent results on thermal decay of gratings, on mechanical reliability of single- and multi-shot gratings, and on the influence of γ-irradiation will be presented.
This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e. the strong non-uniformity of the strain field) the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth may not be valid. Compact tension specimens including a controlled notch shape are fabricated with embedded optical fiber Bragg gratings at identical locations but with different gauge lengths. The spectra in transmission varies between such specimens for given loading conditions. This variation is shown to be due to the difference in gauge length. By using the strain field measured on the specimen surface with electronic speckle pattern interferometry and a discretized model of the grating, the spectra in transmission are then verified analytically. Thus, by considering the non-uniformity of the strain field, the optical fiber Bragg gauge functions well as an embedded strain gauge near the stress concentration. Due to the distributed nature of the measurements within a specific gauge length, the optical fiber Bragg gauge has a large potential to measure debonding in fiber reinforced composites.
The mechanical resistance of UV irradiated optical fibers has been investigated by dynamic fatigue tests. The intrinsic Weibull distribution of the pristine and chemically stripped fiber were almost identical. The high germanium doped fiber was homogeneously irradiate using a KrF excimer with different pulse energy densities and different total doses, both corresponding to typical Bragg grating fabrication conditions. The Weibull distribution of the irradiated fibers depends on the pulse energy density and the total irradiation dose. Both a strong broadening and a reduction of the fiber strength were observed. With increasing total irradiation dose, the median breaking stress of the fiber decreases from 4.8 GPa for the pristine fiber to 2.5, 2.2, and 1.8 GPa for pulse energy densities of 50, 100 and 200 mJ/cm2, respectively, for a total dose of 1 kJ/cm2. At higher total irradiation levels the median breaking stress and hence the strain were found to be independent of laser fluence with a value of 2.7 GPa for 200 mJ/cm2.
Miniature all-fiber phase modulators have been fabricated by integrating piezoelectric actuator coatings with telecommunication optical fibers. Evaporation and sputter deposition processes were used to grow thin film PbZrxTi1 - xO3 (PZT) and ZnO actuator structures directly on the surface of optical fibers. When an electric field is applied to the piezoelectric coating, a strain is produced in both the coating and the optical fiber. The strain induced in the optical fiber causes a change in the refractive index through the photoelastic effect. Electric field tuneable optical phase modulation was achieved by utilizing the optical path length changes in the optical fiber that are induced by the integrated actuator structure. Processes similar to those used for coating optical fibers were also used make micro-tubes of PZT and ZnO between 20 and 30 micrometers in diameter. The piezoelectric layers were deposited onto polyester fibers, which were subsequently burned away to leave behind a micro-tube. Actuator structures have been produced and show promise for micro-electro- mechanical applications.
Optical low coherence reflectometry (OLCR) is applied to the precise characterization of Bragg gratings in optical fibers. Grating with lengths below two millimeters have been fabricated in standard telecommunication fibers. Their OLCR spectra can be used to determine the induced index changes, which range from 0.64*10-3 to 1.16*10-3, with an accuracy of a few percent. These changes are obtained by adjusting the position of the oscillation minima in the OLCR spectra.
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