Temperature and transverse load sensitivities of fibre Bragg gratings (FBGs) fabricated in a range of commercially available high birefringent (HiBi) fibre were measured and compared for the 1550 nm wavelength region. The highest transverse load sensitivity, of 0.23 ± 0.02 nm/(N/mm), and temperature sensitivity, of 16.5 ± 0.1 pm/0C, were obtained with FBGs fabricated in elliptically clad and Panda fibres respectively. The greatest differential transverse load and temperature sensitivities were measured between the eigen axes of the bow tie and elliptical clad fibres respectively. The FBGs fabricated in bow tie fibre were successfully used to monitor the transverse strain development during the cure process of glass fibre/epoxy composites and of unreinforced resins. It was observed that the development of transverse strain was sensitive to the degree of cure of the resin. A FBG fabricated in single mode fibre was also used to monitor the predominantly axial strain development, during the cure of a glass fibre composite, for comparison with transverse strain measurements.
Fiber Bragg gratings (FBGs) written into polarizations maintaining (PM) fibers have been employed for multi-axis strain and temperature sensing. In this paper we report the development of a FBG interrogation system that is capable of detecting independently the two orthogonally polarized signals reflected from a PM FBG. The interrogation system imposes no limitation on the measurable strain range. This relaxes the requirements on the bandwidth of the grating spectrum, allowing the use of a shorter PM FBG (larger bandwidth) for sensing applications where a higher spatial resolution is required. In particular, this system is useful in the cases where the spectra of the reflected polarized signals are broadened or split, resulting from strain-induced structural changes of the PM fibre, or caused by the application of a non-uniform strain on the FBG.
In this paper, an all fibre compatible laser Doppler velocimeter featuring a fibre-optic filter device for frequency discrimination is presented for the first time. The technique is based on the fabrication of a Fabry-Perot filter using in-fibre Bragg gratings for measurement of the in-line velocity component. The choice, design and fabrication of the in-fibre filters are discussed. Problems that affect the stability of the system are addressed. These include the design and implementation of two-independent active feedback control loops for the argon-ion laser wavelength and phase of the fiber-optic filter. An optical fibre-linked probe head, constructed in-house, is used to couple the laser beam at 514.5 nm to and from the measurement volume. Fast photomultiplier tube detectors are used and signal capture and processing is performed on a digital storage oscilloscope (300 MHz) or on a PC. The system is successfully demonstrated by measuring the velocity of a rotating disc (10-cm diameter) over a velocity range of +/- 15 ms-1 with a resolution of 0.2 ms-1.
Long-period fiber gratings (LPGs) written into commercially available boron co-doped fibers operating at wavelengths < 1.1 micrometers are shown to exhibit high temperature- and bending-sensitivities. Each resonant attenuation band of such a LPG was observed to split into two when the LPG was bent. The split attenuation bands' separation increased significantly with increasing bend curvature, while the central wavelengths of the split bands provided a measure of temperature. This effect is exploited to allow simultaneous measurement of temperature and bending in smart structure applications. The demonstrated novel sensor system is simple and low cost.
When a long-period fibre grating (LPG) is bent, each resonant attenuation band of the transmission spectrum is observed to split into two. The wavelength separation of the split attenuation bands increases significantly with increasing bend curvature. Based on the recent observation of this novel effect1, a high sensitivity structural bend sensor has been developed. The LPGs were bonded to the surface of a steel plate using an unsaturated polyester adhesive, and their bend sensing characteristics examined. Pairs of concave and convex testing jigs with a bending curvature ranging from 0.1 to 2m1 were used to apply accurate bending curvature to the steel plate. The LPGs responses to bending in air and in the polyester adhesive are compared.
Fiber Bragg gratings were fabricated in commercially available photosensitive optical fiber, using a phase mask and UV irradiation at 245.6 nm and 266 nm. Prior to irradiation, the acrylate buffer coating was removed from the fiber using different stripping methods: chemical solvent, hot-acid and heat. The tensile strength of the fiber was measured at various stages of the fabrication process: with buffer coating intact, with the buffer coating chemically removed, and following UV irradiation. A Fabry-Perot demodulation system monitored the wavelength shift of the reflected peak from the FBG during the tensile loading, from which the strain to failure of FBG sensors has been determined. Each set of strength data has been plotted by applying the Weibull statistics. The results indicate that the removal of the buffer coating degrades the strength of optical fibers. The Weibull analysis indicates two failure mechanisms. Bare fibers show a high Weibull modulus, above 24, if no mechanical flaws have been introduced, while the fibers with surface flaws fail at low stress with the Weibull modulus as low as 2.78. UV irradiation has been found to further reduce the fiber strength. Most irradiated fibers fractured within the sensor gauge length. It is believed that a stress concentration or surface stress is induced by the UV energy, which results in mechanical degradation.
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