Chen-Chun Ye

Simultaneous temperature and bend sensing with long-period fiber gratings

Long-period gratings (LPGs) written in commercially available boron-codoped fibers operating at wavelengths of <1.1mum are shown to exhibit high temperature and bending sensitivities. Each resonant attenuation band of such LPGs was observed to split in two when the LPGs were bent. The split attenuation bands separation increased significantly with increasing bend curvature, and the central wavelengths of the split bands provided a measure of temperature. We exploit this effect to allow for simultaneous measurement of temperature and bending in smart-structure applications. The demonstrated novel sensor system is simple and low cost.

Characterization of the response of fibre Bragg gratings fabricated in stress and geometrically induced high birefringence fibres to temperature and transverse load

The transverse load and temperature sensitivities of fibre Bragg gratings (FBGs) fabricated in a range of commercially available stress and geometrically induced high birefringent (HiBi) fibres have been experimentally investigated. The wavelength reflected by the FBG in each polarization eigenmode was measured independently and simultaneously using a custom designed interrogation system. The highest transverse load sensitivity, of 0.23 ± 0.02xa0nm/(N/mm), was obtained with HiBi FBGs fabricated in elliptically clad fibre. This was higher than for any other HiBi fibre, which, coupled with the small diameter of the fibre, makes it a good candidate for an embedded or surface mounted strain sensor. The highest temperature sensitivity of 16.5 ± 0.1xa0pmxa0°C−1, approximately 27% greater than any other fibre type, was obtained with the HiBi FBG fabricated in Panda fibre. HiBi FBG sensors fabricated in D-clad fibre were the only ones to exhibit identical temperature sensitivities for the slow and fast axes (11.5 ± 0.1xa0pmxa0°C−1).

A polarization-maintaining fibre Bragg grating interrogation system for multi-axis strain sensing

In this paper we report the development of a FBG (fibre Bragg grating) interrogation system, that is capable of detecting independently the two orthogonally polarised signals reflected from a FBG fabricated in a PM (polarisation maintaining) fibre. The interrogation system imposes no limitation on the measurable strain. It is possible to detect a strain gradient by measuring independently the spectral broadening of the two polarised signals. The system has been used to fabricate and characterise FBGs in different PM fibres. As a demonstration of the feasibility of the interrogation system for multi-axis strain sensing, the transverse strain sensing characteristics of PM FBGs are presented.

Strain development in curing epoxy resin and glass fibre/epoxy composites monitored by fibre Bragg grating sensors in birefringent optical fibre

Fibre Bragg gratings (FBGs) fabricated in linearly birefringent fibres were embedded in glass fibre/epoxy composites and in the corresponding unreinforced resin to monitor the effective transverse strain development during the cure process. The optical fibres containing the FBG sensors were aligned either normal or parallel to the reinforcement fibres in unidirectional glass fibre/epoxy prepregs. The chemical cure kinetics of the epoxy resin system used were studied using differential scanning calorimetry, in order to investigate the correlation between the strain monitoring results and the evolution of the curing reaction. A non-parametric cure kinetics model was developed and validated for this purpose. The effective transverse strain measured by the FBGs demonstrated high sensitivity to the degree of cure as a result of the densification of the resin caused by the curing reaction. The effective compressive transverse strain developed during the reaction, and thus the corresponding sensitivity to chemical changes, was higher in the case of the sensing fibre aligned normal to the reinforcement fibres than in the case of the sensor fibre parallel to the reinforcement fibres. Small but measurable sensitivity to cure induced changes was observed in the case of the unreinforced resin.

A polarisation maintaining fibre Bragg grating interrogation system for multi-axis strain sensing

In this paper we report the development of a FBG (fibre Bragg grating) interrogation system, that is capable of detecting independently the two orthogonally polarised signals reflected from a FBG fabricated in a PM (polarisation maintaining) fibre. The interrogation system imposes no limitation on the measurable strain. It is possible to detect a strain gradient by measuring independently the spectral broadening of the two polarised signals. The system has been used to fabricate and characterise FBGs in different PM fibres. As a demonstration of the feasibility of the interrogation system for multi-axis strain sensing, the transverse strain sensing characteristics of PM FBGs are presented.

Thermal dependence of the strain response of optical fibre Bragg gratings

The temperature dependence of the strain sensitivity of fibre Bragg gratings written into a number of different fibre types has been investigated. It was found that the strain response changed on average by 0.21 ± 0.03 fm µe−1 °C−1 over a range of temperatures between 100–400 °C. These results were in agreement with predictions based on material parameters.

The influence of hydrogen loading and the fabrication process on the mechanical strength of optical fibre Bragg gratings

Abstract This paper investigates the influence of hydrogen loading and other stages of the fabrication process on the mechanical strength of fibre Bragg gratings. Following UV irradiation, tensile tests were carried out on Ge–B codoped photosensitive fibres with and without hydrogen loading. Fibre Bragg gratings (FBGs) were written using a range of UV wavelengths, namely 246, 255 and 266 nm. The tensile strength of the optical fibres was determined in their as-received status and following the various stages of FBG fabrication. The mechanical strength was assessed using Weibull statistics. The results indicate that the strength of FBGs is influenced by the UV irradiation parameters and by the hydrogen-loading process. FBGs fabricated using shorter UV wavelengths and low pulse power intensity exhibit a high mechanical strength. The FBGs written in hydrogen loaded fibres have less than 50% of the strength of FBGs that have not been hydrogen loaded. Fibre fracture morphology observed by scanning electron microscope reveals fracture mechanisms of FBGs, which are correlated with the structural change of the silica fibres induced during the FBG inscription process. Combined with surface information gathered by atomic force microscopy, fracture mechanics is applied to understand the mechanisms of strength degradation caused by the FBG inscription process.

Ultrasonic sensing using Yb3+/Er3+-codoped distributed feedback fibre grating lasers

Yb3+/Er3+-codoped distributed feedback (YbEr-DFB) fibre grating lasers have been investigated for the detection of ultrasonic Lamb waves generated in an aluminium plate. Two sensor configurations have been investigated. A path-length-imbalanced readout Mach?Zehnder interferometer was used to measure the laser frequency shift induced by ultrasonic vibration. The resolution to out-of-plane displacement was 3 ? 10?15?m?Hz?1/2 for a continuous ultrasonic wave at a frequency of 470?kHz. Ultrasonic waves were also detected by directly monitoring the laser output power. This simplified configuration is useful for tone burst AE signals generated by a low energy impact.

Bend sensing in structures using long-period optical fiber gratings

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.

Influence of process route on mechanical and sensing performance of fiber Bragg grating optical sensors

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.