This paper reports on two techniques for creating Fabry-Perot cavities in conventional single- and multi-mode optical fibres. The authors have reported previously on the design and fabrication of extrinsic fibre Fabry-Perot interferometric multi-functional sensors. Here, the authors report on two novel techniques for creating intrinsic fibre optic sensors based on the Fabry-Perot etalon. The first technique involved the use of hydrofluoric acid to preferentially etch the core of the optical fibre. This technique is simple to carry out and provides a cost-effective means for manufacturing intrinsic fibre Fabry-Perot sensors. In the second technique, a 157 nm excimer laser along with a custom-designed beam delivery system was used to ablate (micro-machine) near-paralleled walled cavities through the diameter of the optical fibre (outer diameter of 125 μm). The paper details the experimental methodology and the associated instrumentation for the two techniques. The acid etched and laser ablated cavities were characterised using a 3-D surface profiler, optical and scanning electron microscopy. The feasibility of using these cavities as intrinsic fibre Fabry-Perot strain sensors is demonstrated. This was achieved by surface-mounting the acid etched cavities on to composite tensile test specimens. The output from the optical fibre devices was compared with surface-mounted electrical resistance strain gauges.
Optical fiber-based extrinsic Fabry-Perot interferometric (EFPI) sensors have been extensively deployed for sensing a number of measurands including temperature, strain, vibration and pressure. Their circular cross-section has made it relatively simple and attractive to embed them in advanced fibre reinforced composites (AFRCs) such as glass and carbon fibers. However, a typical construction of an EFPI consists of two optical fibers that are positioned and secured within a precision bore capillary. The relative outer diameters of the various key components are as follows: capillary = 300 micrometers ; optical fibre = 125 micrometers ; carbon and glass fibers = 8 and 14 micrometers respectively. This mismatch in relative diameters of the reinforcing and the sensor fibers can result in significant spatial distortion of the former. The location of the embedded sensing fibre in relation to the reinforcing fibre layers can also lead to the formation of resin-rich regions in the AFRC. These factors can have a detrimental effect on the compressive properties of the material. Therefore, there is significant attraction in reducing the overall diameter of the sensor. In this current paper, the feasibility of reducing the diameter of EFPI sensor design to that of the optical fibre is demonstrated via two techniques. The first technique involved the use of hydrofluoric acid to etch and create the Fabry-Perot cavity. In the second technique, the feasibility of using laser ablation to fabricate the Fabry-Perot cavity in silica and sapphire substrates is presented. The optical fibre-based Fabry-Perot cavity produced via acid etching was interrogated using white light interferometry.
The processing of advanced fiber reinforced composites requires accurate data on the temperature and chemical composition of the resin system as a function of time. This paper reports on a preliminary study to evaluate two multi- functional sensor arrangements to facilitate in-situ chemical and temperature monitoring in epoxy resin-based thermosets. Sensor arrangements evaluated for chemical analysis include evanescent wave and transmission-based near-infrared spectroscopy; and for temperature monitoring optical fiber Bragg gratings. A comparative analysis was undertaken to ascertain the relative merits of each sensor configuration.
The authors demonstrate that several optical fiber Fabry-Perot sensors can be multiplexed in series for axial strain monitoring at each individual sensor. White light interferometry was employed using the laser-referenced Michelson interferometer of a standard Fourier transform spectrometer as a receiving (interrogating) interferometer. The primary aim was to demonstrate that at least six fiber Fabry-Perot transducer interferometers (sensors) can be multiplexed in series provided that each sensor has a unique optical cavity length within the multiplex. The resulting differing optical path differences at each fiber Fabry-Perot sensor give rise to sharp correlation features (side-bursts) at unique positions in the time domain as observed in the interferogram. An optical cavity length change due to an axial strain perturbation is observed as a change in the position in the time-domain of the side-burst feature associated with the fiber Fabry-Perot sensor. This paper demonstrates that multiplexed strain metrology in the quasi-static regime using fiber Fabry-Perot sensors is possible with a measurement range of typically 0 to 4000 microstrain and a strain resolution of better than 10 microstrain.
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