In order to measure strain independently from temperature, hybrid solution based on a polarimetric and chirped Fiber Bragg Grating (FBG) sensors is proposed. The sensor is designed in a reflective configuration, where the chirped FBG is written on a highly birefringent (HB) fiber. The FBG act as a sensing element and also as a mirror for the polarimetric sensor. Information from both polarimetric and FBG part of the sensor can be determined independently from spectral analysis of the reflected light. Strain and temperature sensitivity of the proposed sensor solution is measured. Relation between both sensitivities are different for the FBG and the polarimetric sensor. Taking advantage of this, both temperature and strain can be determined by using only one sensing fiber.
A strain sensor that operates in the intensity domain by converting the wavelength information from the fiber Bragg grating sensor, into intensity variation is presented in this paper. The fiber-optic sensor system involves a highly birefringent fiber as a demodulation system and a FBG sensor which is used for strain measurement.
Composite structures are made from two or more constituent materials with significantly different physical or chemical properties and they remain separate and distinct in a macroscopic level within the finished structure. This feature allows for introducing highly birefringent polymer microstructured optical fibers into the composite material. These new fibers can consist of only two polymer materials (PMMA and PC) with similar value of the Young modulus as the composite material so any stresses induced in the composite material can be easily measured by the proposed embedded fiber optic sensors.
Results of our research on embedded highly birefringent polymer microstructured fibers are presented. A composite material sample with fibers embedded between two layers of a multi-layer composite structure is fabricated and characterized. Temperature sensitivities of the polymer fibers are measured in a free space and compared with the
fibers embedded in the composite material. It appeared that highly birefringent polymer microstructured fibers exhibit a strong increase in temperature sensitivity when embedded in the composite material, which is due to the stress-induced changes in birefringence created by thermally-induced strain.
Silica-based HB fibers have severe limitations due to their coating layers while embedded into a composite: the hard coating layer easily transmits radial stress to the sensing fiber and changes its birefringence. Two coating layers – hard and soft – attached to the HB fiber do not influence fiber birefringence since the second (soft) layer can easily absorb any lateral force. On the other hand, a soft coating does not provide any proper transmission of the longitudinal strain. Additionally, fused-silica fibers have an upper strain limit of approximately 2% strain. In highly loaded engineering structures structural monitoring strain is becoming increasingly important. Hence, soft polymer materials used in the manufacturing process of highly birefringent microstructured polymer optical fibers (mPOFs) can solve this limitation. In this paper we present interactions between a composite material and mPOFs during the manufacturing process. The lamination process can dramatically change the group birefringence of the mPOFs. Measurements for fiber embedded in composite materials and fiber in free space were made and compared. A simple explanation of these differences is presented at the end of the paper.
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