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An optic fiber-based sensor design is presented, suitable for use at high temperatures (up to 750 C) for measurement of the frequency of dynamic strain having wide magnitude range. The design and optimization of the location of the fiber sensor on a diaphragm structure, in two interferometer configurations is analyzed. The complete signal extraction and evaluation with signal processing electronics and DSP algorithm is realized in low power (under 15 mW). The challenge in the implementation of this technology is the need to perform under difficult industrial conditions namely, very large temperature range, high pressure dynamics, good chemical corrosion resistance, high measuring range, high sensitivity, vibration immunity, and low-power operation.Index Terms-Dynamic strain frequency measurement, flat diaphragm, linear phase demodulation, low-power signal processing, optic fiber interferometer, pseudo-heterodyne demodulation.

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Microstructured fibres (MSFs) reveal unique properties including endlessly single-mode operation from ultraviolet to infrared wavelengths, very high birefringence or nonlinearity, very large or very small effective mode field area, and many others. The size, shape and the location of the air holes allow for tailoring MSF parameters in a very wide range, way beyond the classical fibres, what opens up the possibilities for various applications. Due to their advantages MSFs obtain growing attention for their perspectives in sensing applications. Different MSF sensors have already been investigated, including interferometric transducers for diverse physical parameters. Until now, there have not been any publications reporting on the sensing applications of MSF Mach-Zehnder interferometers, targeting the mechanical measurements of vibrations, dynamic or static pressure, strain, bending and lateral force.Moreover, a critical feature opening the prospective of optical fibre transducer to successfully accomplish a particular sensing task remains its cross-sensitivity to temperature. Studied MSF is made of pure silica glass in the entire cross-section with a hexagonal structure of the holes. Consequently, there is no thermal stress induced by the difference in thermal expansion coefficients between the doped core region and the pure silica glass cladding, in contrast to standard fibres.In this paper we present the experimental comparison of mechanical and temperature sensitivities of MachZehnder interferometer with replaceable FC connectorized sensing fibre arm, such as: off-the-shelf endlessly single mode MSF or standard telecom single mode fibre. Experimental results clearly show very low cross-sensitivity to temperature of studied MSF compared with standard fibre. Additionally, microstructured fibre Mach-Zehnder interferometer with standard FC receptacles allows using different fibres as sensors with the same device. Moreover, investigated interferometer consumes in total extremely low electric power (< 20 mW) due to the implementation of exceptionally effective data analysis electronics and VCSEL as the light source.