Abstract

A new, to the best of our knowledge, type of liquid level sensor with $\pi$-phase-shifted long-period fiber grating operating near the phase-matched turning point is proposed. The sensor introduces a $\pi$-phase shift at the center of the grating, which can generate three attenuation peaks due to the mode interference effect in the transmission spectrum of the sensor operating near the phase-matched turning points. According to the different liquid level and temperature sensing characteristics of three attenuation peaks, simultaneous detection of liquid level and temperature can be realized through wavelength modulation and intensity modulation detection. In this paper, the structural parameters of this liquid level sensor, such as grating period and grating length, are designed optimally based on the fiber grating coupled mode theory, and the wavelength spacing of the attenuation peaks on both sides and the intensity of the center attenuation peak are analyzed in relation to the liquid level and temperature. The simulation results show that the liquid level sensitivity is 5.58 nm/mm and ${-}{1.316}\;{\rm dB/mm}$ for wavelength modulation and intensity modulation, respectively, for the liquid level variation range of 0 to 18 mm; and the temperature sensitivity of wavelength modulation and intensity modulation is ${-}{0.139}\;{\rm nm}/^\circ{\rm C}$ and 0.171 dB/°C, respectively, over the temperature variation range of 20°C to 80°C. Therefore the simultaneous detection of liquid level and temperature can be realized by establishing a measurement inversion matrix. Compared with other liquid level sensors, this sensor is expected to be widely used in the field of liquid level sensing due to its simple structure, high liquid level sensitivity, large liquid level measurement range, and narrow monitoring peak bandwidth.

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