The liquid level monitoring with sub-millimeter resolution by discrete wavelength sampling method

The liquid level fiber sensor with a measurement range of about 50 cm is fabricated through a kind of single-no-core-single mode (SNS) structure. It is found that the change of liquid level will only induce the wavelength shifts of the whole interference spectrum. The liquid level monitoring with sub-millimeter resolution can be achieved by the proposed discrete wavelength sampling method. In the experiment, the transmit interference spectrum is sampled by a tunable laser with a step of 0.5 nm, then constructed by cubic spline interpolation algorithm. The wavelength shifts can be obtained through a minimum variance demodulation algorithm. Experiment results exhibit an excellent linear response (R2 = 0.99993) and the high sensitivity response to different liquid refractive index (RI). And the sensitivity of the sensor is 23.38 pm/mm with the liquid RI of 1.3505. In particular, the sensing resolution of the liquid level can reach 0.5 mm. This anti-electromagnetic interference technique with such high resolution and large sensing range can find the promising applications of monitoring the left fuel in the aircraft tanker, the surface evaporation in the water resource cycle, etc.