Intensity modulated optical liquid level sensor based on cascaded multimode-single-mode-multimode fiber structure

In this paper, a novel fiber-optic liquid level sensor is proposed and experimentally completed through a cascaded multimode-single-mode-multimode (MSM) structure with the same length of 25 mm. Two typical in-line Mach-Zehnder interferometers (MZIs) are respectively formed in each MSM structure, but more cladding modes are excited in the second MZI. A multimode-interference-like fringe then is observed, due to the mismatched energy of two MZIs. The comprehensive tests are performed in terms of liquid (water) level and temperature. The experimental results show that obvious intensity variation is demonstrated in a continuous liquid level measurement, but with a slight wavelength drift (about ±0.522 nm). The liquid level sensitivity reaches 0.261 dB/mm in the range from 0 to 40 mm with the linearity of 0.98. Inversely, the temperature response exhibits a flat wavelength shift. The wavelength sensitivity is 43.27 pm/°C and the intensity change is merely 0.01 dB/°C In the range from 24 to 58 °C. So the calculated temperature crosstalk is about 0.04 mm/°C and the measured error is compressed within 0.02%. It is obvious that, owing to such low crosstalk, our senor has the potential in high discriminative simultaneous measurement of engineering application.