A Miniature Fiber-Optic Photoacoustic Probe Coated with MoS2-PDMS for Water Level and Oil-Water Interface Detection

We demonstrate a miniature broadband (~13.3 MHz) fiber-tip photoacoustic probe with $\pmb{\sim 68.32-\mu\mathrm{m}}$ thick MoS2-polydimethylsiloxane (PDMS) composite for accurate measurement of water and oil-water interface levels. The composite was applied to the tip of a double-clad fiber with an outer diameter of $\pmb{125 \ \mu \mathrm{m}}$ using dip coating. The propagation time of an ultrasonic wave generated by a 5-ns pulsed laser excitation in liquids was extracted to determine water level or silicone oil-water interface in a glass tube. An excellent goodness of $\pmb{\mathrm{fitt}(R^{2}\approx 0.996)}$ was achieved with a high accuracy of $\pmb{\sim 1.5\mu \mathrm{m}}$ in the tested water-level range of $\pmb{0-2181\mu\mathrm{m}}$ . Then the proposed technique exhibited an average relative error of 0.68% for the oil-water interface detection, along with a good fitting coefficient of 98.7 ^% in the range of 0–8 mm. The presented photoacoustic probe therefore shows a great potential in compact accurate liquid level monitoring.