Michelson liquid-level sensor based on cascaded no-core fiber and single-mode fiber structure

Abstract

A novel Michelson sensor is proposed and applied to detect the liquid level. The sensor was fabricated by a section of no-core optical fiber (NCF) which is sandwiched between two single-mode fibers (SMFs). One single-mode fiber (SMF1) acts as light guide, and the tip of the other single mode fiber (SMF2) is coated with silver film as mirror. The high-order modes are excited when light travels from SMF1 to NCF and couple into the SMF2 at the NCF-SMF2 interface. The core and the cladding modes are reflected by the silver film of SMF2 and then couple twice at the NCF-SMF2 interface to form Michelson interference. Through simulation optimization and experimental verification, the optimized lengths of 15 mm and 30 mm are determined for NCF and SMF2, respectively. Interestingly, the NCF and SMF2 have the similar sensing responsivity in the liquid level sensing measurement. In the range of 15−90 °C, the sensor exhibits a temperature-insensitive characteristic. In the 0–44 mm liquid level range, the sensitivities of the deionized water, 5 % NaCl, 10 % NaCl and 15 % NaCl aqueous solutions are 216.16, 232.22, 253.41 and 276.87 pm/mm, respectively, and the refractive index sensitivity is 2223.5658 pm/mm/RIU. The sensor has clear interference fringes, simple structure, high liquid level sensitivity, good linearity and temperature insensitivity. It has the potential application in the field of small-scale liquid level monitoring.