In-fiber Mach-Zehnder interferometer based on fiber core etched air-bubble and core diameter mismatch for liquid refractive index sensing

A kind of optical fiber liquid refractive index sensor is proposed based on fiber core etched air-bubble and core diameter mismatched in-fiber Mach-Zehnder interferometer. A core etched standard single-mode fiber is spliced to another core etched standard single-mode fiber (SMF) to form an air bubble at the connecting point, and a 20 mm thinned fiber is cascaded (TCF) by fusion splicing method, which is 20 mm apart from the bubble, then a leading-mode fiber is spliced to the thinned fiber to form a structure of SMF-(air-bubble)-SMF-TCF-SMF in-fiber Mach-Zehnder interferometer liquid refractive index sensor. The air-bubble and the two fiber core diameter mismatched points serve as optical couplers for modes conversion. The transmission spectrum of sensor is studied by experiment. Results shows that the peak power changes with respect to surrounding refractive index with good linearity. The sensitivity of the sensor is 142.537 dB/RIU in the range of 1.3511.402 with linearity of 0.996, making it a good candidate for bio-chemical measurements.