Highly Improved Refractive Index Sensitivity in Fiber Mach-Zehnder Interferometer Based on Two Tapers

An optical fiber Mach-Zehnder interferometer based on two cascaded single-mode fiber tapers with an open cavity is proposed and demonstrated for refractive index sensing. The device is fabricated by firstly employing heating and pulling method to form two cascaded fiber tapers, followed by using femtosecond laser to inscribe a group of vertical line structure in the fiber cladding between the two separated tapers, and finally creating an open cavity with the help of hydrofluoric acid corrosion. The experimental results obtained show that the refractive index sensitivity achieved is ~1107.14 nm/RIU (refractive index unit) in the refractive index range of 1.338 - 1.368, which is more than 40 times larger than that of the Mach-Zehnder interferometer based on pure two tapers. This low-cost and highly sensitive optical fiber refractive index sensor has high potential in chemical or biochemical sensing applications.