Narrow rectangular slit in optical fiber core for ultrasensitive refractive index measurement

Optical fiber Mach-Zehnder interferometer based on a narrow rectangular slit in optical fiber core is presented for ultrasensitive refractive index measurement. The narrow rectangular slit plays the role as an air-cavity and is fabricated by using femtosecond laser to inscribe a group of waveguide array crossing the fiber within a rectangular area, followed by fast hydrofluoric acid etching. The light beam traveling in the fiber core is divided into two parts, one goes through the air-cavity and the other still travels via the remaining fiber core. The two parts of light beam recombine at the air-cavity end and generate a Mach-Zehnder interference pattern. The refractive index sensitivity achieved by such an interferometer device is -13954.05 nm/RIU (refractive index unit), within the refractive index range between 1.3333 and 1.3433. The device is featured with simple structure, compact size, strong robustness, low cost and convenient operation.