Miniaturized Tapered Photonic Crystal Fiber Mach–Zehnder Interferometer for Enhanced Refractive Index Sensing

Miniaturized Mach–Zehnder interferometer (MZI) is constructed using sharply tapered photonic crystal fiber (PCF) for highly sensitive ambient refractive index (RI) measurement. The sensor is fabricated by fusion splicing a small stub of PCF between standard single-mode fibers with fully collapsed air holes of the PCF in a splicing region. The influence of sharp tapering of the PCF is then studied using two different hydrogen flame diameters of 1 and 3.5 mm at the tip. Tiny flame geometry enables the sharp tapering of the PCF for a short fiber length and provides higher RI sensitivity. It appears that sharp tapering has a greater impact on RI sensitivity enhancement compared with decrease in taper waist diameter. The MZI with the taper waist diameter of $65~\mu \text{m}$ and the length of 3.8 mm offers an RI sensitivity of 334.03, 673.91, and 1426.70 nm/RIU within the RI range of 1.3327–1.3634, 1.3634–1.3917, and 1.3917–1.4204, respectively.