Ultracompact and high sensitive refractive index sensor based on Mach–Zehnder interferometer

•We have demonstrated an ultracompact and high-sensitive refractive index sensor based on MZI structure.•The sensor realizes relatively high sensitivity and resolution.•It has the advantages of real-time monitoring, high precision, low cost, and simple fabrication process.

We report an ultracompact and high sensitive refractive index (RI) sensor based on Mach–Zehnder interferometer (MZI) structure. The sensor was assembled using flexible poly(trimethylene terephthalate) nanofiber with a diameter of 700 nm. The valley wavelength shifts of the output spectra of the sensor caused by the variation of ambient medium around the MZI structure region were measured. In the RI range from 1.34 to 1.39, the highest measured sensitivity of the sensor is about 1100 nm/RIU (RI unit), which is 6 times higher than that of S-like abrupt taper based MZI sensors, and the RI detection limit is about 1.8×10−6. The sensor allows the construction of a low-cost, compact, simple, and highly sensitive sensing system.