Highly sensitive temperature sensor based on an ultra-compact Mach–Zehnder interferometer with side-opened channels

We demonstrated an ultra-compact and highly sensitive temperature sensor by using an in-fiber Mach–Zehnder interferometer (MZI) with side-opened channels. The MZI was constructed by off-center splicing a thin piece of microstructured optical fiber (MOF) between two single-mode fibers. Then, two side-opened channels were drilled through the MOF along the transverse direction by using a femtosecond laser to let the liquid with a high thermo-optic coefficient enter into the air cavity of the MZI. Due to the large effective refractive index (RI) difference between the core mode and the cavity mode excited by the offset splicing point, the scale of the MOF-based MZI can be reduced to several hundred micrometers. Experimental results show that the MOF-based MZI infiltrated with isopropanol has a temperature sensitivity up to 3.8 nm/°C with small strain cross-sensitivity of 0.0001 °C/μϵ in the temperature range of 22°C–34°C, which makes it a competitive fiber sensor in highly sensitive temperature sensing applications, including healthcare and consumer electronics.