Ultra-compact in-core-parallel-written FBG and Mach–Zehnder interferometer for simultaneous measurement of strain and temperature

An ultra-compact in-core-parallel-written fiber Bragg grating (FBG) and Mach–Zehnder interferometer (MZI) for simultaneous measurement of strain and temperature is described. The FBG and MZI are written spatially parallel in the same section of fiber core using a femtosecond laser, forming an ultra-compact device, which is different from the previously developed axial cascade of different structures. Due to the weak coupling between the FBG and the MZI, their individual extinction ratios are traded off by optimizing their writing position and separation, and extinction ratios of 5.9 dB for the FBG and 10 dB for the MZI are achieved. Experimental results show that the FBG and MZI have different sensitivities for strain and temperature, allowing this device to measure strain and temperature simultaneously. In addition, since both the FBG and MZI are written in the fiber core, this ultra-compact device is proven to be impervious to ambient humidity, making it a promising candidate for accurate industrial strain and temperature measurements.