Optimization of a long-period grating-based Mach–Zehnder interferometer for temperature measurement

An approach to optimize the design of the long-period grating pair as a temperature sensor device is presented, implemented by using a long-period grating (LPG) pair with a small separation (of around 2 mm) and scaling down their physical length by a factor greater than 2. The technique allows the interferometer formed not only to measure temperature variations over distances as small as the overall length of the grating pair (18 mm) but also to reduce the cladding losses between the LPGs forming the pair. This approach enhances the sharpness of the interference fringes (IFs) and the pits (Pts) in the transmission spectrum and, as a result, a high resolution sensor is obtained. The LPG pair is fabricated in the appropriate photosensitive single mode/core fibres, without being restricted to the use of dual core or other special fibres, thus exploiting the sensitivities of various fibres and reducing the overall system cost. In this work, the effectiveness of this technique is demonstrated by fabricating a small-scale LPG pair in a boron–germanium co-doped single mode fibre, with particular attention being paid to the higher order cladding modes. The sensitivity of the device thus created is 0.31 nm/°C with a root-mean-square (rms) deviation of 0.28 nm in the wavelength measurement, which corresponds to a temperature variation of approximately 0.9 °C. This was achieved while using a relatively low-resolution (0.6 nm) Optical Spectrum Analyzer to detect the wavelength changes of the device and was further improved to 0.7 °C when using an OSA with a resolution of 0.1 nm.