Sensitivity enhancement of long period gratings for temperature measurement using the long period grating pair technique

An approach to enhance the sensitivity achievable with long period grating (LPG) technology for temperature measurement, by using a LPG pair technique to create Mach-Zehnder interferometers written into B–Ge co-doped optical fibres, is presented. The separation of the single LPGs constituting the pair is kept very short with, as a consequence, LPGs being written with high coupling coefficients, implying a significant change in the differential effective group refractive index of the fibre. This allows the temperature-induced wavelength shift of the interference fringes (IFs) of the LPG pair to change at a faster rate than the LPG envelope, due to the consequent increasing phase change of the core modes, with respect to the cladding modes within the grating region as a function of temperature and wavelength variations. A brief theoretical explanation is given and an experimental demonstration is shown by comparing the characteristics of two separate LPG pairs (LPGP1 and LPGP2), where LPGP1 comprises two ‘strong’ LPGs, while LPGP2 comprises two ‘weak’ LPGs. Results obtained have shown that the sensitivity of the change of the IF position in LPGP1 occurs at a faster rate than its envelope, whereas for LPGP2, this shifts at a similar rate to the envelope of the spectrum. A simple mathematical approach is suggested to calculate the wavelength shift based on the phase change variation in LPGP1, using basic Fourier analysis. The sensitivity enhancement obtained in the experimental results from LPGP1 to LPGP2 was determined to be ∼50%, from a phase shift of 2°/°C for the IF of LPGP2 to a phase shift of 3°/°C for the IF of LPGP1, with a root-mean-square (rms) deviation of 1.9°, corresponding to a rms error in temperature of 0.6 °C.