High Accuracy Measurement of Salinity and Temperature Based on Tilted Grating Concatenated Sagnac Interferometer and ResNet Network

The measurement of salinity and temperature is vital for ocean observation. However, due to the cross-sensitivity phenomenon of fiber sensors, traditional fiber-based demodulation algorithms are difficult to measure the dual-parameter accurately and simultaneously. In this research, a cascaded structure of a tilted fiber Bragg grating (TFBG) and a Sagnac interferometer (SI) is employed to monitor salinity and temperature simultaneously. The transfer learning-based ResNet networks (TFRes) is proposed to demodulate two variables at the same time. A total of 1680 spectral samples were collected for training and testing when the sensor was immersed in sodium chloride solution with different salinities ranging from 0% to 25% with 5% intervals and temperatures ranging from 20 °C to 32 °C with 3 °C intervals. The SI exhibits a temperature sensitivity of −1.458 nm/°C, accompanied by a mean absolute error (MAE) of 0.28 °C. The sensitivity of the TFBG core mode to temperature is 0.009 nm/°C, and the sensitivity of one of the cladding modes to salinity is 0.004 nm/%. After training through the TFRes, we successfully achieved precise demodulation of salinity and temperature variations by analyzing the composite reflection spectra of SI and TFBG. The MAE amounted to 0.07304 °C for temperature and 0.07285% for salinity, outperforming traditional analyzer demodulation methods by a factor of four. The monitoring experiment of dual parameter simultaneous changes is conducted, with a temperature error of 0.18 °C and a salinity error of 0.1%. The designed sensing system is poised to play a significant role in marine physical quantity monitoring.