Seawater Temperature Measurement Based on a High-Birefringence Elliptic Fiber Sagnac Loop

A new method for seawater temperature sensing based on a high-birefringence elliptic fiber (HBEF) Sagnac loop is proposed in this letter due to the linear relationship between the change of temperature and the shift of interference peak. The transmission spectra of HBEF Sagnac loop at different seawater temperatures are studied theoretically and experimentally, which show good agreement with each other. The results show that sensing sensitivity increases with the increasing HBEF length and the decreasing wavelength, which is just opposite to the dependence of free spectrum range on the fiber length and the probing wavelength. Thus, there may exist an optimal fiber length and probing wavelength for the highest sensitivity and the largest detection range. By optimizing the parameters of probing system, the sensitivity of -472 pm/°C is obtained for temperature sensing, and the detection range from 10 °C to 30 °C is kept with the wavelength of 1310 nm (communication window) and the fiber length of 98.9 cm. Seawater temperature sensors demonstrated here show the advantages of small size, low cost, easy fabrication, high sensitivity, and easy integration with fiber systems.