Seawater Temperature Measurement System Based on Wavelength Sweeping and Cross Correlation Algorithm

A real-time seawater temperature measurement system based on a wavelength swept distributed feedback (DFB) laser is developed. The
$\pi $
phase-shifted fiber Bragg grating (
$\pi $
-FBG) with a narrow bandwidth of 12 pm is used as the sensing probe. The wavelength is swept by linearly and periodically modulating the driving current of the laser with a sawtooth wave signal. The absorption line of
$\text{H}^{{13}}\text{C}^{{14}}\text{N}$
gas cell provides a standard wavelength to calibrate the swept wavelength in real time to reduce the measurement error induced by the noises or drifts of the laser. Resonance difference between the measurement and the reference in time domain is obtained via a series of digital signal processing such as normalization, derivation, low-pass filtering, and cross correlation operation. Experimental results show that the sensitivity of the sensor is 16.22 pm/°C, the dynamic range is about 55 °C, the resolution is as high as 0.002 °C, and the thermal response time is
$\sim $
50 ms. The sensor system is compact, simple, high resolution, and large-dynamic range that has potential application for ocean temperature monitoring.