Temperature Sensing Using Frequency Beating Technique From Single-Longitudinal Mode Fiber Laser

In this paper, a high-resolution fiber temperature sensor is proposed and demonstrated using the frequency beating technique. The sensor uses a constant wavelength (CW) at 1539.96 nm as the reference signal for the frequency beating technique. The sensor signal is provided by a fiber Bragg grating (FBG) tuned single-longitudinal mode (SLM) fiber laser, which consists of a 0.5-m long highly doped Zirconium-Erbium doped with an erbium concentration of 3000 ppm as the gain medium. The signal of the SLM, which is generated by the FBG in response to external temperature changes, is mixed with the CW signal using a 3-dB fused coupler into a 6-GHz photodetector to generate frequency beating. The typical response of the system is about 1.3 GHz/°C, with nominal temperature measurement resolutions of 0.0023°C being achieved, taking into account the resolution bandwidth of 3 MHz of the radio frequency spectrum analyzer.