Fig. 1. Experimental setup of the proposed temperature sensor utilizing a passively MLFL combined with the beat frequency demodulation system.

Fig. 2. Output characteristics of passively MLFL when the ratio of l to L is 5 m/12.5 m. (a) Optical spectrum; (b) pulse sequence; (c) autocorrelation curve; (d) RF spectrum.

Fig. 3. (a) Measured repeated spectrum of the monitored BFS every 10 min for 50 min; (b) frequency stability of the monitored BFS at 5 GHz, 10 GHz, and 15 GHz, respectively.

Fig. 4. Measured spectra of BFS around (a) 5 GHz and (b) 15 GHz under different temperatures and the linear fit between the frequency shift and temperature around (c) 5 GHz and (d) 15 GHz when the ratio of l to L is set to 5 m/12.5 m, respectively.

Fig. 5. Measured spectrum of the BFS around 10 GHz when the ratio of l to L is (a) 5 m/12.5 m and (b) 10 m/17.5 m, and (c) the response curve of the frequency shift to temperature variation.