Fig. 1. Schematic diagram of the proposed fiber laser displacement sensor.

Fig. 2. Fiber laser cavity’s excited longitudinal modes controlled by π-FBG’s bandwidth.

Fig. 3. (a) Optical spectra of Filter, FBG and π-FBG; (b) optical spectrum of the compound ring laser sensor cavity.

Fig. 4. Beat frequency spectra of BFS.

Fig. 5. (a) Beat frequency spectrum of straight cavity; (b) beat frequency spectrum of compound ring laser cavity.

Fig. 6. Beat signal spectra shifts as a function of displacement variations.

Fig. 7. Measured results analysis of the displacement sensor.