Fig. 1. Experimental setup of optic-fiber axial strain sensor based on multi-longitudinal mode beat frequency with Vernier effect

Fig. 2. Optical spectrums of the FBG and the FBG-based fiber linear-cavity fiber laser

Fig. 3. BFSs of linear-cavity 1 and 2

Fig. 4. Response of the BFSs of linear-cavity 1 to axial strain

Fig. 5. BFS when the linear-cavity 1 and 2 closed

Fig. 6. Response of the vernier envelop to axial strain under peak extraction method

Fig. 7. The frequency difference between peak extraction and RMS method and the response of the vernier envelop to axial strain under RMS method

Fig. 8. The stability of the BFSs at a constant axial strain