Fig. 1. Sensing principle of four-channel detecting phase demodulation φ-OTDR based on 3×3 Michelson interferometer

Fig. 2. Scattering model of optical pulse

Fig. 3. Model of phase difference

Fig. 4. Diagram of demodulation principle

Fig. 5. Four-channel detecting phase demodulation system

Fig. 6. (a) Acquired three-channel signals E1, E2, E3 at 56 m; (b) comparison of E0 and calculated M, N

Fig. 7. Demodulation results under the unbalanced length of (a) 20 m, (b) 30 m, (c) 40 m, (d) 50 m

Fig. 8. Mean-subtracted signal of amplitude detection

Fig. 9. Relationship between demodulated phase and driven voltage of PZT