Abstract:  In order to reduce the false alert or missed alert which is caused by polarization degradation of
interferometric optical-fiber early-warning system, the modulation process of intrusion signal is analyzed based on
the structure model of Mach Zehnder interferometric system. Two kinds of modulation signal distortion are
studied, and the solution, 3 ×3 coupler demodulation method is derived in detail. To meet the demand of
engineering application, a parameter correction method which can corrected demodulation distortion due to
unbalanced coupler is put forward. Simulation result in Simulink showed that the demodulation method can
restore phase signal accurately while the 3×3 coupler is unbalanced or polarization degradation occurred or the
signal contains high order harmonics and it is useful for engineering application of this system.

Key words:  optical-fiber sensor, demodulation, 3×3 coupler, polarization degeneration