Influence of extinction ratio of the modulator on fiber-optic distributed disturbance sensor

The signal models of the fiber-optic distributed disturbance sensor (FDDS) based on the optical time-domain reflectometry (OTDR) under the unsatisfactory modulator conditions are established in accordance with coherent lightwave theory. According to the mathematical models, the effects of the extinction ratio (ER) of the modulator on the sensor are simulated and investigated by experiment. The simulation results demonstrate that the Rayleigh backscattered coherent trace generated by the leaked light waves from the unsatisfactory modulator is an important source of noise. Thus the ER of the modulator is a critical factor that results in the location errors and the reduction of the signal-to-noise ratio (SNR). The SNR is less than 2 dB by subtraction of the traces with disturbances from the traces without disturbances and by moving averaging and moving differential methods, when the extinction ratios of the laser are less than 10.2 dB and 7.8 dB, respectively. It means that the two location methods fail to locate the disturbance. The voltage of the driver for the acousto-optic modulator (AOM) is adjusted to change the ER of the modulator thanks to the variation of the diffractive efficiency. Numerical results are proved by the experiment and the investigation results are helpful to select the modulator and improve the location precision of the sensor.