Suppression of fading in Ф-OTDR systems based on phase-modulated optical frequency combs

A new method of suppressing the fading effect of phase-sensitive optical domain-of-view reflectometer (Φ-OTDR) by using a phase-modulated frequency comb is presented in this paper. The distribution of fading points is different, so frequency division multiplexing (FDM) radiation can be used to suppress the fading effect. In the experimental system of this paper, firstly, frequency comb continuous light is generated by phase modulation; Then, different Rayleigh backscattering (RBS) intensity distributions are generated by pulse detection light modulating and injecting into the sensing fiber. Finally, the intensity distribution of RBS is extracted by amplitude estimation method. The phase is processed to eliminate the missing truth. The fading suppression is realized by our system, and the suppression effect is evaluated. By using a constant amplitude optical frequency comb with seven frequency components, the fading probability of the system is drastically reduced from 5.49%-9.83% to 0.08%. The method proposed in this paper has larger modulation bandwidth and more flexible frequency combination scheme, which can better suppress the fading effect. This method does not sacrifice the response bandwidth of the system, and can accurately control the phase delay, so it is very helpful to fully suppress the fading effect.