The phase-sensitive optical time domain reflectometer (Ф-OTDR) system can quantitatively characterize the vibration events along the sensing fiber by extracting the phase information of Rayleigh scattered light, which features a fast response up to milliseconds or sub-milliseconds and a detection sensitivity of nanostrains. Ф-OTDR has been widely used in sensing scenarios such as seismic wave monitoring, perimeter intrusion monitoring, and pipeline leak monitoring. Due to the interference fading effect in the Ф-OTDR system, the phase information extracted at the lower signal amplitude is distorted, which results in the incorrect response of the intensity and frequency of the vibration event. Then, it introduces frequent false alarms in practical engineering applications. Over the past decade and beyond, tremendous efforts have been devoted to addressing this issue, typically including the utility of special optical fibers (i.e., seven-core fiber and periodic ultra-weak Bragg grating array), complex demodulation algorithm (i.e., phase-shifted double pulse method and tracking algorithm), and multi-frequency pulse modulation [i.e., phase-shifted double pulse method and multi-branch acoustic-optic modulator (AOM) modulation]. From an applicative point of view, a simple multi-frequency pulse modulation Ф-OTDR system for suppressing the interference fading effect, which features flexible controlling of the frequency component of the probe optical pulse without sacrificing the response bandwidth and spatial resolution, is still a research gap to date.

In this paper, for the first time (to the best of our knowledge), an AOM for generating multi-frequency probe light is employed in the Ф-OTDR system. The modulation frequency interval and number can be flexibly controlled by an arbitrary waveform generator (AWG) within the operating bandwidth of the broadband AOM. Subsequently, the continuous multi-frequency probe light is modulated into pulsed light through a general AOM. The multi-frequency beat signals can effectively suppress the inherent interference fading effect along the sensing fiber by appropriate filtering, demodulation, and multiplexing; ultimately realizing the high-fidelity demodulation of the Ф-OTDR system. We believe that the proposed scheme can provide a practicable way toward a simple and compact structure, precise phase delay control, and flexible and controllable frequency components without sacrificing response bandwidth and spatial resolution to suppress the interference fading effect in Ф-OTDR.

The multi-frequency modulation principle and experimental schematic diagram are shown in Fig. 2 and Fig. 3, respectively. We utilize the AWG to generate three unequal interval radio freqency (RF) signals, including the frequency components of 340, 390, and 450 MHz. The multi-frequency modulation RF signal is simultaneously loaded on the broadband AOM. Then, the multi-frequency continuous light is modulated by an AOM with a fixed drive frequency of 300 MHz. The multi-frequency probe light has the characteristics of a down-shift frequency of 300 MHz, pulse width of 100 ns, and repetition frequency of 10 kHz. A 2 Gsa/s data acquisition card acquires the multi-frequency beat signals. The 40, 90, and 150 MHz beat signals are obtained by digital bandpass filtering, respectively, as shown in Fig. 4. Figure 5 verifies that the beat signals of different frequency components present various intensity distributions, which also further indicates the different interference fading locations versus different frequency components. A piezoelectric ceramic transducer (PZT) simulates vibration events across the sensing fiber. The experimental results of the vibration events cannot be effectively reconstructed by the generic method with a single-frequency probe Ф-OTDR, as shown in Fig. 6. It can be seen that the reconstructed phase information originating from 90 MHz beat signal is distorted at the positions of the low amplitude points. Figure 7 shows the experimental results of the vibration events via the proposed multi-frequency modulation Ф-OTDR. More specifically, the high-fidelity reconstructed phase information is obtained by multiplexing three-frequency beat signals based on the amplitude intensity evaluation criteria. The demodulation results of 25 Hz are consistent with the frequency of the PZT vibration signal. The axial strain generated by the fiber is 160 nε. The beat signals of three different frequencies are multiplexed by amplitude intensity evaluation for the two reference points of any phase reconstruction interval. The suppressed results of the interference fading effect are evaluated using 10% of the normalized intensity as the threshold. Figure 8 shows that the probability of the interference fading effect is effectively reduced from 17.541% to 1.123%. In addition, π phase shift is introduced into the spectrum of beat signals of 40, 90, and 150 MHz, respectively. The generating time-domain signal intensity distribution is inconsistent with those of the original signals, as shown in Fig. 10. Figure 11 shows that the probability of interference fading effect is further reduced to 0.045% after all six sets of beat signals are multiplexed.

In conclusion, we propose a novel multi-frequency pulse modulation Ф-OTDR scheme based on broadband acoustic-optic modulation. The proof-of-principle experimental results indicate that the probability of coherent fading can be reduced from 17.541% to 0.045%. Furthermore, this method offers the advantages of a simple and compact pulse modulation structure, precise phase delay control, and a flexible and controllable pulse frequency component without sacrificing the response bandwidth and spatial resolution in Ф-OTDR. We believe that our work provides a practical way toward distributed fiber optic acoustic and vibration sensing, such as seismic wave monitoring, perimeter intrusion monitoring, and pipeline leak monitoring.