Due to the low scattering coefficient of Rayleigh backscatterings and fading noise, a distributed acoustic sensing (DAS) system suffers from detection noise of high level and non-uniform distribution along sensing fiber. Advanced fiber microstructure manipulation technology gives birth to a type of quasi-distributed acoustic sensing (qDAS). This technology employs a fiber array consisting of equally spaced weak reflection points such as weak fiber Bragg gratings (wFBG) or microstructures introduced by laser exposure. By the detection of interrogation optical pulses rather than Rayleigh backscatterings, the qDAS is free of fading noise. Additionally, the reflectivity of a weak reflection point is typically 3.16×10^-5 (-45 dB). Such reflectivity not only improves the signal-to-noise ratio of detected optical signals compared with that in a DAS system, but also reduces transmission loss and gets rid of crosstalk induced by multiply-reflected optical pulses due to low reflectivity. Therefore, this type of qDAS is promising in large-scale and high-fidelity sensing applications. Unfortunately, the interrogation optical pulses generate Rayleigh backscatterings during transmission in a fiber array. The Rayleigh backscatterings are collected together with the interrogation optical pulses. By taking an optical pulse of 100 ns in width as an example, the reflectivity of the induced Rayleigh backscatterings reaches 2×10^-6 and is comparable to that of a weak reflection point (3.16×10^-5). Since Rayleigh backscatterings carry sensing information, they introduce crosstalk between adjacent sensing channels in a qDAS system, which severely distorts the sensing signals and degrades the accuracy of sensing event positioning. Taking a wFBG-based qDAS system as an example, we study a method that suppresses the crosstalk induced by Rayleigh backscatterings. We hope that our study can help improve the sensing fidelity, sensing scale, and sensing positioning accuracy of a qDAS system, and expand the applications of qDAS.

The phase interrogation scheme is presented in Fig. 1. A dual-pulse direct detection scheme with a high heterodyne frequency is employed for phase interrogation in a wFBG-based qDAS system, and the polarization switch method is adopted to eliminate polarization fading noise. A phase modulator is applied after a laser source to suppress crosstalk induced by Rayleigh backscatterings. Driven by an electrical cosinusoidal signal, the phase modulator introduces a cosinusoidal phase modulation Ccos2πfmt to the coherent continuous light wave emitted from the laser source. Two optic probe pulses with equal pulse width W=100 ns, frequency difference Δν=20 MHz, and optical path difference n_eL_MZ are launched in a pair into a wFBG array, where n_e=1.5 and L_MZ=29.8 m are effective refractive index and fiber length difference of M-Z interferometer that generates two optic probes respectively. By carefully designing modulation frequency f_m to satisfy πfmτ=kπ,k=0,1,2,?, crosstalk can be suppressed, where k is integer, τ=neΔL/c, and ΔL=2L-LMZ. Additionally, larger magnitude C of phase modulation leads to better crosstalk suppression.

The validity of the proposed approach is experimentally verified. A fiber array consisting of seven wFBGs is under interrogation and a PZT is placed between the second and third wFBGs to simulate a cosinusoidal acoustic signal oscillating at 1 kHz. wFBGs are equally spaced by L=15.5 m to make ΔL= 1.2 m. An electrical cosinusoidal signal oscillating at f_m and a magnitude of 2 V is applied to the phase modulator to induce cosinusoidal phase modulation. By the design of f_m=166 MHz and ΔL=1.2 m, πfmτ=0.996π is realized. The demodulated signals along the fiber array are obtained, and the mean power spectrum density (PSD) over ten measurements at 1 kHz along the fiber array is presented (Fig. 7). Compared with the crosstalk of -21.82 dB without cosinusoidal phase modulation, the crosstalk in the case of cosinusoidal phase modulation is reduced to -35.22 dB. Under power boost of 10 dB, the electrical cosinusoidal signal is applied to the phase modulator, and the PSD at 1 kHz along the fiber array is also presented (Fig. 7). It is shown that the crosstalk is further suppressed by 23.85 dB and is -45.67 dB. The experimental results reveal that the cosinusoidal phase modulation approach can reduce the crosstalk, and the larger magnitude C of phase modulation leads to better crosstalk suppression.

We propose a cosinusoidal phase modulation approach to suppress crosstalk induced by Rayleigh backscatterings in a qDAS system. The hardware implementation of this technology is simple, and only a phase modulator and a cosinusoidal electrical signal source are applied behind the laser. Cosinusoidal phase modulation is adopted to the high-coherent laser, and crosstalk can be suppressed by carefully setting the phase modulation frequency. Larger phase modulation amplitude leads to better crosstalk suppression. The validity of the proposed approach is confirmed by 23.85 dB crosstalk suppression in the experiment. The proposed cosinusoidal phase modulation approach for crosstalk suppression is suitable for a variety of phase demodulation methods such as high-frequency heterodyne, PGC and 3×3. Meanwhile, this method will suppress crosstalk in a qDAS system based on weak reflection point, reduce system detection distortion, and improve qDAS detection ability. In addition to crosstalk, Rayleigh backscatterings act as intensity noise in a qDAS systems and deteriorate detection noise. The proposed cosinusoidal phase modulation approach also optimizes detection noise by suppressing the intensities of Rayleigh backscatterings. Relevant studies have been carried out, and the noise suppression effect will be quantified and evaluated in our future work.