Distributed Acoustic Sensing in Geophysics. Группа авторов

light and ensure low transmission loss within the optical fiber, we propose a special sensing fiber named as DMOF for DAS. The design schematic of the sensing fiber is shown in Figure 3.2, which introduces successive microstructures with refractive index modulations in the fiber core through UV laser light exposure. The microstructures can be treated as small and strong scatters and distributed along the optical fiber with the same interval. Ultraweak fiber Bragg grating (UWFBG) inscribed on the fiber is one type of microstructure that has been recently employed to enhance the SNR for fiber optic DAS (Ai et al., 2017; Zhu et al., 2015). However, the spectra of UWFBG will shift with temperature and strain change (Yang et al., 2016), and consequently the uniformity of the SNR enhancing effect along the optical fiber is difficult to control. To overcome the effect of environmental change along the optical fiber, Rayleigh backscatters with only an increase in the SNR, but colorless, are desirable, which can be created by continuous UV laser light exposure alone without periodic refractive index modulation.

Figure 3.2 Schematic of DMOF.

Then, the stabilization effect of DMOF is systematically investigated through numerical simulation. To analyze the influence of the scattering light amplitude on the intensity distribution along the fiber, one microstructure is assumed to be inserted at the center of a 2‐m‐long SMF corresponding to the injected pulse width of 20 ns, serving as a stronger scatter. Random temperature fluctuation or strain effect is applied on the fiber section, resulting in the random phase change. When the intensity of the backscattering light from the microstructure is set to 0 dB, 3 dB, 7 dB, and 10 dB higher than the intensity from the SMF without microstructures, the dynamic intensity distributions around the scatter over 1,000 traces appear to be more and more stable, which are shown in Figures 3.3a–3d, respectively. Figure 3.3a shows that, when the scatter is weak, the intensity at any position is not stable, and the fading points along the fiber randomly move for different traces, which is a fatal defect for low‐frequency acoustic sensing. The intensity distribution, especially the intensity at the scatter marked by the black dotted line, becomes more stable and grows stronger with the enhancement degree of the scatter. Hence, the intensity fading is gradually eliminated, and the SNR and long‐term stability are improved step by step.

      3.2.3. Fabrication and Performance Test of DMOF

The microstructured optical fiber is fabricated by a continuous online UV‐inscription system, which consists of a fiber winding module, UV laser source, laser collimation module, and computer control unit; see Figure 3.4 for details. The fiber winding system is based on the reel‐to‐reel process of fiber with large winding velocity control (from 1 mm/min to 10 m/min) and uniform stress control (from 0 to 100 N). The fiber used during the fabrication process is coated with a UV transparent silicone layer, which allows the microstructure fabrication process without removal of the fiber coating. The laser system we used was a conventional 248 nm pulsed excimer laser with a maximum pulse energy of 300 mJ and a large beam size of 26 mm × 12 mm, which can ensure highly effective UV exposure in the fiber core with only a single pulse of radiation, and acceptable fiber vibration during the winding process. Moreover, scattering intensity of the microstructure point over a large range of intensities could be controlled by the UV pulse energy. Finally, a microstructure with arbitrarily spatial distribution along the fiber could be designed by a computer control unit. The scattering intensity of each microstructure can be monitored by an OTDR system with an ultrashort pulsed tunable laser.

Figure 3.3 Simulated intensity distribution along fiber when the intensity of the backscattering light from the microstructure is, respectively, enhanced by (a) 0 dB, (b) 3 dB, (c) 7 dB, and (d) 10 dB higher than that from standard SMF.

Figure 3.4 The block diagram of the continuous online DMOF fabrication system.

To test the optical characteristics of the DMOF, the light of an amplified spontaneous emission (ASE) source is injected into the fiber. The spectra of backscattering light in the SMF and DMOF are, respectively, observed through an optical spectrum analyzer and illustrated in Figure 3.5a. It is clear that the spectrum of backscattering light from the DMOF is colorless across the C band (1525–1656 nm), and its intensity has been improved by more than 10 dB from that of the SMF. Since the microstructures are weakly reflective and just located at local points, the total insertion loss is as low as to be neglected. Moreover, the intensity stability at certain points in the fiber without and with microstructures is, respectively, monitored during 100 s and presented in Figures 3.5b and 3.5c. Compared with the random fluctuation of the intensity distribution in the SMF, the DMOF keeps a much higher SNR and stability, both in spatial and time domains, which are in consistent with the simulation results.

Figure 3.5 Comparison between the DMOF and the SMF: (a) Spectra of the backscattering light in the SMF and DMOF; the 100 s intensity distribution records of a 10‐m‐long section of SMF (b) and DMOF (c).

      3.2.4. System Configuration and Working Principle of the DMOF‐DAS

The experimental setup is illustrated in Figure 3.6a. A typical coherent OTDR structure is adopted and the DMOF is used as the functional unit test (FUT) for sensing, both of which can improve the SNR of the acoustic sensing signals. A 40‐mW laser source with a narrow linewidth of less than 1 kHz is split into two parts by the coupler with a splitting ratio of 1:99. One part serves as the local oscillate signal, and the other part is modulated into pulse with a duration time of 20 ns and frequency shifted with 200 MHz by the acoustical optical modulator (AOM). The erbium‐doped fiber amplifier (EDFA) amplifies the average power of the pulse and pours the pulse into the FUT through the circulator. The backscattering light from the DMOF, which carries external acoustic information, is mixed with the local light and detected by the balanced photodetector (BPD) to generate the heterodyne beat frequency signal. It should be noted that, owing to the high SNR of backscattering light from the DMOF, only one amplifier is needed and inserted between the AOM and optical circulator, which is simpler than the SMF‐based DAS interrogator. Then the electrical signal from the BPD is collected by a DAQ card. In order to sample the coherent signal of 200 MHz precisely, the acquisition speed of the DAQ is set as high as 2 GS/s. Then the acquired data are multiplied with the reference signal as the in‐phase and quadrature (IQ) demodulation scheme to extract the phase information along the fiber.

Figure 3.6 Working principle of DMOF‐DAS: (a) System configuration and (b) phase extraction workflow.

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