Fig. 1. (a) Geometric model of DAS helically wound vector detection optical cable; (b) Plane unfolding of helically wound vector detection optical cable structure; (c) Normalized sensitivity curve of helically wound vector detection optical cable as a function of incident angle

Fig. 2. (a) Geometric model of DAS elliptical fiber winding vector detection optical cable; (b) Elliptical fiber winding vector detection optical cable vector detection elements; (c) Normalized sensitivity curve of elliptical fiber winding vector detection optical cable as a function of incident angle

Fig. 3. The flowchart of vector space demodulation algorithm based on minimum distance matching

Fig. 4. (a) Schematic diagram of elliptically wound cable structure; (b) Schematic diagram of the experiment setup; (c) Schematic diagram of the distributed acoustic sensing system based on backscattering enhanced optical fiber

Fig. 5. Response linearity testing of the three sensing channels of elliptically wound cable. (a) Incident angle \begin{document}$ \varphi $\end{document}=0°; (b) Incident angle \begin{document}$ \varphi $\end{document} =60°; (c) Incident angle \begin{document}$ \varphi $\end{document} =120°, the variation of the system response \begin{document}$S( \varphi) $\end{document} with \begin{document}$ \varphi $\end{document} after normalization while θ is fixed at 90°; (d) α winding angle of 60°; (e) α winding angle of 45°; (f) α winding angle of 30°

Fig. 6. The demodulated error of uniformly distributed in all directions

Fig. 7. Time-domain waveforms of sound pressure in x and y directions demodulated by the system when the incident angle φ is (a) 8° and (b) 60°, respectively; (c) The demodulated errors in the x-direction sound pressure amplitude and y-direction sound pressure amplitude at different incident angles φ