DAS system intro(Intensity)

2022/07/12 DAS 共 1043 字,约 3 分钟

Two types of DAS system

  • Intensity-based system
    • first gen
    • not quantitive
    • cheaper & robustness
    • Amplitude of Rayleigh backscattered energy
  • Phase-based system
    • second gen
    • quantitative
    • expansive

Intensity based system

Principle of intensity-based system

The perturbed fiber will have a different intensity image:

Experiments on intensity-based system

2-m fiber -> non-linear and random. Not easy to use.

Phase based system

Complicated, but could provide quantitative result.

Principle

The first pulse: \(\delta\phi_1(z,t_1)=\phi(z+\delta z, t_1)-\phi(z,t_1)=2k\Delta z+c_0\)

The second pluse: because of the strain in fiber, there will be a distortion \(\delta z\).

\[\delta\phi_2(z,t_1)=\phi(z+\delta z, t_2)-\phi(z,t_2)=2k\Delta z+2k\delta z+c_0\]

The differential phase: \(d\phi(z, t_1) = \delta \phi_2-\delta \phi_1=2k\delta z\)

Note that \(\delta z\) is much smaller than \(\Delta z\). Change in length will change in refractive index(assume it’s linear).

Then, \(\delta \phi = \frac{4\pi nL_G}{\lambda}[\frac{\Delta x}{x}+\frac{\Delta n }{n}]\)

–> \(\delta \varepsilon_{xx}(t, x_j)\approx 1.16\times 10^{-10}\delta \Phi(rad)\)

Note that this is happened in one time and one channel.

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