The Study On Dual-parameter Distributed Sensing

Distributed Optical-fiber Sensor has become a technology attracting a widely attention for its ability of sensing the measured field by detecting the continued space. There are many sensing mechanisms for Distributed Optical-fiber Sensing, some of which have formed mature products. And in the Distributed Optical-fiber Sensing based on Brillouin scattering, the Brillouin scattering signal has many advantages, such as its light-power is stronger, its work-wave is at the range of long-wave-band, and its attenuation and scattering are both smaller, all of which enable the Brillouin DOFS more proper to sense at long distance; Besides, Brillouin scattering is sensitive to the temperature and strain, thereby it is able to realize dual-parameters of the long distance. In the time domain, there are two different techniques to realize the Brillouin DOFS technique, one is Brillouin Optical Time Domain Analysis (BOTDA), and the other is Brillouin Optical Time Domain Reflection (BOTDR). In the BOTDA DOFS system, because stimulated Brillouin scattering is polarization related, in BOTDA sensing system, the steady measurement of sensing signals is seriously affected by the random change of polarization state of pump light and probe light, which causes the measured signal fluctuant. Aim at such polarization-induced fading phenomena of BOTDA, all kinds of polarization-induced fading eliminating techniques are introduced in detail in our article, such as, using polarization-maintaining fiber, polarization tracking technique and polarization expanding receiving technique. Polarization expanding receiving technique is a novel technique aiming to solve the polarization-inducing fading of the BOTDA system. After theoretical analysis and simulating analysis and experimental validating, we make sure that the technique is workable and effective. Actually, it can control the noise within 4%. However, in BOTDR DOFS technique, since what is detected is coherent signal produced by the coherent interaction between spontaneous Brillouin scattering and local referred light. The power of such coherent signal depends not only on the power of the two light waves, but also on the polarization states of the two light waves (the power of the coherent signal reaches the strongest when the two polarization is consistent and the weakest when the two polarization is perpendicular). Such phenomena also causes the polarization-referent disturb of detective signal. Aim to the phenomena existing in the BOTDR system, some polarization-induced fading eliminating techniques are introduced in detail as well, such as, polarization controlling technique, polarization collecting receiving technique and time-domain receiving technique. Except for the polarization-induced fading eliminating techniques, the selecting of numerical disposal method for receiving discrete frequency shift spectrum and power spectrum is also an important factor to ensure the distilling of dual-parameters (temperature and strain) can reflect the correct information of the outside environment. The numerical disposal method includes the interpolation method and curve fitting method. piecewise cubic spline interpolation, polynomial three times curve fitting and based on Gauss curve fitting all content the changing characteristic of the datum, and therefore, they are all considered as be effective methods to distill the parameters from the receiving signal.