###### Abstract  
The coherent optical time domain reflectometer (COTDR) is a very important instrument for distributed temperature/strain measurement along the sensing fiber with a large dynamic range and high accuracy. The length of the sensing fiber and the temperature/strain measurement range limit the system performance, especially the measurement efficiency. So, a COTDR system is constructed, and the characteristics of the obtained coherent Rayleigh noise (CRN) are analyzed. Then, in consideration of the temperature/strain measurement range, accuracy, and time efficiency, the temperature/strain demodulation algorithm in noise conditions is studied. With different noise coefficients, the array length with 11, 21, 31, 51, and 101 independent frequency sweeping points are adopted to calculate the cross-correlation coefficients along a standard reference array with 301 independent frequency sweeping points. The results demonstrate that the array length has little influence on the signal processing time, but it can decide the measurement accuracy. To balance the system measurement efficiency and accuracy, it is inferred that, for a sensing fiber with a length of 100 km or more, the optimal independent frequency sweeping points are 101 and the trace average number is 1000. Rayleigh scattering; distributed optical fiber sensor; power line monitoring +
Footnote †: journal: _Sci_  
OPGWs and OPPCs can be used as a sensor, and the sensing nodes along the optical fiber are infinite in theory by distributed sensing technology. Typically, the optical time domain reflectometer (OTDR) is adopted to monitor the breakpoints or corrosion due to hydrogen absorption in OPGW [5; 6; 7]. In recent years, other light-scattering measurement-based distributed optical fiber sensors have also been adopted widely, for temperature, strain, vibration, and galloping monitoring [8; 9; 10; 11; 12]. When light propagates in optical fiber, it will generate Rayleigh scattering light, spontaneous Brillouin scattering light, and Raman scattering light. The frequency of the Rayleigh scattering light is the same as the original light, and the frequencies of Brillouin and Raman scattering lights shift about \(\pm\)11 GHz and \(\pm\)13.2 THz, respectively. Based on the optical time domain reflecting technology, Raman scattering light was first used for distributed temperature monitoring, as the position where the probe pulse transmits and light scattering happens can be accurately located. The Raman Stokes light and anti-Stokes light are only sensitive to the temperature parameter [13], so the Raman OTDR becomes an ideal distributed temperature sensor and is widely used now. However, as the spontaneous Raman scattering light is quite weak, the measurement range is generally lower than 10 km with a spatial resolution of about 1 m. Additionally, Raman OTDR with a much high dynamic range has been reported [11], but it is not enough compared with the Brillouin or Rayleigh scattering-based distributed temperature sensors [14; 15; 16]. Generally, the measuring distance of the Brillouin OTDR is several tens of kilometers. It is based on the linear relationship between the center frequency shift of the Brillouin spectrum and the temperature/strain. As the dynamic range of the Brillouin OTDR is limited by the scattered signal power and the sensitivity of signal receiving and processing units [14], it is quite difficult to enhance the measuring distance to 100 km with high spatial resolution, for example, 1 m. The Rayleigh scattering in optical fiber has a much higher power level than that of the Brillouin and Raman scattering, so the measuring distance can be easily improved to more than 100 km [17]. By the characteristics of the coherent Rayleigh noise (CRN) [10], the Rayleigh scattering signals from different sweeping frequencies at each position along the sensing fiber can be used to measure the temperature/strain, as the frequency shift of the CRN trace also linearly changes with the temperature/strain sensed [18; 19; 20].