Performance Enhanced POTDR Systems Based On Communication Coding And Decoding Technique

Optical time domain reflectometry(OTDR) system has been widely used in the field of characterization of optical fibers, because of its convenience and availability. The signal-to-noise ratio(SNR) is dominated by the peak power and pulse widths of the probe pulse and the performance of photodetector. In this case, when the peak power of the probe pulse and the sensitivity of photodetector can't be improved easily, the SNR of the OTDR system can be effectively improved by increasing the pulse width. But the spatial resolution will deteriorate along with the expanding of pulse width. In order to improve the SNR without sacrificing the spatial resolution, various communication coding techniques have been proposed and applied in conventional OTDR system and distributed sensing systems based on OTDR technology. Biorthogonal Codes was used in conventional OTDR system,and the SNR was enhanced up to 9.04 dB. Distributed temperature sensing based on Raman scattering using OTDR coding is demonstrated and Brillouin-based distributed temperature sensor employing pulse coding provides up to 7 dB SNR improvement.In a polarization optical time domain reflectometry(POTDR) system, as a result of imperfect manufacturing and the influence of external environmental factors, the birefringence of the fiber is randomly distributed which makes the polarization states of signal light from different part of the sensing fiber change randomly. Although the POTDR suffers from the same low SNR problem as other OTDR systems, the application of coding techniques in POTDR system is not straightforward because the polarimetric measurement has a strong correlation with the coded pulse width and pulse sequence. The performance of polarimetric measurement is largely constrained by the low signal to noise ratio(SNR) due to the weak Rayleigh backscattering in practical use and the degradation of the degree of polarization(DOP) of signal light. It will be indispensable to improve the SNR without sacrificing the DOP of backscattered signal for a sufficient dynamic range.In this thesis, a Simplex coded POTDR system has been proposed and demonstrated for the first time to the best of the authors' knowledge. The main work is as follows:(1)Using the derived quantitative relationship between signal DOP and the entire performance of the POTDR system, we found a positive correlation between each other,which provides a basis for choosing an appropriate communication coding and decoding technique.(2)Based on Stokes vectors and Mueller matrix, a wave plate model is employed for numerical simulation to study the relationship among signal's DOP, width of single probe pulse and the bit width/coding length of Simplex code. The results show that the coding length has nothing to do with the signal's DOP changing and longer bit width cause temporal depolarization. So the Simplex coding technology can be used in POTDR system to overcome the tradeoff between SNR and the resolution without introducing any extra temporal depolarization by setting longer bit width and shorter coding length.(3)Establish the POTDR system based on Simplex code. The simulation results have been verified experimentally by using 7-to-511 bits Simplex codes and excellent coding gain has been achieved, which agrees with theoretical analysis. Applying 511-bit Simplex codes,a coding gain of 10.125 dB has been demonstrated. Benefiting from these advantages, the birefringence change caused by polarization controller(PC) between two sections of single mode fiber has been detected and located without performing any average processing. The results have proved practical value of Simplex coded POTDR system. We believe that the sc-POTDR technology will pave the way to the long distance instantaneous application of intrusion sensing and vibration frequency extraction.