| Optical Time Domain Reflectometer(OTDR)is the specialized instrument recommended by the International Telecommunication Union for fiber optic ranging and fiber fault detection.With the rapid development of fiber-to-the-home and tight star networks,there is an urgent need for OTDR with large dynamic range and high spatial resolution.Compared with the traditional pulse flight method OTDR,the chaos OTDR uses the wide-band chaotic light source to replace the traditional pulse light source,and divides the generated chaotic signal into two: reference signal,probe signal,and obtains the information of the fiber link through cross-correlation operation.Chaos OTDR has the advantage of high spatial resolution independent of distance,and has broad application prospects in fiber network detection in the future.However,chaos OTDR encounters the following limitations in practical applications: 1.The implementation of high spatial resolution of cm order must use GHz bandwidth signals,which requires broadband detectors and high-speed data acquisition cards,resulting in expensive chaos OTDR and the sacrifice of sensitivity of detectors.2.The chaotic signal generated by the semiconductorlaser has obvious relaxation oscillation characteristics,and the signal energy is mainly concentrated near the relaxation oscillation frequency,resulting in low energy in the low frequency band.In practical applications,limited by the bandwidth and low-pass filtering characteristics of electronic devices such as detectors,the energy utilization of chaotic signals is insufficient,which limits the dynamic range of chaos OTDR.Aiming at the two problems existing in chaos OTDR,this thesis proposes a simple fiber ring and significant-bit correlation method to optimize the chaos OTDR.By introducing a simple fiber ring into the chaotic light source,the low-band energy of the chaotic signal is improved,and the relaxation oscillation characteristic is eliminated.The experimental results show that the dynamic range of chaos OTDR based on the simple fiber ring is greatly improved.Through the significant-bit correlation method,the spatial resolution of the chaos OTDR breaks through the limitation of the bandwidth of the electronic device such as the detector,and fully exploits the high spatial resolution advantage of the chaos OTDR.Focusing on the above contents,the related work and main achievements of this thesis are as follows:(1)A simple fiber ring structure is proposed to improve the low-band energy of external cavity optical feedback chaos.It is verified from both theoretical and experimental aspects.Under an effective bandwidth of 200 MHz,the dynamic range of chaos OTDR based on fiber ring is increased by 5dB.(2)An improved cross correlation method is proposed: significant-bit correlation method.The theoretical and experimental verification of the signal bandwidth enhancement after the reconstruction of extracting N LSBs(N least significant bits)is carried out,and the mechanism of bandwidth enhancement is explored.The chaos OTDR system based on significant-bit correlation method is built.The spatial resolution improvement is experimentally verified.Under200-MHz detection bandwidth and 6-LSBs correlation,the spatial resolution of chaos OTDR is increased by 5 times.Multiple reflection events were measured.In addition,we also theoretically and experimentally verified the linearity of the chaos OTDR system with significant-bit correlation. |
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