Precise Fault Monitoring For Optical Access Network Utilizing Correlation Detection With Complex Signals

With the development of optical communication technology, the optical access network regarded as the basic unit of optical signal transmission has been massively deployed. And that makes everyone enjoy the convenience of optical communication. The optical fiber distribute more densely in optical access network, and the technique of fiber-to-the-home limits several fibers laid in few meters or even shorter range. It requires a high level on measurement technique. If the resolution of the measurement cannot distinguish each fiber in several meters, it maybe cause unnecessary waste of human and material resources for the misjudgment of the fault fiber when the fault occurs. In addition, the operator pay more attention on the influence of the fiber fault due to the improvement of the transmission rate. Therefore, in order to minimize the influence of the communication failure, reduce the testing and maintenance time is the most direct solution, while reinforcing the wiring system.The maintenance time is directly related to the accuracy of fault location during the maintenance. The optical fiber in transmission line is the most frequently problem in the optical communication system. The operator always utilize the optical time domain reflectometer to locate the faults which occur in optical fiber. However, due to the instrument using the principle of pulse-time-fly, the location accuracy decrease rapidly when implement a long range measurement(the accuracy decrease to several ten or hundred meters when the detection range longer than ten kilometers). This makes the operator must search the fault in the range of the accuracy, and that prevents the progress of the maintenance. Although many methods have been proposed to locate the fault position, it is difficult to achieve high accuracy in a large range with simple structure. Since the development of the optical access network, optical fiber deploys densely and extensively. Therefore, the service provider desiderates a simple and practical monitoring system which can be strictly controlled to save the operational expenditure. Recently, some researchers find that the correlation detection technology can achieve high spatial resolution in large measurement range, but it has a very strict requirement for the probe signal. The periodicity will limit the measurement range, and the bandwidth of the signal will limit the spatial resolution. Therefore, in the correlation detection technology, the detection signal requires random oscillation, high bandwidth and simple generation structure. The optical access network develops three main structures: active optical network, wavelength division multiplexed-passive optical network and time division multiplexed-passive optical network. Different network structures require different monitoring systems. In this paper, we propose three monitoring systems for each kind of network with high accuracy location function, based on correlation detection technology. These monitoring system has simple structure and good prospect. The main researched contents and their innovation points are as follows: 1. Start with the existing fiber fault detection technology, we describe the advantages of correlation detection comparing with other methods in principle, and demonstrate its requirements for the probe signal.2. In active optical network, we propose to use the communication traffic signal as probe signal with random oscillation and high speed rate. We have achieved the fault detection with several centimeters accuracy in a real optical network, using the correlation detection technology. 3. In wavelength division multiplexed-passive optical network, each branch has its own unique wavelength, and the wavelength of the probe signal must cover all branches’ wavelength in order to realize the network monitoring. Therefore, we proposed to use a super-luminescent diode with high integrated form to monitor the fault in the network. The wavelength of super-luminescent diode can achieve the requirement of the network and the oscillation of the output signal is random. We realize fault monitoring with high spatial resolution in experiment.4. In time division multiplexed-passive optical network, it is difficult to distinguish each branch because of its broadcast signal distribution form. We proposed to use a length information to mark each branch. Employing a semiconductor laser serving as the probe light source subject to self-feedback which is caused by reflection of fiber Bragg grating inserted in each branch or Fresnel reflection at fiber fault. The feedback light will induce the laser to generate chaos and the output chaotic laser will contain all external-cavityinformation. According to the disappearing cavity information of the marked fiber Bragg grating, we can distinguish the fault branch, and the fault position can be acquired by the cavity information of Fresnel reflection at fiber fault. Our proof-of-concept experiment demonstrates the identification of the faulty branch and the location of the fault point with several millimeters’ accuracy simultaneously.