Research On Distributed Long - Distance Fiber Distributed Interferometry

The object detection of the distributed optical fiber vibration technology is widespread. There is a wide application in many important areas of the relationship between the beneficial to the people’s livelihood. Because of the fast response, high sensitivity of the technology, it has been selected as the object of study in this paper. According to the common problem of polarization stability and the distance of the measurement, a scheme with polarization maintaining road is proposed, and a method to increase the distance of the measurement is also proposed. based on the theoretical analysis, the feasibility of these schemes is verified by the by experiments. The main work and achievements completed are as follows:1. A new Interference light path structure with polarization-maintaining fiber, which has reliable polarization stability, is presented.. The feature of this scheme is that the sensing fiber is the single mode fiber, the influence of its polarization to the interferometer is eliminated by ta Faraday rotation mirror. But the other optical path is structured with the polarization maintaining fiber. And with the key component-Faraday rotator, the interference is realized. Because the use of single mode optical fiber as the sensing part, it has Practicality.2. The relationship between signal and interference phase difference is analyzed. And the mechanism of the key component3x3optical fiber coupler is analyzed theoretically. The algorithm of phase retrieve and the location are also discussed.3. By the study of the scattering light in optical interference path:1) Point out the parasitic interference generated by the scattering light along the optical fiber is a important factor affecting the fiber long distance detection. And through theoretical derivation, the formula of the scattering light pharasitic interference is given.2) Through the analysis of signal modulation characteristics of traditional phase generated carrier technology, a method, which reduce the influence of optical parasitic interference by phase generated carrier (PGC) technology with a carrier at the end of sensing fiber, is proposed.3) Combined with the characteristics of the interference structure making use of the3x3coupler’s phase feature to retrieve the phase, presented a characteristic PGC modulation and demodulation technology, which Different from the traditional way of PGC. This method is a kind of demodulation which can be deployed at the remote place without using the reference signal. So it can be use at any condition, unrestricted by the optical line routing. This scheme gives full play to the advantages of the system using the phase characteristic of the3X3coupler to retrieve the phase difference. As the carrier is loaded at the end of the sensing fiber, The amplitude of the carrier is fixed, not changing with the sensing fiber length. And because of the modulation of the carrier, the Interference signal is in full amplitude. So, the amplitude parameters for the phase retrieve can be directly obtained from the interference signals, which simplified the solving process.4. Through the experiments, it shows that:the polarization maintaining interferometer with the single mode optical fiber has reliable stability and its positioning distance is up to80km. With the characteristic PGC technology, its positioning distance is up to100km. The experimental result shows the combination of these two technologies could form a new technology with high practical value, worthy of further research and application.The main innovations are as follows:(1) Present a polarization maintaining interferometer with the single mode optical fiber as the sensing fiber. This is a balance between system stability and practical technology.(2) Points out that the parasitic interference caused by the scattering light is an important limiting factor for the position distance of single core positioning technology. The formula of the scattering light pharasi tic interference is given by derivation theoretically.(3) A method, which reduces the influence of optical parasitic interference by phase generated carrier (PGC) technology with a carrier at the end of sensing fiber, is proposed.(4) According to the optical path structure characteristics, a characteristic modulation and demodulation method, which is different from the traditional PGC technology, is proposed. By setting the carrier amplitude to a particular value, of the he particular,, and with the special demodulation at the remote site from the carrier loading position, the characteristic PGC technology the restriction of the traditional PGC technology. It is feasible when the two end of the sensing fiber is far away. And also the technique is inseparable with the3×3couple in the interferometer.. The advantage of3×3couple is brought into full play here.The combination of schemes of (1) and (4) will provide a high stability technique for long distance distributed positioning measurement.Through the analysis, some other new conclusions and methods are be put forward, such as the use of lithium niobate modulator unconventionally, a simple analysis method of the initial phase of interferometer with the3×3coupler, and so on.