Fiber Coupling Techniques, Eavesdropping System

Techniques of fiber detection and anti-detection are of great importance in the field of national defense for all the countries. In the beginning, the development history and the current development of the technique of fiber detection are introduced briefly. Then, discussed in detail are strong coupling theory concerning the FBT coupler and the weak coupling theory concerning the HF acid etching and the side-polishing coupler. From the analysis of fiber coupling equations, the solution of the equation and the formula of the coupling coefficient are derived. Meanwhile, the paper gives a detailed discussion on the coupling principles and also the fabricating processes of these three traditional fiber couplers.The HF etching method to fabricate the coupler is studied. The etching reactor is developed to obtain reproducible tapers of desired diameter and length. An approach for on-line monitoring of etching using the power meter is demonstrated. Based on the experimental data, the relationship between the leak-out optical power and change of time and also that between the leak-out power and width of the remaining width of cladding, and the fiber length exposed to HF acid are summarized. To solve the problem of low coupling ratio, a new method is put forward- the whole etching method.In this thesis, the side-polishing coupler is designed; the polishing process and some points for attention are presented. From the experimental data, the graphs concerning the leak-out power and the coupling ratio to the change of the remaining width of the cladding are drawn. The results of the experiment are analyzed, referring to the theory. In light of such problems as the long production period and the difficulty in controlling the planeness while polishing the surface of the coupler, a new design of the coupler is proposed. Meanwhile, the paper also discusses some methods to improve the polishing processes, i.e. using the liquid-drop method to monitor the remaining width of the cladding in time.Finally, the feasibility of the three kinds of coupling method—FBT, HF acid etching and the side-polishing are discussed and compared.