Theory Research On Twin-core Fiber And Development Of Novel Twin-core Fiber Full-access Connector

ABSTRACT:The application potential of twin-core fiber (TCF) in the field of optical communication and optical fiber sensoring is great, due to its simple manufacture and stable construction. However, the absence of available TCF full-access connector between one TCF and two single-core fiber (SCF) blocks the further development of TCF in various application researches. With the supporting of National Natural Science Foundation of China, the lateral coupled-mode theory of optical waveguide, which is the theory bases of TCF, the coupling characteristic of TCF and the TCF full-access connector are researched in detail by theoretical analysis and experiments in this dissertation. The main achivements of the dissertation are listed as follows:1) Base on the basic idea of the lateral coupled-mode theory of optical waveguide, a novel non-orthogonal coupled-mode equation is derivated from Helmholtz equation. Compared with the traditional equation, the novel equation is certificated to be correct.2) The change of coupling length versus center distance of two cores in TCF is analyzed by the TCF coupled-mode theory. The birefringence of eigen mode and the polarization dependent property of coupling length in TCF is analyzed by fiber finite element method. The stress-induced birefringence is much larger than the geometrical birefringence. The coupling coefficient of fast axis is greater than that of slow axis. The polarization sensitivity of the coupling angle will become greator if the center distance of the two cores in TCF is extended.3) A few kinds of twin-core fiber with circular, dumbbell and racetrack cross section are fabricated with side-polised preforms or side-grooved preforms. A modify system of coupling coefficient of TCF is built with CO2laser. In the experiment, the coupling angle of TCF changes by about104.7rad/m, and the change amount is not effected by wavelength significantly.4) The temperature sensing characteristic in the range of-30℃～175℃is tested. The mutual coupling coefficient will be decreased if the temperature rises. And the temperature sensitivity of the coupling angle will become greater as the temperature rises.5) A novel, low loss, low cross-talk, all-fiber, full-access connector is proposed and demonstrated experimentally. With the novel connector, two close cores of TCF, with the center distance smaller than20μm, can be accessed simultaneously. For the simulated example by finite element method, the center distance of the two optical fields is increased from16μm to more than90μm by passing the novel connector, the cross-talk is only-37dB, and the power loss is only0.056dB. A piece of collinear four-core fiber and a piece of Gemini fiber are fabricated by groove&stack&draw technology and fused-strech technology respectively, which proves the feasibility of the novel connector partially. with two single-mode SCF pigtails, and the working principle can be explained by directional coupling theory. The two close optical fields launched from the TCF are separated into two far apart optical fields, so that each optical field could be butt-coupled into one of the cores of one gemini fiber segment.