Fiber Sensor And Tunable, Single Longitudinal Mode Fiber Laser Based On Modal Interference

Modal interference has been extensively investigated since it was proposed and has been widely used as optical coupler, switcher and wavelength demultiplexer. The optical devices based on modal interference have been widely investivated in recent years due to the perfect sensing characteristic. Beside these, the modal interferometers also have shown good filer characteristic and have been used in the fabrication of multi-wavelength, tunable fiber laser.Under the supports of National Nature Science Foundation of China, National973Program and the Innovation Foundation for Excellent Doctorial Candidantes of Beijing Jiaotong University, the thesis is devoted to investigate the modal interferometers based on no-core fiber, core-offset jointing structure and up-taper structure. The main innovative achievements are listed as follows:1. Proposed a tunable fiber laser based on the refractive index sensing characteristic of no-core fiber basic modal interferometer. The theory of the multimode interference has been analyzed in detail, and the sensing characteristics of the multimode interferometer have been studied in experiments. According to the high refractive index sensing characteristic of the modal interferometer, a tunable fiber laser has been proposed and a laser output with the lasing wavelength ranged from1531.97nm to1563.85nm has been achieved.2. A tunable, switchable fiber laser based on the core-offset structure has been proposed. Here a core-offset structure basic modal interferometer corroperated with a Sagnac loop interferometer was act as the tunable filter in the laser. The lasing wavelength was selected by the Sagnac loop interferometer and the tunable ability was achieved by using the curvature sensing characteristic of the core-offset structure. By adjusting the bending curvature of the modal interferometer, the output lasing wavelength can be tuned in a range of8.3nm.3. A novel modal interferometer based on an up-taper structure and a core-offset structure has been proposed. The coupling coefficient and the curvature sensing characteristic of the structure have been studied in experiment and a curvature sensing sensitivity of-12.95nm/m-1has been achiebed.4. Proposed a multi-wavelength, switchable, single longitudinal mode fiber laser based on the cascaded up-taper and core-offset structure. Here the novel modal interferometer was used as the comb filter. By adjusting the polarization controller, the output laser can be switched among single-waveleng, dual-wavelength and three-wavelength. In addition, by inserting an unpumped erbium doped fiber in the cavity, a single longitudinal mode fiber laser with the3dB band-width of5.7KHz was achieved.