Research On Single Longitudinal Mode Fiber Laser Based On Multiple Fiber Rings

Fiber lasers have the advantages of natural compatibility with optical fiber components,low noise,easy maintenance and so on.Thus they are widely used in the fields of optical fiber communication,optical fiber sensing,laser distance measuring,etc.As an important research aspect of fiber lasers,single longitudinal mode fiber lasers have become the significant light sources in the applications of fiber-optic time and frequency synchronous transfer,optical fiber sensors and coherent optical communication systems due to their high monochromaticity,narrow linewidth,and high coherence.The output performances of a single longitudinal mode fiber laser,especially the linewidth,will influence the application value.The narrower the linewidth of the laser output,the higher its application value.Therefore,it is of great significance to carry out corresponding theoretical and experimental research on single longitudinal mode narrow linewidth fiber lasers,for improving the application value.Fiber ring filter,as a comb filtering fiber element,is simple in structure and flexible to adjust the filtering performance.Put it into the laser cavity and the longitudinal modes in the cavity can be filtered,making it easier to output single longitudinal mode and narrow the linewidth.The research work of this thesis is expended around single longitudinal mode fiber laser,theoretically researching the fiber ring filtering technique,proposing and designing a single longitudinal mode fiber laser based on multiple fiber rings filter,and achieving single longitudinal mode and narrow linewidth output.The main research contents of this thesis are as follows:1.The structures and principles of the single-coupler fiber ring filter,the double-coupler fiber ring filter and the Mach-Zehnder filter constructed by coupler in the all-fiber narrow band comb filters are researched,and the filter functions considering the coupling loss of the coupler and the internal fusion loss of the filter are derived.At the same time,the influence of the coupling loss and coupling coefficient of the coupler,as well as the length and internal fusion loss of the filter on the filtering performance of the three filters is analyzed in detail by numerical simulation,providing theoretical guidance for practical applications in the laser resonator.2.A single longitudinal mode narrow linewidth fiber laser based on multiple fiber rings filter is proposed and experimentally verified.The multiple fiber rings filter is composed of a single-coupler fiber ring filter nesting with a double-coupler fiber ring filter.The structure and principle of laser linewidth measurement with delayed self-heterodyne method are researched,and the optical path of delayed self-heterodyne is designed and constructed to observe the single longitudinal mode output of the laser and measure the linewidth.In addition,the Lorentz fitting is utilized to optimize the linewidth measured by delayed self-heterodyne.The research results of the thesis show that the filtering bandwidth of fiber ring filter is greatly affected by the couling coefficient of the coupler and the loop length of the filter,and the experimentally constructed single longitudinal mode fiber laser based on multiple fiber rings filter operates in single longitudinal mode with linewidth of about 1.84 kHz after Lorentz fitting,confirming the feasibility of the proposed laser to achieve single longitudinal mode and narrow linewidth output.The research results are of great significance for the application value improvement of single longitudinal mode fiber laser.