Research On Narrow Band Mode Selection Based On Whispering Gallery Mode Microcavity

With the development of optical communication, optical sensor, military and medical treatment, high performance fiber source becomes a research focus, especially for narrow line width fiber laser. Narrow line width fiber laser has some great properties, such as narrow line width, flexible wavelength, compact equipment and high output power. So it becomes a major study object in optical fiber sensor, radar and optical communication. And new requirements are put forward.Whispering gallery mode(WGM) microcavity has many advantages, such as high quality factor, small mode volume, low loss and low-cost. So it becomes an ideal narrow band filter. In this paper, narrow band mode selection property of WGM microcavity has been mainly discussed. The origin, development, classification and applications of WGM microcavity have been introduced. The coupling method and theoretical model of WGM microcavity have been elaborated. Homogeneous dielectric microsphere coupled with taper fiber has been simulated through FDTD software.Based on the above, two ways to improve the narrow band mode selection property have been proposed. Firstly, a new structrue of WGM microcavity has been proposed. That is to use two cascade microspheres to couple with taper fiber. The simulation result shows that compared with homogeneous dielectric microsphere, the side mode suppression ratio of two cascade microspheres system has been increased, and the line width has been declined. So, its’ property of narrow band mode selection has been improved. Secondly, a double-layer WGM microcavity has been proposed. That is to plate a layer on the surface of homogeneous dielectric microsphere. According to the simulation result, when the layer thickness of coated microsphere is 100 nm and the refractive index is 2.4, the side mode suppression ratio of the coated microsphere system has been increased, the line width has been declined, and the maximum energy density has been raised. So, it has smaller mode volume and better narrow band mode selection property. According to all simulation results, we find that both two ways can improve the mode selection property of WGM microsphere.Finally, the experimental system of WGM microsphere coupled with taper fiber has been designed and constructed. Through the coupling experiment, the resonance spectrum has been achieved. The free spectrum range is about 1.32 nm, and the line width is about 0.8 pm. The experiment shows that WGM microsphere can be used for narrow band mode selection.