Double Ring Cavity Single Frequency Brillouin Fiber Laser And Its Applicationy

Brillouin fiber laser,which is based on stimulated Brillouin scattering(SBS)effect in optical fiber,has wide application prospect in fiber optic gyroscope(BFOG),optical fiber sensing,coherent optical communication and other fields due to its advantages of narrow linewidth,low threshold and stability.Single frequency Brillouin fiber laser(SFBFL)is the typical representative.Compared to other optical fiber lasers,SFBFL has the advantages of high coherence,narrow line width,large wavelength tunable range which is especially suitable for the field of high frequency microwave signal optical generation.However,this kind of laser suffers from the disadvantage of low pump efficiency.In this thesis,a double ring cavity single frequency Brillouin fiber laser is proposed and experimentally studied,the double ring cavity can increase the number of pump light cycles and improve the light conversion efficiency.The experiments focus on the performance improvement of the laser and its application for microwave signal generation.The main work of this paper is as follows:(1)Firstly,the research background and significance are briefly introduced.Then the classification,progress application and of Brillouin fiber laser(BFL)are summarized.Finally,the basic principle of BFL is theoretically analyzed.(2)In order to cover the low optical conversion efficiency of the traditional single-ring cavity SFBFL,we propose a double-ring cavity single-frequency Brillouin fiber laser scheme.The impact of the coupling ratio of fiber coupler and the optical polarization state on the laser performance are studied experimentally.The experimental results show that the proposed scheme has lower threshold,the coupling ratio of the coupler and the polarization state of the light have a certain effect on the performance of the laser.Under the same pump power,the output power of the laser reaches the highest value when the coupling ratio is 7:3.The polarization state of light may affect the output power and wavelength stability of the laser.The output power of the laser linearly increases when the second control loop of the polarization controller(PC)rotates from0~o to 90~o.While the output power linearly decreases when the second control loop of the PC varies from 90~oto 180~o.The output power reaches its maximum at rotating angle of 90~o.Under such conditions,the laser shows better wavelength stability.The experiment also demonstrates that the wavelength tenability can be realized by changing the wavelength of Brillouin pump,and the tuning range covers the whole C-band.(3)The difference of threshold between the proposed SFBFL and traditional single ring cavity Brillouin fiber laser with different cavity length is experimental studied.The results show that the Brillouin threshold power can be effectively reduced by using the proposed double ring cavity structure.When the length of SMF in the ring cavity is 2 km,5 km,7 km and 10km,the Brillouin threshold can be reduced by 6.3mW,6.2mW,6.2mW and 3.1mW in our scheme,respectively.It indicates that the proposed SFBFL has higher optical conversion efficiency.(4)Stable microwave signal generation is realized by beat frequency of the proposed SFBFL and the Brillouin pump.The frequency tunability of the microwave signal can be achieved by controlling the temperature of SMF in Ring cavity.The tunable accuracy is1.13MHz/?,1.0mhz/?,1.03mhz/?and 0.98mhz/?when the cavity length is 2 km,5 km,7km and 10km,respectively.The tuning range is determined by the temperature range of the SMF.The impact of the cavity length and the polarization state on the linewidth and stability of microwave signal is also studied in experiment.The results show that the linewidth of microwave signal is gradually broadened with the increase of cavity length.Adding a polarization controller in the ring cavity can effectively reduce the linewidth of microwave signal.