Research On The Brillouin Few-mode Fiber Lasers Based On Mode Selective Coupler

With the development of metaverse,cloud computing and other technologies,the demand for communication capacity is also increasing.In order to improve the transmission capacity of fiber communication system and to break the limit of single mode fiber transmission capacity,the mode division multiplexing multiplexing transmission based on mode dimension of few-mode fiber is becoming hot topics.One of the key technologies is the study of laser light sources that can provide higher-order modes.In this thesis,we analyze the Brillouin scattering characteristics of different modes in few-mode fiber,and built a Brillouin fiber laser that can output both fundamental mode and higher-order mode.The main research contents of the thesis are as follows:（1）Taking the three factors into consideration,including the laser linewidth,the pulse width and the two guiding modes in the fiber,present the difference of pump light on the Brillouin scattering between the LP₀₁ and LP₁₁ in the few mode fiber,the numerical results are obtained:（1）For different laser linewidth,the linewidth and peakvalue of Brillouin scattering have the same tendency for LP₀₁ mode and LP₁₁ mode;but the difference is about 0.2 MHz and 0.07 dB,respectively,when the laser linewidth is less than 1 MHz;and difference of the two modes decreases slowly until the same,when the laser linewidth is wider than 1 MHz.（2）With the pulse width widening,the Brillouin linewidths for the LP₀₁ and LP₁₁ modes both decrease sharply,almost to the nature linewidth of fiber,but the linewidth difference increases;the peakvalue difference of modes LP₀₁ and mode LP₁₁ raises.（2）In order to solve the simultaneous output of LP₀₁ mode and LP₁₁ mode,a Brillouin few-mode-fiber laser with ring cavity structure is built.that the LP₀₁ mode and LP₁₁ mode lasers have 0.12 nm wavelength increasement to the pump light;22.69dB and 74.827 dB signal-noise-ratio is obtained for the two lasers,and the optical efficiencies are 4.603×10^-3%and 7.827%,respectively;the two modes laser have 0.54nm and 0.026 nm wavelength stability,and 3.277 dB and 1.262 dB intensity stability.