Research On Transverse Mode Controllable Fiber Laser

TMoi mode and TE₀₁ mode are two vector modes in the second-order mode group of optical fiber.These two modes have the advantages of stable polarization and high energy concentration after focusing.So,they have significant application value in optical tweezers,particle acceleration,material processing,high resolution microscope and so on.At present,the way most lasers output TM₀₁ mode(TE₀₁ mode)is based on mode conversion,so both the output power and mode purity are limited by mode conversion efficiency.In this paper,the method of achieving the laser directly oscillates in TMoi mode(TE₀₁ mode)and separately outputs TM₀₁ mode(TE₀₁ mode)through controlling the transverse mode of few-mode fiber laser has been studied.Firstly,two fiber structures are designed in this paper,which can realize the effective separation of degenerate vector modes(TM₀₁ TE₀₁ and HE₂₁)in LP₁₁ mode;also,the vector modes transmitted in these two fibers and the power filling factor along the radial direction of these fibers of each vector mode are solved by numerical method.Secondly,these two fibers are applied to fiber laser respectively,and combined with the power distribution of the vector mode in the fiber,the scheme of single-ring and double-ring erbium doping distribution are proposed.Based on the fiber laser rate equations,the forward and reverse power of the pump and signal modes at various positions in the fiber laser resonator are calculated by numerical method.Then,the effects of fiber length,erbium doping concentration and pump power of fiber laser on the output power and mode purity of the TM₀₁ mode and TE₀₁ mode of the erbium-doped fiber laser based on two different fibers are analyzed.Finally,the effects of spontaneous emission noise on the controllability of transverse mode in fiber laser and the output power and mode purity of TM₀₁ mode and TE₀₁ mode are analyzed.The research results indicate that these active erbium-doped fiber can make the fiber laser directly oscillate in TMoi mode(TE₀₁ mode).Under conditions that the mode purity is high enough,for the erbium-doped fiber laser based on anti-parabolic fiber,when it outputs TE₀₁ mode and TMoi mode,the slope efficiency of it is as high as 65.06%and 62.15%,respectively.Meanwhile,the slope efficiency of the erbium-doped fiber laser based on ring fiber is as high as 67.5%and 67.46%,respectively.In addition,by designing different erbium doping distribution,the proposed erbium-doped fiber laser can also output HE₁₁,HE₂₁,HE₃₁,and EH₁₁ modes separately,and the slope efficiency is higher than 60%when the mode purity is high enough.Therefore,by designing different erbium doping schemes,the transverse mode controllable of the fiber laser can be achieved.Without introducing any mode conversion device,the method proposed in this paper has higher laser slope efficiency and output mode purity than the existing methods of laser outputting TM₀₁ mode and TE₀₁ mode.The research results in this paper have great guiding significance for the design and manufacture of fiber laser with high power and high purity vector mode output.