Research On The Key Component Of All-fiber Laser With Cylindrical Vortex Beam Output Based On A Mode Selective Coupler

Cylindrical vector beam has the characteristics of axially symmetric polarization and intensity distribution.Under strong focusing conditions,it will form a unique and strong local longitudinal electric field,so has unique application advantages in industrial processing,material handling,particle manipulation,optical microscopy and other fields.The mode selection coupler is the key device to realize the all-fiber mode conversion.It has the advantages of simple structure,small size,high conversion efficiency,high mode purity and excellent fiber compatibility.Therefore,it can be said that the mode selective coupler is currently the most promising technical solution to obtain stable and reliable all-fiber laser with cylindrical vortex beam output.However,the conventional pre-tapering mode selection coupler still has some technical defects such as complicated manufacturing process and poor stability.In this paper,a new type of pre-tapering-free mode selection coupler is designed and manufactured using the beam propagation method and the fusion taper technology,which are both based on the mode coupling theory.Finally,the main technical parameters of the coupler are tested by building a narrow-linewidth all-fiber laser experimental platform with cylindrical vortex beam output,and the LP₁₁ mode output with high mode purity and high stability is obtained.The main research contents and specific methods of this paper are as follows:1.Starting from the few-mode fiber mode theory and coupling mode theory,the working principle and structural characteristics of the mode selection coupler were studied.The relationship and distribution characteristics of the low-order vector mode and linearly polarized mode in the few-mode fiber were analyzed respectively.Meanwhile,the variation of coupling coefficient and power transmission distribution with the structural parameters of the mode selective coupler were reveals.2.For the first time,a novel pre-tapering-free mode selection coupler constructed by using Nufern 1060XP fiber and Corning SMF-28e fiber as the SMF and FMF in 1μm is proposed.Based on the beam propagation method,the energy flow process in mode coupling process was simulated and analyzed via the model built with Rsoft Component Suite software.And the coupling structure parameters that can realize the theoretically highest conversion efficiency from LP₀₁ mode to LP₁₁ mode were obtained through reasonable design:6.06μm in the diameter of the SMF/FMF,1125μm（or an integer multiple of it）in the length of the coupling region,and 6μm in the core spacing.3 Based on the multifunctional optical fiber taper and fusion system in our laboratory and the optimized coupling structure parameters,the experimental production and packaging of the pre-tapering-free mode selective coupler was completed using the fusion taper method.4 The all polarization-maintaining narrow-linewidth linearly polarized single-mode CW/pulsed fiber laser with an average power of 56.1/31.8 m W respectively were built.Injecting into the pre-tapering-free mode selection coupler,the main technical parameters including the mode conversion efficiency,mode purity,coupler ratio,additional loss,power stability of LP₁₁ mode were measured under CW/pulsed working conditions.Among them,the average LP₁₁ mode purity of not less than 99.6%and the average power stability of less than 0.06 d B are both better than the same type of mode selection coupler.