Study On Focusing Characteristics Of Quasi-single-mode Fiber Laser

In the field of laser processing,it is necessary to focus the laser to the target to obtain a high power density spot.From the laser output from the laser,to the laser focusing system,to the laser transmission in free space,to the evaluation of the laser beam quality at the target,Lots of parameter selection and structural design are involved.Quasi-single-mode fiber laser is widely used in laser processing because they can obtain higher power,but there are many modes in the fiber,and the existence of high-order modes will affect the beam quality.In this paper,the mixed modes in the quasi-single-mode fiber laser are taken as an example to analyze the influence of the ratio of the fundamental mode in the mixed mode and the phase difference between the modes on the focusing characteristics.And combined with the specific focusing system,the influence of thermal deformation of the optical element after heat absorption on the focusing characteristics is studied.Using average power density,M~2factor and focus shift as beam quality evaluation criteria,the focused beam quality is evaluated.The main research contents included in this paper are as follows:1.Taking the LP₀₁mode,LP₁₁mode and LP₀₂mode in the quasi-single-mode fiber laser as examples,in the case of incoherent superposition and coherent superposition,the effects of the fundamental mode ratio and the inter-mode phase difference in the mixed modes of the fiber on the focusing performance is studied when the laser passes through an ideal optical system,and the focused beam quality is evaluated by using the average power density,M~2factor and focus shift.The results show that in incoherent superposition,the focusing characteristics of the mixed modes gradually become better with the increase of the fundamental mode ratio.In the coherent superposition,when the phase difference takes different values,the average power density of the focused spot has a maximum value with the change of the ratio of the fundamental mode,the variation of average power density,M~2factor and focus shift with phase difference has periodicity,reasonable control of fundamental mode ratio and inter-mode phase difference can improve average power density and reduce focus shift.2.According to the index of the focusing system in practical application,three optical system models are given,and the performance of the focusing system is evaluated by the focusing spot size,the wavefront diagram and the diffraction encircled energy.Due to the simple structure,small focusing spot size,small wavefront aberration and high energy concentration of the double-mirror system,this system is selected as the optical system model used in the simulation in this paper.In addition,the influence of the deformation of the optical elements in the focusing system after heat absorption on the focusing characteristics of the mixed modes is studied.The variation law of average power density,M~2factor and focus shift after mixed modes focusing under the influence of thermal effect is analyzed.The simulation results show that the existence of thermal effect makes the average power density of the focusing spot decrease,the M~2factor increases,the focus shift increases,and the focusing effect becomes worse.3.In order to verify the accuracy of the results of the focusing characteristics of the mixed modes in the simulation and the focusing characteristics under the influence of thermal effect,we built a test platform for the focusing effect of quasi-single-mode fiber laser.The experimental results show that the measured M~2factor of the quasi-single-mode fiber laser after focusing is close to the simulation results.After the high-power laser irradiates the optical system,the measured M~2factor increases and the average power density of the focused spot decreases,which is consistent with the trend of the simulation data.Due to the deterioration of the fiber quality of the fiber laser and the initial aberration of the processing and adjustment of the optical system,the experimental results have certain errors,and the experimental plan needs to be improved.