Research On Coherent Beam Combination Technology Based On Photonic Lanterns

Fiber lasers are widely used in defense and industrial manufacturing with their compact structure,good beam quality and high conversion efficiency.However,with the evolution of precision in manufacturing and the further development of defense,the demand for high-power,high-beam quality fiber lasers is also increasing.Although a single fiber laser can obtain a higher power output by optimizing the structure,its improvement is limited by factors such as nonlinear effects and thermal damage of the fiber.To solve this problem,laser beam combination technology came into being,which can obtain a higher power laser output by combining several medium-power lasers.Although this method can achieve high power output,when the combined beam power increases to a certain extent,the heat load of the active fiber will increase sharply,and while reducing the field area of the fiber mold,it will also bring about the effect of transverse mode instability,that is,energy transfer will occur between the transverse modes inside the fiber,so that the output power is unstable and the beam quality becomes poor.As a new type of optical fiber beam combiner,the photonic lantern has lower loss and can achieve mode control.When it is used for laser beam combination,on the one hand,can improve the beam combination efficiency,on the other hand,can improve the threshold of transverse mode instability with its mode control.Therefore,it is of great significance to study coherent beam combination technology based on photonic lanterns.In this paper,we will carry out detailed theoretical,simulation and experimental research on coherent beam combination technology based on photonic lanterns,and the specific content and innovation are as follows:Firstly,this paper introduces the research status of coherent beam combination technology,and points out that although the use of existing coherent beam combination technology can achieve high power output,it is impossible to avoid the effect of mode instability in the beam combination process.According to this,the research content of this paper is proposed:the mode control of a new optical fiber beam combiner-photonic lantern is studied,and the advantages of coherent beam combination based on photonic lanterns'mode control are analyzed.Secondly,the related theory of coherent beam combining technology is introduced.Several methods of coherent combining and their advantages and disadvantages are introduced in detail,and the principle of coherent combining based on fiber couplers and fiber combiners is expounded,which provides a theoretical basis for coherent combining using photonic lanterns.At the same time,the key indicators for judging the quality of the bundle are expounded,and an appropriate judging method is selected as the index of the subsequent experimental results.Then,the photonic lanterns are simulated and experimentally made.Firstly,based on finite element analysis,the structural design and simulation modeling of three-mode non-modal photonic lanterns are carried out,and the simulation results verify the rationality of the design structure.Then,the effect of cone length and tapered ratio on the transmission efficiency of photonic lanterns was analyzed by simulation,and the results showed that the transmission efficiency of photonic lanterns gradually increased with the increase of cone length and the increase of tapered ratio.Finally,photonic lanterns with different cone lengths and tapered ratios were made according to the designed structure,and the transmission efficiency of photonic lanterns was tested.The experimental results were consistent with the laws shown in the simulation results.Finally,simulation and experimental analysis of coherent beam system based on photonic lanterns were carried out.First of all,the experimental system was simulated by using the beam propagation method,and the mode control was realized based on the simulated transmission matrix of the photonic lantern,and the system of beam convergence efficiency and beam quality were measured.simulation results show that by controlling the input power and phase of the photonic lantern's input,we can achieve the mode control of non-mode selected photonic lanterns,so that it outputs pure LP01mode,at this time the system beam convergence efficiency is as high as 92%,the output beam spot presents a base mold form,the output beam~2 factor tends to 1.Then,a coherent beam experimental system based on photonic lanterns was built,and the mode control was realized based on the actual transmission matrix of the photonic lantern,and the beam combination efficiency and beam combination quality of the system before and after the mode control were compared.The experimental results showed that the combined beam efficiency of the system under the mode control was greatly improved compared with the modeless control,which was affected by the welding loss and the loss of the photonic lantern.The maximum beam combination efficiency is54.69%,the beam combination efficiency after loss removal can reach 97.28%,the far-field spot of the output beam shows the form of the base mode,and the LP01 mode in the beam accounts for up to 91.48%,and the beam quality is high.