Study On Beam Shaping And Coupling Of Megawatt High-Peak Power Laser In Flexible Transmission Setup

Laser cleaning is based on the selective absorption of large pulse laser energy on the surface of the object to be cleaned,and it is a new type of cleaning method that achieves no contact and no chemical reagents for the dirt and paint attached to the surface of the workpiece.Because laser cleaning has the characteristics of high efficiency,high precision,non-contact,low pollution,and no secondary emissions,it has achieved rapid growth in many fields in recent years.Especially for high-value military targets,such as aircraft skins and tank outer surface,it ensures the rapid cleaning of high-value targets,greatly reducing the maintenance cost and time period of fighter aircrafts and tanks.Due to its novelty and importance,the Ministry of Science and Technology of China organized relevant organizations to implement the "High-power Laser Cleaning Equipment Application Demonstration" project under the National Key R?D Program of China in conjunction with military academies and laser equipment manufacturers.The target of the project is to achieve beam shaping?transmission and flexible transmission of megawatt-level ultra-high peak power laser pulse in multi-mode laser and remote cleaning operations.So that the setup can be applied to the demonstration of online laser cleaning of fighter skins to meet the country's demand for efficient and damage-free maintenance of fighters.In the mean time,the technology helps to eliminate the oversea-domestic gap on the civil aviation aircraft maintenance and break foreign monopoly.Optical fiber has the characteristics of compact and reliable structure.It is a good medium for flexible transmission of optical pulses.However,how to couple high peak power and high average power nanosecond laser pulses into the optical fiber without damage has become an important core technology of online laser cleaning devices.The research and development of fiber-coupled flexible transmission systems for high-power pulsed lasers has far-reaching significance.The focus of this paper is to address the problems of air breakdown and optical damage in the process of coupling megawatt-level high peak power kilowatt-level high average power short pulse lasers to large-core special fibers.The micro lens array beam homogenization and optical end caps are combined.The technology eliminates the problems of high peak power and high light intensity in laser focusing and subsequent processes,and realizes the coupling of high peak power short pulse laser beams to large core special fiber.With the help of optical fiber's waveguide capability and flexible transmission function,a set of fiber coupling device was designed and developed for solid-state lasers with peak power of 10 MW and average power of 1500 W,realizing the long-distance flexible transmission of high peak power and high average power laser beams.For this reason,in theoretical analysis and numerical simulation,this paper uses the mixed mode of Zemax optical design software to combine sequence and non-sequence to study and analyze the spot shaping,coupling and loss of the new laser in the system.After studying and analyzing different types of fiber coupling solutions,on the basis of theoretical analysis and coupling simulation,the method of utilizing the micro lens array mirror group to homogenize the beam,couple the beam into the multi-mode fiber was adopted.A beam with beam diameter of 12 mm is injected into a multi-mode fiber with a core diameter of 1000?m and high-efficiency coupling is achieved.The theoretical coupling efficiency of this method can be as high as 98%.Furthermore,with Zemax and Origin software,we comprehensively analyzed the dependence of the coupling efficiency on different errors such as longitudinal and lateral offset and tilting angle between the beam axis and the multi-mode fiber axis.The dependence relationship provides valuable technical reference for setting the assembly tolerance of the multi-mode fiber coupling system in engineering applications and the process of alignment.In the experiment,according to the results of theoretical analysis and computer simulation,we use Solidworks and CAD drawing software to design the mechanics to couple of the high-peak pulse power,high-average power laser beam into multi-mode fiber.The experimental test result shows that,when the axial offset is less than0.35 mm,the longitudinal deviation is less than 0.11 mm,and the angle deviation is less than 0.25°,the fiber coupling efficiency is up to about 91%,and there is no laser breakdown or optical damage in the high peak power and high average power laser coupling both in the air and to the optical path.The performance of the setup satisfies the the technical requirements of high peak power,high average power laser aircraft skin cleaning system on coupling technology.The above experimental results are the highest power level reported in China for the efficient coupling of high peak power lasers to multi-mode fibers.It has thus solved a major engineering problem for the completion of a major national scientific and technological projects.