Research On Automatic Low-order Aberration Correction Of Slab Laser

Solid-state slab lasers have been widely used in a variety of fields including industrial processes and scientific research for the advantages of compactness,long lifetime,all-electric operation and so on.However,at high average powers,the waste heat is deposited throughout the volume of the lasing material.This waste heat leads to thermo-optical and thermo-mechanical effects(e.g.thermal lensing and thermal stress),which bring great limitations to beam quality.How to achieve high output powers with excellent beam quality has always been a major scientific problem in the field.In the thin direction of the gain medium,the laser beam propagates along a zigzag path and thus dramatically alleviates the thermally induced aberrations.However,in the wide direction of the gain medium,the thermal effects are not compensated as well,which deteriorates beam quality.Among the entire aberrations in the slab lasers,the main components are astigmatism and defocus.These two low-order aberrations may range up to several tens of or even above 100 microns,which are far beyond the strokes of conventional deformable mirrors.Consequently,it is difficult to improve these large magnitude low-order aberrations directly through adaptive optics.In addition,a large aspect ratio of the slab is required to cool down its thermal load effectively.This leads to the output beam in a rectangular aperture with high aspect ratio(normally 1:5),while square apertures are preferred in many applications.In order to solve these problems,taking the low-order aberration correction technique of slab laser as the main line and the improvement of the output beam quality as the ultimate goal,this paper focuses on how to correct low-order aberration,maintain the targeted beam size simultaneously and obtain the near-diffraction-limit beam quality finally.The main content can be divided into the following four parts:Firstly,the aberration correction technology of slab lasers is reviewed and analyzed.Based on the aberration characteristics and application requirements of slab lasers,an analytical model-based low-order aberration correction method is proposed to establish the relationship among aberrations,targeted beam size and positions of the lenses.Thus,the problems of large magnitude low-order aberration and high aspect ratio beam size can be solved simultaneously.Secondly,aiming at the multiple output powers and aberration variations at a specific power,two correction strategies are developed based on the correction method proposed in this paper.In order to calculate the positions of each lens based on the size and aberrations of the input beam and targeted beam size for the two correction strategies,we deduce an analytical model which takes the beam divergence angle as an intermediate variable to characterize the low-order aberrations of slab lasers,the targeted beam size as the constraint condition and ray tracing as the tool.These methods provide the basis for realizing low-order aberration automatic correction and beam size matching.Finally,several simulations and experiments are conducted to verify the effectiveness of these methods.Thirdly,the research of automatic low-order aberration correction system is carried out based on a conduction-cooled end-pumped continuous-wave slab laser in our laboratory.This system is developed by taking the constraint model correction method as core,the beam divergence angle as feedback and the high-precision coaxial adjustment system as support.The initial size of the input beams is 1.8mm×11mm,and peak-to-valley(PV)value of the wavefront ranges up to several tens of microns.After automatic correction,the dimension reached nearly 22mm×22mm as expected,and PV values of the wavefront are less than 1.87?m.The experimental results show that the beam can be reshaped and corrected at the same time,demonstrating the automatic low-order aberration correction system can be used in the practical slab lasers.Finally,in order to further improve the beam quality of slab lasers,a hybrid adaptive aberration correction system which combines automatic low-order aberration correction system and adaptive optics system is developed.The adaptive aberration correction system has been successfully applied to the 5J/6.6ns/200 Hz nanosecond slab lasers developed by the Key Scientific Equipment Development Project of China.After low-order aberration correction,the beam size is transformed from 7mm×35mm into 42mm×44mm and the beam quality ? is improved from 18.42 to about 2.86 times of the diffraction limit.To further improve beam quality,a continuous surface deformable mirror with 59 actuators is adapted in the AO system.After further correction,the beam quality ? is improved to 1.64 times of the diffraction limit.A near diffraction-limit output beam is achieved for the first time in such a high-power,high-repetition rate nanosecond Nd:YAG solid-state laser ever built.