The Study Of Yb:YAG Thin-disk Laser With Minimal Heat Generation Based On Anti-Stokes Fluorescence Cooling Effect

In solid-state lasers,the waste heat generated inside gain medium plays an important role in limiting the improvement of output laser performances.In the perspective of heat dissipation,thin-disk architecture with high surface-to-volume ratio can increase the thermal management,decrease the thermal residual stress and ensure uniform temperature distribution.In the perspective of heat generation mechanism,the quantum defect of Yb:YAG thin-disk laser decreses by 80%when pumped by the 1030nm light,and the gain medium has a minimal heat generation.At the same time,the anti-Stokes fluorescence cooling occurs in thin-disk,which helps to reduce the fractional thermal load still further.We believe that the Yb:YAG thin-disk laser has a minimal heat generation inside gain medium when pumped by1030nm laser.However,due to the low absorption efficiency of 1030 nm laser for single-pass pumping in Yb:YAG thin-disk crystal,it is critical to improve the absorption efficiency.In order to increase the absorption efficiency of 1030nm pumped light,the incra-cavity and multi-pass pumped thin-disk laser are studied in both theory and experiment.The detail contents are:(1)The heat generation mechinasm of Yb:YAG thin-disk is analyzed.A numerical thermal load model of Yb:YAG thin-disk laser with minimal thermal load is proposed.The results show that quantum defect is the main heating source,whereas anti-Stokes fluorescence cooling effect is the unique cooling source.In consideration of the impacts of average fluorescence wavelength and radiative quantum efficiency on thin-disk thermal load,decreasing average fluorescence wavelength and increasing radiative quantum efficiency can decrease the fractional thermal load of gain medium.In addition,an iterative algorithm of one dimensional heat dissipation is proposed to calculate the central temperature,paving the way for studing the temperature characteristics of Yb:YAG thin-disk with minimal heat generation.(2)The study of intra-cavity pumped Yb:YAG thin-disk laser with minimal heat generation has been carried out in detail.This scheme enables a quantum defect of 1.69%,indicating a minimal heat generation inside thin-disk.A dynamic numerical model of two coupled thin-disk is proposed,and the influences of the self-sustained effect between two thin-disks on the temporal characteristics and stability of output laser are analyzed.Based on the dynamic numerical model of intra-cavity pumped laser and experimental results,the spot size ratio between two thin-disks has an important impact on output laser types.In this paper,we designed a 4f imaging based cavity,which ensures stable and identical spot size on two thin-disks.The experimental results show that the 4f imaging based cavity can suppress the self-sustained pulsations in the intra-cavity pumped Yb:YAG thin-disk laser,and the mode stability also benefits from it.(3)The study of multi-pass pumped Yb:YAG thin-disk laser with minimal heat generation has been carried out in detail.This scheme improves mode stability and has a plethora of advantages,including high efficiency,low waste heat generation,and so on.The origination and components of additional thermal load inside thin-disk are studied in detail.On the side of Yb:YAG thin-disk,there are Cu,In,Au,and Ta elements,which origin from the impurities during the assembling process of thin-disk.Considering the additional thermal load,the fractional thermal load of Yb:YAG thin-disk is revised.The additional thermal load and fractianl thermal load of thin-disk is then calculated based on the numerical model.In a32-pass pumping scheme with a spot size of 1.8 mm,the additional thermal load and fractional thermal load of thin-disk are 0.12%and 0.8%,respectively.The gain medium has a minimal heat generation.(4)The optimization of Yb:YAG thin-disk laser with minimal heat generation is carried out by solving the thermal load and quasi-three-level rate equations.The number of pump passes and the thickness of thin-disk have been optimized.The spot size of the pump spot has been enlarged.The dependence of threshold on temperature has also studied deeeply.As a result,a 60-pass pumping scheme with 0.24 mm thickness thin-disk and 2.6mm spot size delivered an average output power of 6.32 W operated at fundament mode with M_x~2=1.137,M_y~2=1.139,and the optical to optical efficiency and slope efficiency were 18.1%and 31.4%,respectively.Considering the power scaling law,a k W-level Yb:YAG thin-disk laser with minimal heat generation can be implemented.In summary,the Yb:YAG thin-disk laser with minimal heat generation based on anti-Stokes fluorescence cooling effect paves the way for realizing efficient thermal management in high-power solid-state lasers,and it is of great significance.