热透镜效应对Er^(3+),Yb^(3+)∶glass激光器输出能量的影响

Influence of Thermal Lens Effect on Output Energy of Er^(3+),Yb^(3+)∶Glass Laser

通过实验和理论相结合的方式,从热透镜效应角度研究了Er^(3+),Yb^(3+)∶glass在激光二极管(LD)端面抽运条件下的激光输出能量,建立起LD抽运重复频率、Er^(3+),Yb^(3+)∶glass热光系数与空腔运转斜率效率、调Q输出脉冲能量之间的关系。采用平凹腔结构,并以Co^(2+)∶MgAl_(2)O_(4)作为调Q开关,设计了LD端面抽运被动调Q激光器,实验结果发现,当热焦距大于前腔面光斑半径最小值对应的热焦距时,空腔运转斜率效率与材料的热光系数呈负相关,调Q输出脉冲能量与热光系数及抽运重复频率均呈负相关。基于理论研究,通过热焦距公式、稳定谐振腔的矩阵计算法以及被动调Q的速率方程进行数值模拟,解释并验证上述实验现象。结果显示,可通过降低抽运重复频率和选择低热光系数的Er^(3+),Yb^(3+)∶glass等方式降低热透镜效应实现高脉冲能量的输出。

Objective Since Er^(3+),Yb^(3+)∶glass is used in laser diodes(LD)pumped lasers,several studies on thermal properties of Er^(3+),Yb^(3+)∶glass have been reported.However,there are still few related reports on the effect of pumping repetition frequency and the material thermo-optic coefficient on output energy.The pumping repetition frequency of LD and thermo-optic coefficient of Er^(3+),Yb^(3+)∶glass affect the thermal focal length and ultimately change the output energy of the Er^(3+),Yb^(3+)∶glass laser.Thus,in this study,we analyze the effect of pumping repetition frequency and the thermo-optic coefficient on output energy to guide material selection and output energy analysis for the endpumped high output energy Er^(3+),Yb^(3+)∶glass laser.Methods In this paper,we design a passively Q-switched laser with LD end-pumped to measure output energy using aplano-concave cavity structure and Co^(2+)∶MgAl_(2)O_(4) as the Q-switching crystal(Fig.1).The measuring thermal focal length experiment is based on the critical cavity method and uses a flat-flat cavity to measure the thermal focal length.In the measurement of slope efficiency,the variation of the output energy with pumping power is measured without Co^(2+)∶MgAl_(2)O_(4) in Fig.1under the same pumping conditions.Additionally,the thermal focal length formula,the matrix calculation method of the stable cavity,and the rate equation are used to analyze the phenomena in the above experiments.Results and Discussions In a passively Q-switched LD end-pumped laser,we obtained that the output pulse energy of EY1,EY2,and EY3decreases from 417,401,and 378μJ to 356,337,and 329μJ as the pumping repetition frequency increases from 10Hz to 28Hz(Fig.2).First,we verified the phenomenon that the thermal focal length decreases with increasing pumping repetition frequency theoretically and experimentally to analyze the above phenomenon.Second,by numerical simulation of the resonator,it is found that in the plane-concave cavity,the beam radius of the front surface of the resonator decreases with increasing thermal focal length when the thermal focal length is greater than f_(Tm)(the thermal focal length corresponding to the minimum beam radius of the front surface of the resonator)(Fig.5).Finally,the rate equation is solved using the Runge-Kutta algorithm to obtain the trend of the output pulse energy gradually decreasing with increasing pumping repetition frequency(Fig.6).This is because the output pulse energy is positively correlated with the beam radius of the front surface of the resonator.After performing the free-running operation,the slope efficiencies of the three Er^(3+),Yb^(3+)∶glass samples,EY1,EY2,and EY3were 22.16%,22.82%,and 23.14%,respectively(Fig.7).Their thermo-optic coefficients are34×10^(-7),29×10^(-7),and 24×10^(-7)K^(-1),respectively.The Q-switched output pulse energy after adding Co^(2+)∶MgAl_(2)O_(4) to the above non-saturable cavity are 323,363,and 391μJ(Table 3).When the thermal focal length increases with a decrease in the thermo-optic coefficient(Fig.3),the cross-sectional area of the laser beam in Er^(3+),Yb^(3+)∶glass increases accordingly.The inversion population participating in stimulated radiation in Er^(3+),Yb^(3+)∶glass also increases,which increases the slope efficiency.Moreover,since the Q-switched output energy is positively correlated with the beam radius of the front surface of the resonator,the Q-switched output energy also shows an increasing trend.Conclusions In this study,we reveal the relationship among thermo-optic coefficient,pumping repetition frequency,slope efficiency,and Q-switched output pulse energy of an eye-safe solid-state laser with Er^(3+),Yb^(3+)∶glass as the gain medium experimentally and theoretically,when the thermal focal length is greater than the value at the minimum point.The thermal focal length in Er^(3+),Yb^(3+)∶glass decreases with an increase in the pumping repetition frequency,which decreases the cross-sectional area in the gain medium,and finally reduces the Q-switched output pulse energy.Thus,the Q-switched output pulse energy is negatively related to the LD pumping repetition frequency.Since the same pumping conditions,Er^(3+),Yb^(3+)∶glass with lower thermo-optic coefficient has a larger thermal focal length,larger cross-sectional area of the stimulated radiation beam in Er^(3+),Yb^(3+)∶glass,higher slope efficiency in a non-saturable cavity,and higher Q-switched output pulse energy.Thus,the Q-switched output pulse energy is negatively correlated with thermo-optic coefficient and pumping repetition frequency.Therefore,we can reduce the thermal lens effect by reducing the LD pumping repetition frequency and selecting low thermo-optical coefficient Er^(3+),Yb^(3+)∶glass to achieve high output pulse energy,which will help human eye-safe solid-state lasers use this material further to develop the ranging,lidar,and medical fields.