Influence Of Concave Cavity Mirror On Spherical Aberration Of LD End-Pumped Laser

Solid-state lasers with high beam quality and high power are the frontier subjects of all-solid-state laser research.Due to the existence of thermal effect,the beam quality of the laser deteriorates and the output power decreases,which is the main factor limiting the output power of the fundamental mode.Therefore,the research on effectively reducing the thermal effect and its influence has also become an important direction in solid-state laser technology.In the oscillating process of the beam,the concave mirror will introduce a negative spherical aberration term,and the spherical aberration introduced by the concave mirror is superimposed on the thermal spherical aberration in the resonant cavity.Without additional optical components,the thermal effect of the laser crystal can be compensated,and then the output laser with high beam quality and high power can be obtained.Compared with the current structure of inserting compensation elements or multistage compensation in the resonant cavity.This method to reducing the influence of thermal effects can not only improve the beam quality,but also ensure that the resonant cavity has a low misalignment sensitivity and reduce the cavity loss.In this paper,the influence of concave cavity mirror on the spherical aberration of LD end-pumped laser is studied theoretically and experimentally.This provides a new idea for the optimization of output characteristics of high-beam quality and high power solid-state lasers,and provides a theoretical basis for effectively reducing the thermal effect of lasers.The main research contents include the following three aspects: Based on the thermal effect theory,the thermal spherical aberration characteristics of laser crystals are analyzed,and the influencing factors of thermal spherical aberration are discussed;The dynamic model of the resonant cavity with concave mirror is established and its characteristics are analyzed;The influence of concave cavity mirror on the spherical aberration of the resonator is analyzed and verified.In order to study the effect of spherical aberration on the laser.In this paper,based on the thermal effect theory,the spherical aberration generated by the laser crystal is theoretically analyzed.The temperature distribution of laser crystal is analyzed based on heat conduction equation.The analysis of the spherical aberration characteristics under the thermal lens effect of the laser crystal shows that the pump unevenness factor β will affect the position of the spherical aberration zero point.When β < 0,the greater the absolute value of β,the higher the pump power corresponding to the position of the zero of the spherical aberration.When β > 0,the position of the zero of the spherical aberration does not exist.The spherical aberration model of concave cavity mirror is established,and it is found that a negative spherical aberration is introduced when the curvature center is located at the aperture.The dynamic model of the cavity with concave mirror is established,and the dynamic stability range of the cavity is simulated and analyzed by using the concept of equivalent g parameter.The influence of concave cavity mirror on the thermal spherical aberration of laser crystal is analyzed.The compensation effect of concave mirror as the output mirror on the thermal spherical aberration is discussed,and the change of the beam quality of the resonant cavity is analyzed.Based on the above research,concave mirror is used as output mirror of resonator,the influence of concave mirror on the spherical aberration of resonant cavity was experimentally studied.The spherical aberration generated by the concave mirror under a certain pump is superposed with the thermal spherical aberration of the crystal,so as to reduce the influence of the thermal spherical aberration on the beam quality reduction.The influence of concave mirrors with different parameters(curvature radius and transmittance)on the thermal spherical aberration is designed and verified.In the experiment,concave mirrors with transmittance of 32 % and 15 % and curvature radius of 500 mm and 5000 mm were used as output mirrors.The experiment of plane-concave asymmetric cavity was set up to measure the wavefront information of the crystal end face and the output mirror end face,and analyze its change with the pump power.The experimental results show that under the condition of the same pump power and the same transmittance,the smaller the curvature radius of the concave cavity mirror,the better the compensation effect of the thermal effect,and the better the output laser beam quality of the resonator;Under the condition of the same curvature radius,the greater the transmittance of concave mirror,the better the compensation effect of thermal effect.It is verified that the plane-concave cavity can improve the beam quality of the laser output beam within a certain pump range,which plays a positive role in the spherical aberration of the resonator,consistent with theoretical analysis.