The Study Of The ASE Effect In Thin-Disk Lasers

The thin disk is a solid-state laser based on a thin disk crystal which can promise its high optical-to-optical efficiency,high beam quality,high peak power and high pulse energy.The power of the thin disk laser can be increased by enlarging the pumping spot size.When it realizes the high power output,the large pumping size will induce the serious ASE effect,which decreases the optical-to-optical efficiency.In order to reduce the ASE effect effectively,how to evaluate the ASE effect has become one of the hot issues for the reaserchers.On the other hand,another research hotspot of the ASE effect is its heat generating properties.The ASE photons transmitted to the edge of the thin disk crystal cause an increase in the temperature of the thin disk laser and a decrease in conversion efficiency,and may even cause damage to the thin disk crystal.In order to solve the above problems,the content of this thesis is as follows.Firstly,we used the semi-analytical method proposed by P.Peterson to analyze the relationship between ASE spectrum and ASE intensity in thin disk lasers,and proposed an iterative algorithm to calculate the effective upper level lifetime by fitting the ASE spectral linewidth theoretically.At the same time,in order to modify the spectral bias caused by the reabsorption effect,the influence of the reabsorption on the ASE spectrum was analyzed theoretically,and the correct method was proposed and proofed by the experiment.And then,the modified effective upper level lifetime was calculated,and the cross-check results showed that the corrected spectral linewidth could effectively evaluate the intensity of the ASE effect.In addition,in order to evaluate the ASE effect by using the ASE spectrum accurately,the angular distribution of ASE photons in the air was also analyzed in this thesis.It is proved by experiments that,for the thin disk laser,the angular distribution of ASE photons incident in the air is isotropic for the rough-edge thin disk crystals.Since the edges of commercial thin disk crystals are roughness,the results show that the influence of the measurement angle on the spectral measurement can be ignored when using the ASE spectrum to evaluate the ASE strength.On the other hand,according to the space angular distribution of the ASE photons,most of the photons will propagate to the edge side of the thin disk crystal.Based on the theoretical analysis and the finite element analysis simulation,the heat generated on the edge of the thin disk crystal which induced by the ASE effect was analyzed.Subsequently,in order to eliminate the heat problem on the edge of the thin disk crystal caused by the ASE effect,we have re-polished the thin disk side.The experimental results showed that compared with temperature before re-polishing,the side temperature on the edge was reduced by 50%,and the optical-to-optical efficiency was improved 15%.Therefore,the method of edge re-polishing can effectively reduce the temperature of the thin disk crystal and improve the optical-to-optical conversion efficiency of the thin disk laser.Finally,in terms of engineering,by using the re-polished thin disk crystal,we completed the development of the pulsed thin disk laser in test prototype and engineering prototype,and realized laser operation with average power >300W,peak power >100kW,single pulse energy ~35mJ based on the self-developed pumping module,thin disk crystal and acousto-optic Q-switching module.