【作者】 张萌；

【导师】 储玉喜；

【作者基本信息】 天津大学 ， 光学工程， 2022， 硕士

【摘要】 波长短、单光子能量高的紫外飞秒激光在精密加工、半导体制造、生物医学等领域都有着独特的优势。飞秒激光频率变换产生紫外的转化效率会受到群速度失配的限制。传统补偿时空走离的延迟线方法系统庞大且环境稳定性差,无法适应复杂的工业市场。本文系统研究了双折射晶体在时空走离补偿上的优越性能,并探究了紫外激光光束质量的影响因素。创新性地将双折射补偿板与紧凑稳定的光纤激光器结合,开发了高功率、高稳定性、高光束质量、小巧紧凑的飞秒紫外光源。具体研究内容如下:1.以倍频、和频过程为例分析影响转化效率的因素,同时理论分析了孔径长度、时间走离长度和相干长度对非线性晶体长度的限制。对比了常用倍频与和频晶体的关键参数,分析晶体的时间和空间特性,选出了基于光源参数的最佳晶体。2.理论对比双折射补偿板在正入射和斜入射情况下的补偿效果。斜入射具有时间和空间补偿量的可调性,但二者同时变动降低了可调的实用性。正入射下通过精确设计补偿板的长度和切割角度即可实现对时间和空间的同时补偿。以BBO晶体为例,根据理论模型系统展示了补偿板的设计方法。3.理论设计补偿板参数,为了优化最佳输出功率,实验运用五种不同时间补偿量的补偿板。通过优化补偿板参数和入射光强度,最高获得了2.23 W的紫外激光输出,对应转化效率为23%。台面实验下测量了输出紫外的6 h功率稳定性为0.67%RMS。基于延迟线结构重现三倍频系统,获得了最高1.9 W紫外功率,功率稳定性为1.32%RMS并伴随着0.73%/h的功率下降。4.理论和实验探究了绿光和紫外激光光束质量的影响因素。基于LBO II类相位匹配和频,加入补偿板时最高获得了4.46 W(27.9%)的紫外激光输出。紫外的光束质量为M_x~2=1.513,M_y~2=1.16,相比BBO I类相位匹配下M_x~2=1.927,M_y~2=1.756,LBO的紫外光束质量保持良好。同时实现了倍频晶体、补偿板、和频晶体直接串联的结构,大大缩小了三倍频模块体积。基于补偿板的三倍频系统研究了双折射晶体对延迟线的替代效果,为稳定紧凑的紫外或者深紫外激光器提供了一种补偿思路。更多还原

【Abstract】 The femtosecond ultraviolet(UV)laser sources with short wavelength and high single-photon are of great importance in precision machining,semiconductor manufacturing,biomedical,etc.Unfortunately,the efficiency of frequency up-conversion based femtosecond laser is limited by group velocity mismatch.Traditionally,a delay line is used to control the temporal overlap.The most common draw-backs of such a delay line system lay with their bulky size and environmental sensitivity,which lead to its inability to adapt to complex industrial markets.In this article,we systematically studied the superior performance of birefringent crystals in the compensation of spatial and temporal walk-off,and explored the influencing factors of UV beam quality.On the basis of a femtosecond Yb-doped fiber laser,we designed a high stability,high beam quality,compact and efficient third harmonic generation scheme by a simple compensation plate(CP).The thesis is summarized as below:1.The factors influencing the nonlinear conversion efficiency are analyzed by taking the difference frequency and frequency doubling as examples.The limitations of aperture length,temporal departure length and coherence length on crystal length are theoretically analyzed.The key parameters of frequency-doubling and sum-frequency crystals are compared.After analyzing the temporal and spatial characteristics of crystals,the best crystals based on the parameters of fundamental pulse are selected.2.The compensation effect of birefringence CP is compared theoretically under normal incidents and oblique incidents.Although it is adjustable when the light incident on the crystal obliquely,the coupling effect of time and space departure reduces the practicability of tunability.The thickness and cutting angle of the CP are optimized to reverse both the arbitrary spatial and temporal walk-off under normal incidence.Based on this theoretical model,the design method of CP is presented as an example of BBO.3.The parameters of the CPs were designed theoretically.To obtain optimum efficiency,we used five kinds of CPs with different compensation capabilities.By optimizing the parameters of the CP and incident light intensity,a maximum UV output of 2.23W is obtained,corresponding to a single-pass near-IR-to-UV conversion efficiency as high as 23%.The tabletop UV laser system has a power stability of 0.67%RMS in 6 h with almost no power degradation.By contrast,the maximum UV output power with the complex traditional delay line is 1.9 W,and the power stability is 1.32%RMS with a clear drift at a rate of approximately 0.73%per hour.4.The influencing factors of the green and UV laser beam quality are explored theoretically and experimentally respectively.Based on type II phase matching LBO crystal for Sum-frequency generation(SFG),the maximum UV power of 4.46 W(27.9%)is obtained by CP.The UV beam quality is M_x~2=1.513,M_y~2=1.16.Compared with type I phase matching BBO crystal(M_x~2=1.927,M_y~2=1.756),The UV beam quality of LBO maintains good.At the same time,the cascaded third harmonic generation scheme is realized with second harmonic generation crystal,CP,and SFG crystal,which greatly reduces the length of the THG system.The substitution effect of CP on delay line is studied in the cascaded third harmonic generation scheme,which provides an idea of compensation for stable and compact UV or deep UV lasers.更多还原