【作者】 黄海涛；

【导师】 何京良；

【作者基本信息】 山东大学 ， 凝聚态物理， 2011， 博士

【摘要】 磷酸钛氧钾(KTiOPO4,简称KTP)晶体具有非线性光学系数大,热导率高,失配度小,激光抗损伤阈值高,不潮解及化学机械性能稳定等特点,目前广泛应用于中小功率Nd3+激光的倍频。然而当将其用于大功率1064nm激光倍频以及532nm泵浦的OPO时,都会观察到“灰迹效应”(gray tracking effect)。在形成灰迹的区域,晶体对可见光及近红外波段吸收显著增加,导致非线性频率转换效率显著下降,严重的还会导致晶体发热,造成永久性的损伤。为解决这一问题,人们通过使用特殊的助熔剂和热处理技术并更好地控制晶体生长条件,生长出了抗灰迹KTP晶体(Gray Track Resistance KTP,简称GTR-KTP)。实验结果表明,GTR-KTP较之普通KTP (common KTP,简称CKTP)在吸收损耗控制及抗损伤阈值等方面均有大幅度提高。本文以全固态激光器为载体,从理论及实验上系统研究了GTR-KTP晶体参与的腔内非线性频率转换过程,包括大功率倍频、OPO、SRS及复合频率转换过程。研究表明,由于GTR-KTP较CKTP在可见光及近红外波段的吸收大大减少,其参与的非线性频率转化性能大为改善,如输出功率、转换效率、温度特性及输出光束质量方面等；文中建立了相应的速率方程组对实验工作进行指导；鉴于GTR-KTP优良的二阶及三阶非线性特性,本文还进一步对其参与的并联型复合频率转换进行了有益的探索,拓展了晶体的应用范围。具体内容如下：1.建立了考虑腔内非线性频率转换过程的速率方程组,包括OPO、一阶及二阶SRS过程。为了验证理论模型,以KTA晶体为频率转换介质,实验研究了其参与的IOPO及腔内一阶SRS过程,并将实验结果与理论计算数据进行了比较。结果表明,所建立的速率方程模型可以较好地描述相应的实验过程,这对GTR-KTP参与的腔内非线性频率转换具有一定的理论指导意义。(第二章)2.对晶体进行了抗灰迹性能测试,与CKTP相比,GTR-KTP抗灰迹性能显著改进；比较了GTR-KTP与CKTP在大功率腔内倍频的性能差异。当采用GTR-KTP时,在10kHz重复频率和180W LD泵浦功率下,获得了最大平均功率为40.6W的绿光输出,这比相同实验条件下CKTP的绿光输出功率提高了近50%；研究腔内倍频温度特性时,发现GTR-KTP的温度带宽大大优于CKTP,并且观察到腔内、腔外倍频时晶体的温度调谐曲线有所差异；考虑到晶体对基波及二次谐波吸收不同会导致温度特性的差异,提出了一种定性比较KTP晶体抗灰迹能力的方法,实验结果与预期较为吻合；在以上研究基础上,利用GTR-KTP晶体,试制了一台可长期稳定运转的、输出功率为20W的准连续532nm激光器。(第三章)3.在理论上研究了shared cavity及coupled cavity两种OPO结构下的F-P透过率谱,发现shared cavity结构的透过率带宽远大于coupled cavity结构,相关的实验结果也表明shared cavity OPO输出的信号光线宽确实大于coupled cavity结构的。这为shared cavity OPO可以大幅改善输出信号光功率稳定性提供了合理的理论解释。(第四章)4.考虑到GTR-KTP晶体在近红外波段的吸收也大大小于CKTP,我们比较了二者参与的IOPO运转特性。以LD端面泵浦声光调Q Nd:YAG 1064nm激光器作为激励源并采用coupled cavity结构,在重复频率为15kHz和LD功率为11.4W时,GTR-KTP IOPO输出的1572nm信号光最大平均功率1.2W,相应的光光转换效率10.5%,这较之相同条件下CKTP IOPO的转换效率提高了25%；进一步研究了以LD端面抽运Nd:YAG/Cr4+:YAG键合晶体被动调Q激光泵浦GTR-KTP的shared cavity OPO输出特性。在LD功率为8.4W时,1572nm最大平均输出功率为900mW,而相同条件下CKTP信号光的最大输出功率只有640mW。利用第二章中的IOPO速率方程组,从理论上进一步模拟了该Nd:YAG/Cr4+:YAG激光泵浦GTR-KTP IOPO的输出特性。结果表明,理论与实验可以较好的吻合。(第四章)5.首先测量了GTR-KTP自发拉曼散射光谱,并与相同规格KTA晶体的拉曼光谱做了比较,得到了GTR-KTP的相对拉曼增益系数;利用LD端面泵浦声光调Q Nd:YVO4 1064nm激光器作为激励源,实现了GTR-KTP腔内二阶SRS激光器的高效运转。在重复频率为20kHz及LD功率为9.5W时,1129nm最大平均功率为860mW,相应的光光转换效率及斜效率分别为9.1%和11.6%。与相同谐振腔参数下的CKTP SRS做比较发现,GTR-KTP SRS的转换效率提高了近20%。结合第二章中给出的腔内多阶SRS速率方程组,对该Nd:YVO4/GTR-KTP二阶拉曼激光器进行了理论模拟；进一利用LD端面泵浦Nd:YAG/Cr4+:YAG键合晶体作为激励源,研究了被动调Q下的GTR-KTP腔内拉曼激光器的运转特性。当LD功率为8.1W时,1129nm最大输出功率为420mW,相应的光光转换及斜效率分别为5.2%和11.4%,相应的脉冲宽度及重复频率分别为2.2ns和5.9kHz。(第五章)6.实验研究了GTR-KTP参与的并联型复合频率转换过程。将非临界相位匹配的GTR-KTP及KTA放入一LD端面泵浦Nd:YAG/Cr4+:YAG激光器中,采用shared cavity OPO结构及优化的晶体参数,实现了1534nm及1572nm的双波长同步输出。在LD泵浦功率为7W时,两信号光的平均输出功率均为230mW,相应的脉冲宽度和重复频率均为3.9ns及5.5kHz；分别采用LD端面泵浦Nd:YAG声光调Q及被动调Q激光器作为激励源,在一块GTR-KTP晶体上同时实现了OPO及SRS两个过程,获得了相应的拉曼光及信号光输出。主动调Q情况下,在重复频率15kHz及LD功率为10W时,1129nm及1572nm的平均输出功率分别为150mW和180mW,相应的脉冲宽度分别为22ns及3ns。被动调Q情况下,通过更换适合的腔镜,实现了复合SRS+OPO的高效运转。在LD泵浦功率为8.6W时,一阶拉曼光1096nm和信号光1572nm的平均输出功率分别为1.1W和0.36W,相应的脉冲宽度分别为2.8ns和1.1ns,重复频率均为11.2 kHz。(第六章)论文的主要创新工作包括：1.系统研究了GTR-KTP与CKTP在大功率1064nm倍频中性能的异同,发现GTR-KTP在输出功率、温度特性及光束质量方面较CKTP均大为提高及改盖2.提出了一种定性比较KTP晶体抗灰迹能力的方法。3.首次对shared cavity OPO可以大幅改善输出信号光功率稳定性给出了合理的理论解释。4.以shared cavity结构实现了LD端面抽运Nd:YAG/Cr4+:YAG键合晶体被动调Q激光泵浦GTR-KTP IOPO。在LD功率为8.4W时,1572nm最大平均输出功率为900mW,光光转换效率为10.7%,此为shared cavity OPO下得到的最高效率。5.首次测量了GTR-KTP自发拉曼散射光谱,并得到了GTR-KTP相对与KTA的拉曼增益系数。研究了GTR-KTP参与的SRS转换特性,在LD功率为9.5W时,1129nm最大平均功率为860mW,相应的光光转换效率及斜效率分别为9.1%和11.6%。6.对并联型复合频率转换进行了有益的探索。首次实现了基于GTR-KTP及KTA的复合OPO转换,获得了1534nm和1572nm信号光的同步输出；首次以GTR-KTP为转换介质实现了高效的OPO+SRS转换,在LD泵浦功率为8.6W时,一阶拉曼光1096nm和OPO信号光1572nm的平均输出功率分别为1.1W和0.36W。更多还原

【Abstract】 Potassium titanyl phosphate (KTiOPO4, or KTP) has large second-order susceptibility, large angular (temperature) bandwidths, large thermal conductivity, high damage threshold, resistance to the deliquescence and stable chemical and mechanical characteristic, which make it extensively used as the nonlinear crystal for frequency doubling of medium-low power Nd3+ laser. However, laser-induced damage in KTP, termed gray tracking, is always observed in 1064nm second harmonic generation (SHG) and optical parametric oscillation (OPO) pumped at 532nm. This damage will dramatically increase the absorption of KTP in the visible and infrared band, resulting in sharp decline in frequency conversion efficiency. What is more serious is that the permanent damage will be produced when KTP is overheated. Optimizing the crystal growth conditions and adopting special fluxes as well as heat treatment techniques have been proven effective in improving the resistance of KTP to gray tracking. The gray-tracking resistance KTP (GTR-KTP) has been introduced accordingly. It has been demonstrated that GTR-KTP has improved absorption loss and damage threshold in comparison with the common KTP (CKTP).In this dissertation, by using the all-solid-state lasers, we have theoretically and experimentally studied the intracavity nonlinear frequency conversions based on the GTR-KTP, including high power SHG, OPO, stimulated Raman scattering (SRS) and mixed frequency conversion processes. Compared with CKTP, GTR-KTP has greatly decreased absorption at the visible and infrared wavelengths. This is favorable to improve the performance of nonlinear frequency conversions, such as average output power, conversion efficiency, temperature characteristics and output beam quality. Meanwhile, the corresponding rate equations have been established to simulate the experimental processes. In addition, the mixed frequency conversion processes incorporating with GTR-KTP have also been investigated. The main content of this dissertation includes:1. By introducing the nonlinear frequency conversion losses, the rate equations of intracavity OPO and Raman laser have been given, respectively. To evaluate the established theoretical models, the intracavity OPO (IOPO) and SRS experiments based on KTA crystals have been performed. It was found that the corresponding experimental results were in good agreement with theoretical results, which confirms the applicability of the theoretical model. (Chapter 2)2. The gray-tracking test has been carried out. The results indicated that GTR-KTP had good resistance to the gray tracking in comparison with CKTP. A comparative study of a frequency-doubling 532nm laser based on GTR-KTP and CKTP was also presented. Under the laser diode (LD) pump power of 180W and repetition frequency of 10 kHz, the maximum average output power at 532nm was 40.6W for GTR-KTP, which was increased by 50% compared with that obtained in CKTP. With the intracavity SHG configuration, GTR-KTP was proved to have larger temperature bandwidth than that of CKTP. Moreover, the intracavity SHG temperature tuning curve were found to be different from that obtained with extracavity SHG configuration for the two crystals. Considering the great difference between the two kinds of KTP in absorption at 1064 and 532nm, a qualitative evaluation method for the KTP’s resistance to gray tracking was presented. In addition, a 20W all-solid-state GTR-KTP green laser with long-time stability was made. (Chapter 3)3. By theoretically calculating the spectral transmissions induced by the etalon effect, it has been found that the shared cavity OPO had much wider transmission bandwidth than that of coupled OPO. The corresponding experimental results indicated that the line-width for the shared OPO was apparently wider than that of the coupled OPO. Therefore, the mentioned theoretical analysis can offer an explanation for the improved power stability of shared cavity OPO. (Chapter 4)4. Due to the fact that GTR-KTP has a lower absorption coefficient at infrared wavelengths than that of CKTP, the IOPO performance incorporating the two crystals has been studied. The GTR-KTP IOPO excitated by a diode-end pumped acousto-optic (AO) Q-switched Nd:YAG laser was investigated. Under the incident LD power of 11.4W and repetition frequency of 15 kHz, the maximum signal average output power was 1.2W. This corresponded to the optical conversion efficiency of 10.5%, increased by 25% compared with that obtained in CKTP IOPO. In addition, an efficient GTR-KTP IOPO with the shared cavity configuration and excited by a diode-end pumped composite Nd:YAG/Cr4+:YAG laser was also demonstrated. Under the incident LD power of 8.4W, the maximum average output power of 900mW at 1572 nm was obtained. A theoretical model for this compact GTR-KTP IOPO was also presented. Theoretical analysis on the pulse characteristics of the signal was performed, which showed a good agreement with that obtained experimentally. (Chapter 4)5. The X(ZZ)X spontaneous Raman spectrum of GTR-KTP has been measured, with the Raman gain coefficients relative to KTA given accordingly. A GTR-KTP second Stokes Raman laser intracavity driven by a diode-pumped AO Q-switched Nd:YVO4 laser was demonstrated. With an incident pump power of 9.5W, the GTR-KTP intracavity Raman laser, operating at the repetition rate of 20 kHz, produced the maximum average output power of 860mW at 1129 nm, corresponding to the optical conversion and slope efficiency of 9.1% and 11.6%, respectively. When the GTR-KTP was substituted with CKTP, a lower average output power of 720mW was obtained under the same pump condition and cavity setup as the GTR-KTP Raman laser. A theoretical model for this GTR-KTP SRS laser was also presented. In addition, the GTR-KTP Raman laser intracavity excited by a diode-end pumped composite Nd:YAG/Cr4+:YAG laser was also demonstrated. Under the incident LD power of 8.1 W, the maximum average output power of 420mW at 1129nm was obtained, with the optical conversion and slope efficiency being 5.2% and 11.4%, respectively. The corresponding Stokes pulse width and repetition rate were respectively 2.2 ns and 5.9 kHz. (Chapter 5)6. The mixed frequency conversion processes based on GTR-KTP have been studied experimentally. The synchronized dual-wavelength emissions at 1534 and 1572nm was realized by the mixed OPO conversion in GTR-KTP and KTA crystals. Both the two crystals were inserted into the diode-pumped Nd:YAG/Cr4+:YAG fundamental resonator. At an incident LD pump power of 7 W, the maximum output powers of the two wavelengths were all 230mW, with the corresponding pulse width and repetition rate measured to be 3.9 ns and 5.5 kHz, respectively. When the AO and passively Q-switched Nd:YAG lasers were respectively used as the excitation source, the simultaneous SRS and OPO conversions could be realized in one GTR-KTP crystal. For the AO Q-switching, under the incident LD power of 10W and repetition frequency of 15 kHz, the maximum average output powers of 1129 and 1572nm were150 and 180mW, respectively. The corresponding pulse widths were 22 and 3ns, respectively. For the passively Q-switching, under an incident diode laser power of 8.6 W, the maximum average output powers at 1096 nm and 1572 nm were 1.1 W and 0.36 W, respectively. The corresponding minimum pulse widths at 1096 nm and 1572 nm were 2.8 and 1.1 ns, respectively. (Chapter 6)The main innovations of this dissertation are as follows:1. A comprehensive study of high power SHG conversion based on GTR-KTP and CKTP was presented. It was found that GTR-KTP had advantage over CKTP in the output power, temperature characteristic and output beam quality.2. A qualitative evaluation method for the KTP’s resistance to gray tracking is presented.3. The theoretical analysis on the improved power stability of shared cavity OPO configuration was first demonstrated.4. With the shared cavity OPO configuration, an efficient eye-safe GTR-KTP IOPO excited by a diode-end pumped composite Nd:YAG/Cr4+:YAG laser was demonstrated. Under the incident LD power of 8.4 W, the maximum average output power of 900mW at 1572 nm was obtained, corresponding to a diode-to-signal conversion efficiency of 10.7%. This was the highest conversion efficiency obtained with the shared cavity configuration.5. The X(ZZ)X spontaneous Raman spectrum of GTR-KTP has been measured, with the Raman gain coefficients relative to KTA given accordingly. The intracavity SRS conversion based on the GTR-KTP was studied. With an incident pump power of 9.5W, the intracavity GTR-KTP Raman laser produced the maximum average output power of 860mW at 1129 nm, corresponding to the optical conversion and slope efficiency of 9.1% and 11.6%, respectively.6. Some useful exploring in the mixed frequency conversions has been made. The synchronized dual-wavelength emissions at 1534 and 1572nm was realized by the mixed OPO conversion in GTR-KTP and KTA crystals. In addition, the simultaneous SRS and OPO conversions have been successfully realized in one GTR-KTP crystal. At an incident diode laser power of 8.6W, the maximum average output powers at 1096nm and 1572nm were 1.1 W and 0.36 W, respectively.更多还原