【作者】 李凤琴；

【导师】 彭堃墀；

【作者基本信息】 山西大学 ， 光学， 2012， 博士

【摘要】 全固态蓝光激光器在高密度光学数据存储、激光医学、光谱学、吸收成像和激光冷却等领域都有着广泛的应用。由于利用原子中的光频跃迁作为时间频率基准,比微波频率基准具有更高的准确度和稳定度,光学原子钟的研究已经形成一个重要的研究热点。在众多碱土金属原子的研究中,锶原子具有窄的跃迁频率,可以使时间测量精度大幅度的提高,因此,锶冷原子样品被广泛用于光钟的研制工作中。锶原子的一级冷却利用偶极跃迁(5s2)1So—(5s5p)1P1,即在单态能级1S0→1P1基础上的多普勒冷却,其对应波长为461nm,在本文中,为了满足锶原子光钟实验中一级冷却源的需求,我们采用全固态高功率单频绿光激光器泵浦钛宝石晶体,通过内腔倍频的方式实现了可调谐461nm蓝光钛宝石激光器,为锶原子一级冷却提供了优质的光源,本论文的主要工作如下：1.理论上介绍了钛宝石晶体的物理化学特性以及钛宝石晶体的光谱特性,钛宝石晶体不仅具有宽的吸收光谱和宽带发射光谱(650nm～1200nm),而且都存在比较强的偏振性。因此,实验中采用偏振性比较高的高功率绿光激光器泵浦钛宝石晶体,设计了像散补偿的六镜环形谐振腔结构,实现了两个较小腰斑位置处放置钛宝石晶体与倍频晶体,并合理选择谐振腔参数,在尽量满足最佳模匹配条件的前提下,缩小倍频晶体处的腰斑,实现了高效率的922nm基频光输出,通过谐振腔内插入宽带光学单向器实现了稳定的单频运转。2.介绍了二次谐波理论中倍频效率与相位匹配的关系以及双折射相位匹配与准相位匹配技术的特点。临界相位匹配方式中由于光线传播方向与晶体的一个主轴不平行,容易导致走离效应,从而影响倍频效率的提高；而准相位匹配技术利用周期极化晶体来实现相位匹配,走离角为零,可充分利用晶体的最大非线性系数,可实现非临界相位匹配,具有比较宽的倍频光波段范围。3.理论上介绍了双折射滤波片以及标准具片的调谐特性以及它们在倍频激光器中的压窄线宽的作用。钛宝石晶体具有很宽的发射光谱,双折射滤波片组合在该激光器系统中具有粗调谐的作用,其中最厚一片决定调谐的精度,最薄一片决定调谐的范围；而且为了提高调谐精度,需要在谐振腔内插入薄片标准具,通过改变标准具的角度以及改变标准具的温度来微调谐振腔的腔长,实现精细调谐。4.在上述的理论基础上,我们首先采用非线性系数较高、Ⅰ类临界相位匹配的BIBO作为倍频晶体,通过内腔倍频的方式实现了461nm钛宝石蓝光激光器。为了减小走离效应对倍频转换效率的影响,采用比较短的BIBO作为倍频晶体,当泵浦源为输出功率为8W的单频绿光激光器时,得到了最大输出功率为280mW、中心波长为461.62nm的可调谐单频蓝光输出；利用双折射滤波片组调谐其输出波长,调谐范围为10nm,将蓝光波长调谐至Sr原子的吸收线附近460.86nm,激光器输出功率降至202mW。为了改善激光器的频率稳定性,利用电子伺服系统将激光器锁定在一个精密控温、机械结构稳定的法布里-珀罗参考腔上,10min内蓝光的频率稳定性优于±2.22MHz。但是,仍然受走离效应的影响,蓝光输出光束为一个椭圆高斯光束。5.为了改善激光输出质量,进一步提高蓝光输出功率,我们采用Ⅰ类准相位匹配的PPKTP作为倍频晶体,设计了一种PPKTP内腔倍频的461nm单频可调谐钛宝石蓝光激光器。由于PPKTP晶体对波长小于500nm激光有较强的吸收,不利于提高倍频效率,因此,重新设计了六镜环形腔中凹面镜的曲率半径以及腔长结构,在保证最佳模匹配的前提下,适当增大倍频晶体处的腰斑半径,并且采用长度相对短的PPKTP作为倍频晶体,当泵浦功率为9.2W时,得到了中心波长为460.86nm的蓝光输出,其单频最大输出功率可以达到330mW,同时具有较好的光束质量,改变标准具的温度,实现了蓝光输出波长从460.8263nm～460.8795nm的调谐范围。为了提高钛宝石激光器的稳定性,将激光器频率锁定在F-P干涉仪的中心频率上,15分钟内频率漂移优于±3.47MHz。并且,将该激光器用于锶原子光钟系统中,作为一级冷却光源,得到了垂直入射时锶热原子的荧光谱线。6.设计了连续单横模最高输出12.9W的绿光激光器。分析了激光晶体的热透镜效应,设计了热不灵敏谐振腔结构,在30W的泵浦功率下,采用低掺杂浓度的复合晶体,Ⅰ类非临界相位匹配的LBO作为倍频晶体,获得最高输出12.9W的绿光激光器。创新性的工作：A.设计了紧凑稳定的六镜环形谐振腔结构,在谐振腔内设计了两个较小腰斑分别放置增益介质和倍频晶体,采用钛宝石晶体作为增益介质,实现了基频光中心波长为922nm的可调谐单频钛宝石激光器的稳定运转。B.采用较短的Ⅰ类角度匹配BIBO晶体作为倍频晶体,采用内腔倍频的方式,实现了中心波长为461.62nm的单频可调谐蓝光输出。C.由于周期极化PPKTP利用准相位匹配技术实现相位匹配,走离角为零,有效非线性系数比较大,可在室温下实现非临界相位匹配,因此,为了改善光束质量,进一步提高输出功率,采用准相位匹配的PPKTP作为倍频晶体,通过内腔倍频的方式实现了中心波长为460.86nm可调谐单频蓝光输出,并且光束质量比较好；设计了激光波长调谐系统,实现了蓝光波长460.8263nm～460.8795nm的精细调谐范围,能够满足锶原子一级冷却源的需求。更多还原

【Abstract】 All-solid-state continuous-wave blue lasers have a wide field of applications in high-density optical data storage, biomedicine, high-resolution spectroscopy, laser cooling etc. It is known that atomic clock operating at optical rather than microwave frequencies has higher accuracy and stability. The research of the optical clock based on the laser cooling femotosecond optical comb technic has attracted great interest owing to it’s potential optical advantages. It will be the new time and frequency standards. Because of the narrow transition width of the strontium atoms, the accuracy of the time can be highly enhanced, they have been widely used in the optical clock system. For laser cooling and trapping, the dipole transition between ’So and1P1states of the strontium atoms at461nm is usually adapted. So a single frequency and tunable high power light at461nm is desired in a strontium optical clock. During the period of my Ph. D study, we have realized a blue light source at461nm by intracavity frequency doubling of a Ti:sapphire laser. The accomplished main works are as following:1. We have analyzed the level structure, absorption and emission spectra, physical and chemical properties of the Ti:Sapphire crystal. Based on those theories, an astigmatically compensated double-folded resonator is designed to give two tight-focus regions, where the Ti:sapphire and BIBO crystals are located. And we have resonablely choiced the parameters of the cavity to satisfy with the optimum mode matching. High efficient output at922nm has been realized.2. By analyzing the features of the birefringence phase matching and quasi-phase matching, we get the merits of quasi-phase matching such as free of walk-off, high effective nonlinear coefficient, wide wavelength range of SHG, tuning convenience etc.3. The tuning properties of the Ti:sapphire laser are analyzed. The laser can be coarsely tuned by BRFS inserted in the laser cavity. And the tuning range of the BRFS is decided by the thinnest plate. The tuning precision is decided by the thickest one. And to improve the tuning precision, an angle-tuned thin etalon is inserted for fine tuning. To control the temperature of the etalon, the frequency of the cavity is continuously tuned.4. We demonstrate a tunable continuous-wave single frequency intracavity frequency doubling Ti:sapphire laser, which uses a homemade single frequency green laser as the pump source. The highest output power of280mW at461,62nm is obtained by employing a type I phase-matched BIBO crystal, and the power fluctuation is less than±1%within3hours. A three-plate birefringent filter allows for the tunable range from457nm to467nm, which covers the absorption line of the strontium atoms(460.86nm). The frequency stability is better than±2.22MHz over lOmin when the laser is locked to a confocal Fabre Perot cavity.5. To improve the quality of the spatial mode, we use PPKTP as a laser frequency doubling crystal to generate461nm blue radiation. Output power of330mW at460.86nm has been achieved at the incident pump power of9.2Watts at532nm. The blue laser can be tuned from460.8263nm460.8795nm by changing the temperature of the etalon. The frequency stability is better than±3.47MHz over15min when the laser is locked to a confocal Fabre Perot cavity.6. A maximal output power of12.9W at532nm wavelength is obtained under a pump power of30W. Analyzing the thermal effect of the laser crystal, a CW laser consisting of a low-doped Nd:YVO4/YVO4composite crystal with a three-mirror V-fold thermal unsensibility resonator has been designed and constructed. Using a Ⅰ-type phase-matching LBO as the frequency doubler, a maximal output power of12.9W at532nm wavelength is achieved.The creative works are as following:A. An astigmatically compensated double-folded resonator with six mirrors is employed to give two tight-focus regions, where the Ti:sapphire and doubling crystals are located. The central wavelength of the laser is922nm.B. Using the BIBO crystal as the intracavity frequency doubler, a tunable single-frequency Ti:sapphire laser has been achieved. It’s central wavelength is461.62nm.C. To improve the beam quality, a tunable single-frequency Ti:sapphire laser has been achieved using the PPKTP crystal as the intracavity frequency doubler. It’s central wavelength is460.86nm. The blue laser can be tuned from460.8263nm-460.8795nm by changing the temperature of the etalon.更多还原