【作者】 闫晓娟；

【导师】 贾锁堂； 马维光；

【作者基本信息】 山西大学 ， 光学， 2012， 博士

【摘要】 激光在人们生产生活的各个领域发挥着重要的作用,然而由于增益介质的缺乏,某些特殊波段的激光不能采用直接受激辐射的方法获得,因此需要非线性频率转换技术。非线性频率转换其本质是光与物质相互作用所产生的一种非线性光学效应,该效应突破了传统光学中光波线性叠加和独立性传播的局限性,揭示了介质中光波场之间相位关联、能量的交换的变化过程。非线性频率变换主要包括光的倍频、和频、差频及光学参量振荡。与倍频相比,和频是一种三波相互作用,基频光有两种不同的波长,因此特殊性和复杂性也有所增加,但它可以在倍频技术不能达到的某个短波长处实现相干辐射,因此成为人们研究和探索的热点。非线性光学和频转化效率主要取决于基频光的功率密度,双波长外腔共振技术就是将两基频光同时耦合到外部谐振腔中共振,从而放大基频光功率,并对基频光的腰斑半径进行控制,提高和频转化效率。在本文中,我们主要以938nm和1583nm两基频光和频产生589nm钠黄光为例,对双波长外腔共振和频过程中涉及的基本理论进行简要介绍,对晶体的相位匹配角度和有效非线性系数进行模拟计算,对激光到外腔耦合过程中的模式匹配和阻抗匹配进行详细分析。然后围绕系统的频率级联锁定、外腔的腔参数优化、和频光的波长调谐与锁定三方面展开详细的研究,其具体的研究内容如下：1、从理论和实验上分别对双波长外腔系统的频率级联锁定进行研究,从而使得两基频光能在腔内同时共振。首先对常用的锁频技术进行比较,选择了操作简单并适合双波长外腔系统进行频率级联锁定的Hansch-Couillaud (HC)技术。然后研究单KTP晶体和键合KTP晶体对HC锁频技术的影响。结果表明对于单KTP晶体,激光会在晶体内发生走离,相互走离的两个偏振光束不能在环形腔内共振,从而使得HC技术不能用于Ⅱ类相位匹配的和频过程,而利用键合KTP晶体或者周期极化晶体就可以克服此缺点。实验中,我们将外腔腔模频率锁定到938nm激光器的输出频率上,然后将1589nm激光输出频率锁定到外腔腔模频率上,从而使三者之间级联锁定,实现了两基频光在腔内的共振。此外为了优化锁频系统中的反馈电路,基于基本的控制理论研究了伺服系统的设计原则,并通过测量激光-腔系统的传递函数设计出了伺服反馈电路,提高了稳频性能。2、从理论和实验上分别对双波长外腔共振和频系统的外腔参数进行优化,以满足最佳的Boyd-Kleinman (BK)因子与系统的阻抗匹配。首先建立了双波长外腔共振和频产生的理论模型,分析了BK因子与腔增强因子对和频转化效率的影响,然后详细研究了影响BK因子的因素,建立了BK因子与腔长参数的依赖关系,详细研究了影响腔增强因子的因素,建立了腔增强因子与入射腔镜反射率的依赖关系,从而通过计算模拟优化了腔长参数与入射腔镜镀膜参数,提高和频转化效率。实验上利用938nm激光与1583nm激光在蝶形环形腔内的PPLN晶体中共振和频获得了波长为589nm,功率大于0.2W的钠黄光，在考虑了两基频光到环形腔的耦合过程中不完全的模式匹配以及晶体和出射腔镜对和频光的损耗之后,实验得出的结果与理论计算的结果符合的很好。3、从理论和实验上分别对双波长外腔共振和频系统产生的和频光进行波长调谐、频率锁定和参数测量。首先从理论上分析了双波长外腔系统在级联锁定的情况下的频率扫描。为了将和频光锁定到标准的参考频率上,详细研究了和频光通过气体吸收池之后不同温度下的直接吸收光谱和饱和荧光谱。实验上,我们将扫描信号加载到调谐范围较大的938nm基频激光器上,通过扫描其频率来获得和频光的波长调谐；利用高精细度的F-P腔对和频光的线宽进行测量；通过温控装置的设计,测量了不同温度下Na原子的直接吸收光谱和饱和荧光谱；利用波长调制的一次谐波技术将和频产生的589nm激光锁定到Na的D2a线上；最后测量了频率失锁到锁定误差信号的变化,反映了锁频前后和频光的频率稳定度。实验结果表明在双波长外腔级联锁定情况下589nm和频光的最大扫描范围为2.2GHz,线宽为7MHz,将589nm和频光精确锁定到Na的D2a线上,将频率稳定度由400MHz改善到了4MHz。在上述的研究工作中,属于创新性的内容有以下几方面：1、对双波长外腔共振和频系统进行了详细地研究,在基频光受限的情况下分析了影响和频转化效率的因素,通过对腔长参数的优化获得了最佳的BK因子,通过对入射腔镜反射率的优化使系统达到完全的阻抗匹配,提高了腔增强因子,从而提高了和频转化效率。2、分析了单块晶体中两相互垂直偏振光的走离效应对HC锁频技术的影响,并设计利用键合晶体或者周期极化晶体来克服这种影响,使得相互垂直的两束偏振光能够基于HC锁频技术在双波长外腔系统中共振,实现Ⅱ类相位匹配的和频。3、利用HC锁频技术实现了两基频光到环形腔的频率级联锁定,即先将环形腔腔模频率锁定到激光器Ⅰ的输出频率上,然后将激光器Ⅱ的输出频率锁定到环形腔腔模频率上,从而实现了两基频光在腔内的共振。4、基于双波长外腔频率级联锁定技术实现了和频光的波长调谐,只要通过调谐主激光的频率,就可以实现和频光的可调谐输出。5、采用1583nm和938nm两波段在双波长外腔共振系统中进行高效和频,产生589nm的钠黄光,其中1583nm激光器为光纤激光器,938nm激光器为外腔二极管激光器,所用的非线性晶体为PPLN晶体,938nm激光的和频效率达到42.8%更多还原

【Abstract】 The laser plays an important role in many fields of industry and daily life. However, due to the lack of appropriate gain mediums, some special bands cannot be obtained directly by laser radiation. The nonlinear frequency conversion technique will be need. The nature of the nonlinear frequency conversion is a nonlinear optical effect produced in the process of the interaction between laser and medium. The nonlinear optical effect breaks with conventional limitations of linear superposition and independent propagation of lightwaves and reveals the exchanges of energy and phase association of lightwaves. Nonlinear frequency conversion technique includes second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG), and optical parametric oscillator (OPO) and so on. Compared with SHG, SFG is a three-wave interaction because the wavelengthes of two fundamental lights are different. Therefore it is complex. However, SFG can achieve coherent radiation in the specific short-wavelength that can not be achieved by SHG and become a focus of research and exploration. The sum-frequency conversion efficiency is directly proportional to the product of two fundamental laser power densities. The doubly resonant technique in an external cavity is that the two fundamental lights should be frequency resonant with the external cavity simultaneously. In the application, the laser power in cavity will be amplified and the waist radius in cavity will be controlled, thereby improving the conversion efficiency of SFG.In this thesis, the example treated here is589nm generation by mixing938nm and1583nm fundamental lights. The basic theories of SFG in a doubly resonant external cavity were briefly analyzed, the phase matching angle and effective nonlinear coefficient of nonlinear cry stral were calculated, the mode matching and impedance matching in the process of coupling a laser beam to external cavity were detailedly analyzed. Then we introduce our works mainly around the serial locking of the sum-frequency system, the optimization of external cavity parameters and the wavelength tuning, frequency locking of sum-frequency output. The key parts contain following aspects:1. The serial locking of the sum-frequency system based on doubly resonant external cavity have been studied theoretically and experimentally. First, the common frequency locking techniques were compared. The Hansch-Couillaud (HC) frequency locking technique was selected for achieving the serial locking of the sum-frequency system based on doubly resonant external cavity. Then the HC frequency locking schemes based on a single KTP crystal and diffusion bonded KTP crystals were analyzed. The results show that two orthogonal linearly polarized components are seriously departed in a single KTP crystal and are not resonant with the external cavity simultaneously. That makes HC frequency locking schemes can not be used for type II phase matching SFG. The use of diffusion bonded KTP crystals (or periodically poled crystal) can overcome this drawback. In experiment, the cavity mode frequency of bow-tie cavity was locked the frequency of938nm laser firstly, and then the frequency of1583nm laser was locked to the cavity mode frequency of bow-tie cavity. The serial locking of three components were realized. In addition, in order to optimize the feedback circuit of the frequency locking system, the design principles of servo system were studied based on the basic control theory. The servo feedback circuit was design by measuring the transfer function of the laser-cavity system and the performance of frequency stabilization was improved.2. The cavity parameters of SFG in doubly resonant external cavity have been optimizated theoretically and experimentally. In theory, a theoretical model of SFG in a doubly resonant external cavity is established. The influence of BK factor and cavity enhanced factor on SFG conversion efficiency was analyzed. Then the factors that influence BK factor, the dependent relationship of BK factor and cavity structure parameters were detailed studied. Similarly, the factors that influence enhancement factors, the dependent relationship of enhancement factors and the reflectivities of the input couplers were detailed studied. Thereby the cavity structure parameters and the reflectivities of the input couplers were optimized by computational simulation and the conversion efficiency of SFG was improved. In experiment, the continuous-wave589nm emission with an output power of larger than0.2W has been obtained by mixing the938nm and1583nm fundamental lights in a bowtie-configured cavity containing a PPLN crystal and using the optimum cavity parameters that obtained by previous theoretical calculations. The experimental results and the prediction agree well considering the imperfect mode matching and reflection losses of cavity mirror and PPLN crystal.3. The wavelength tuning, frequency locking and parameter measurement of sum-frequency output generated in a doubly resonant external cavity have been studied theoretically and experimentally. First, the frequency scanning is analyzed theoretically when doubly resonant external cavity system is serially locked. In order to lock the frequency of sum-frequency output to reference frequency, the direct absorption spectrum and saturated fluorescence spectrum produced by leading the sum-frequency output to the gas cell was studied in detail. In experiment, a scanning signal was added to the port of frequency modulation of938nm laser source with large tuning range. The frequency of sum-frequency output can be scanned as soon as we scan the938nm frequency. The linewidth of sum-frequency output was measured by scanning the cavity length of a high-finesse F-P cavity. The sodium absorption spectrum and saturated fluorescence spectrum at different temperatures were measured by the design of the temperature control device. The frequency locking of589nm sum-frequency output to sodium D2a line was realize using wavelength modulation frequency locking technique. Finally, the variation of error signal with frequency locking was measured and was compared with the variation of error signal without frequency locking. The results show that the maximum scanning range of589nm frequency is about2.2GHz, the linewidth is7MHz. When the frequency of589nm sum-frequency output is locked to sodium D2a line, the frequency stability is improved from400MHz to4MHz.The creative works are as follows:1. The sum-frequency in doubly resonant external cavity system is studied in detail. When the fundamental beam is fixed, the factors that influence the SFG conversion efficiency is analyed. The optimum BK factor can be obtained by optimizing the cavity length parameters, the perfect impedance matching can be achived and the cavity enhanced factor can be improved by optimizing the reflectivities of the input couplers, thereby improving the SFG conversion efficiency2. The impact of walk-off effect of two orthogonal linearly polarized components in a single crystal on HC frequency locking technique is analyzed. This impact can be eliminated by using bonded crystal or periodically poled crystal. Therefore the type II phase matching SFG can be achieved in the doubly resonant external cavity based on HC frequency locking technique.3. The phase correlated locking of a doubly resonant external cavity are realized based on HC frequency locking technique. The cavity mode frequency of bow-tie cavity is locked the frequency of938nm laser firstly, and then the frequency of1583nm laser is locked to the cavity mode frequency of bow-tie cavity, thereby two fundamental beams resonant with the external cavity simultaneously.4. The wavelength tuning of sum-frequency output is achived when doubly resonant external cavity system is serially locked. The sum-frequency output is tuned as soon as tuning the frequency of the master laser5. The589nm sum-frequency output is generated by mixing938nm and1583nm fundamental lights in a doubly resonant external cavity. The1583nm laser source is a fiber laser, the938nm laser source is a semiconductor laser and the nonlinear crystal is PPLN. The sum-frequency conversion efficiency is42.8%更多还原