【作者】 胡静；

【导师】 袁孝；

【作者基本信息】 苏州大学 ， 光学工程， 2016， 硕士

【摘要】 激光的光束质量会对激光的应用产生直接的影响。由于基模光束的衍射损耗小,能量集中,光束发散角小,在许多应用领域都要求激光器为基模输出。但是激光器的实际工作状态一般处在多模运转状态,含有高阶横模的激光束光强分布不均匀、光束发散角大及相干性差。为了提高和改善激光器的输出光束质量,通常需要对谐振腔进行横模选择。传统的横模选择方法主要依赖改变腔型和使用小孔光阑等,但仍存在腔内损耗较大,模体积小,输出功率小等诸多问题。基于光热敏折变玻璃的体布拉格光栅以其优秀的角度选择性、光谱选择性、以及较高的损伤阈值等优点,广泛地应用于高功率激光系统中。本论文提出采用体布拉格光栅作为谐振腔的横模选择元件,并进行了横模选择的理论研究。首先,利用耦合波理论分析了不同光栅结构参数对体布拉格光栅衍射特性的影响及用于横模选择时的光栅参数设计,并分析了体布拉格光栅的角度选择性对不同高阶横模的衍射损耗和强度分布的影响。其次,结合快速傅里叶变换法和耦合波理论建立了对体布拉格光栅谐振腔模式的求解算法,可计算出不同横模的强度分布和衍射损耗。最后,分别以透射型体布拉格光栅和反射型体布拉格光栅作为平行平面腔的腔内元件,建立了无源体布拉格光栅谐振腔模型。对体布拉格光栅谐振腔的模式进行求解,分析了其横模的模式特性,并对比分析了透射型和反射型体布拉格光栅角度选择性对横模近场及远场分布的影响。结果表明:在体布拉格光栅的角度选择性的作用下,平行平面腔不同横模的强度分布分别得到了优化,不同横模之间的衍射损耗差异增大。匹配谐振腔的腔型、增益以及调节体布拉格光栅的角度选择能力,可有效扩大不同横模间的损耗差异,提高谐振腔的选模性能,从而实现基模输出。更多还原

【Abstract】 The quality of laser beam has direct impact on the the application of the laser.Due to the small diffraction loss,energy concentration and small beam divergence angle,the fundamental transverse mode is required in many laser applications.However,the laser usually operates in multimodes state,and will make the output laser beam with nonuniform distribution,large beam divergence and poor coherence.In order to improve the output beam quality,the transverse mode selection is required in the laser resonator.The traditional transverse mode selection method mainly depends on the resonator parameters or using a small aperture in the resonator,but there are still have some problem need to solve,such as large diffraction loss,small mode volume and small output power.The volume Bragg gratings(VBGs)recorded in a photo-thermo-refractive(PTR)glass has several unique optical properties such as high diffraction efficiency,excellent angular selectivity,high damage threshold,and has many applications in high power lasers.In this thesis,volume Bragg gratings used in a laser resonator as transverse mode selection elements is proposed.First,the diffraction models for different types of volume Bragg gratings are introduced by the couped wave theory,the influence of different grating structure parameters on the diffraction characteristics are analyzed and introducing the design of grating parameters for transverse mode selection,and the diffraction loss of the different high order transverse modes diffracted by volume Bragg gratings are discussed.Secondly,a solution to calculate output modes of a laser resonator with volume Bragg gratings is established by the Fast Fourier transform method and the couped wave theory.At last,using the transmitting Bragg grating and the reflecting Bragg grating in a plane-parallel resonator as volume Bragg resonator modal respectively,then simulated the output modes of the volume Bragg resonator model,and the characteristics of the transverse modes are analyzed.The influences of angular selectivity of the transmitting Bragg grating and the reflecting Bragg grating on the transverse modes both in the near field and in the far field are discussed.The results show that under the effect of volume Bragg gratings in laser resonator,the intensity distribution of transverse modes is optimized,and the loss differences between different transverse modes are getting large.By matching the parameters of laser resonator,gain and angular selectivity of volume Bragg gratings,the loss difference between different modes enlarged effectively,the performance of the laser resonator in transverse mode selection improved,and the fundamental transverse mode output can be obtained.更多还原