飞秒激光刻写偏振光纤光栅的微观机理及其在线偏振掺镱光纤激光器中的应用研究

Narrow-linewidth, linearly polarized Yb-doped fiber lasers are critical components for many applications, such as nonlinear frequency conversion and beam combining to produce higher power. In this body of project, we set up a physical model that the polarization-dependent effects of fiber Bragg gratings (FBGs) are a result of the elliptical shape of the refractive index modulations (RIMs) that constitute the FBG. The finite element method （FEM）and other theory are used to analyze the relationship between the polarization-dependent effects of FBGs and gratings parameters and fabrication conditions as well. In experiments, laser pulses from a regeneratively amplified Ti:sapphire femtosecond laser is focused into the core and inner cladding to fabricate a FBG and to induce the stress birefringence. The aforementioned theory analysis and experiments results will uncover the formation micromechanism of a polarization-dependent FBG. As a verification，a pair of polarization-dependent typeII FBGs is fabricated into non polarization-maintaining fiber to form the resonant cavity. The wavelength difference between the two orthogonal states of linear polarization reflection can be induced and enlarged to some extents by exposing the inner cladding of the fiber to fs laser, which will form refractive index modified regions in the cladding to induce stress birefringence. A dual-wavelength linearly polarized fiber laser will be studied by using the wavelength-difference-enlarged FBGs to form the resonant cavity, since the two wavelengths will not overlap. Comparing with the ordinary laser scheme which uses the polarization maintaining fiber and volume polarization components to select the polarized state of laser light, the proposed novel integrated linearly-polarized Yb-doped double clad fiber laser that incorporates two polarization-dependent gratings to form the resonant cavity, has more advantages, such as compact and robust platform, low price, higher polarization extinction ratio and few fusion points. Through the theoretical and experimental research of the project, we will obtain some important conclusions that will have the potential to help us design the fiber gratings with controlled polarization characteristics at different wavelengths and integrated linear polarized fiber lasers.

窄线宽、线偏振掺镱光纤激光器在非线性频率转换、相干组束等方面具有重要用途而受到研究者注目。本项目采用有限元法建立光栅区域光纤端面上的折射率椭圆形分布的物理模型，分析光栅的双折射特性与光栅参数及制作条件的关系，实验探索钛宝石飞秒激光制备光栅及在光纤中引起双折射的规律，揭示成栅的微观机理。作为验证，在非保偏增益光纤上写入typeII型偏振光栅对构成激光谐振腔，利用飞秒激光在光纤内包层引入的应力双折射加强偏振光栅的两正交偏振波长差异，实现双波长线偏振激光输出。该新型一体化激光器方案较之普遍采用保偏光纤结合体偏振器件来实现偏振态和波长的选择的方法，具有更多优点：熔接点少、造价低和整机鲁棒性强等特点。通过此项目的理论和实验研究，为设计偏振特性可控、波长可控的光纤光栅和一体化的线偏振光纤激光器提供规律性依据。

本项目采用有限元法建立光栅区域光纤端面上的折射率椭圆形分布的物理模型，分析光栅的双折射特性与光栅参数及制作条件的关系，实验探索钛宝石飞秒激光制备光栅及在光纤中引起双折射的规律，揭示成栅的微观机理。作为验证，在非保偏增益光纤上写入typeII型偏振光栅对构成激光谐振腔，利用飞秒激光在光纤内包层引入的应力双折射加强偏振光栅的两正交偏振波长差异，实现双波长线偏振激光输出。主要研究内容概括为：（1）采用有限元分析软件Lumerical分析了双折射光纤端面的光场分布，利用Comsol分析了双折射光纤的应力分布，揭示了不同应力区域对双折射特性的影响；（2）搭建了飞秒激光的光栅刻写系统，研究了光纤光栅的逆向设计理论；并利用800nm飞秒激光刻写了光纤光栅；（3）以单模保偏双包层掺镱光纤为增益介质，实现了输出功率为１Ｗ的稳定的双波长激光输出，该激光信噪比为４８ｄＢ，斜率效率为３4％，通过腔内增加偏振选择器件，实现了线偏振输出；（4）研究了激光器的热管理，重点分析了半导体激光器封装内的温度仿真，增镱光纤和溶解点的温度分析，揭示了高功率光纤激光器中光纤炸裂的原因；（5）开展了石墨烯、碳纳米管等材料用于可饱和吸收体的研究，实现了1um,1.5um,2um等波段的锁模激光输出。