全光纤轴对称偏振激光器的脉冲技术和偏振调控研究

Polarization is one important property of light. Cylindrical Vector beams (CVBs) have drawn considerable attention recently due to their unique axially symmetry in both field-amplitude and polarization. This polarization symmetry leads to interesting properties that find applications in many areas such as optical tweezers, high-resolution metrology, surface plasmon excitation and material processing. A variety of passive and active techniques have been developed to generate CVBs. Because of their compactness, excellent flexibility and high efficiency, fiber laser CVBs sources have attracted increasing interests. Several fiber laser designs have been successfully demonstrated to generate CVBs. However, volume optical elements or free space propagation were still used in the fiber laser cavity designs, which could increase the complexity of the system, instability of the laser output, and laser threshold..We propose a novel fully integrated all-fiber laser generating CVBs using few-mode fiber grating. The reflection spectra, polarized modes and dispersion characteristics of the few-mode fiber grating will be investigated experimentally and theoretically. The Q-switched (or mode-locked) all-fiber laser generating CVBs is proposed by combination of few-mode fiber grating with Q-switch (or SESAM). The radially and azimuthally polarized pulse can be manipulated and switched by new technique in the fiber laser cavity. We anticipate this novel all-fiber laser approach to find a broad array of applications in areas such as optical imaging, sensing, manipulation and laser nano-fabrication.

偏振是光的主要特性，轴对称偏振光束的电场振动方向在光束横截面上具有轴对称性，光束的波振面呈漩涡状，且中心呈现奇异性。轴对称偏振光束独特的光学特性使其在生物光镊、空间通信、高分辨率显微镜技术、电子加速以及激光加工等领域有着非常重要的应用。轴向偏振激光器全光纤化是目前一个重要的发展方向，目前的轴对称偏振光纤激光器中通常都包含体器件，造成激光器阈值高且结构复杂。本项目在前期工作的基础上，全面研究少模光纤光栅的色散特性、带宽特性、偏振模式与波长对应特性等，开展基于少模光纤光栅的全光纤激光器中轴对称偏振光束的产生机理、调Q技术、锁模技术和偏振调控技术等研究，获得的全光纤轴对称偏振激光器的脉冲输出，拓展轴对称偏振光束的功能并使得很多新的应用成为可能。

轴对称偏振光束在众多领域具有广泛应用并有其独特优势，本项目从理论和实验上对基于少模光纤光栅的全光纤激光器中轴对称偏振光束的产生机理、产生技术和偏振调控技术进行多维研究，研制了各类全光纤轴对称偏振连续输出激光器、调Q激光器和锁模激光器。取得主要成果如下：①采用耦合模理论数值模拟了少模光纤光栅的响应特性，采用三维结构的功率传输方程对基于少模光纤光栅全光纤轴对称偏振光激光器进行系统分析。②研制了基于少模光纤光栅的低阈值单波长全光纤掺镱光纤激光器并实现了高纯度的轴对称偏振光束输出；在激光腔内加入可调谐滤波器，搭建了一种“横模-波长”关联可调的光纤激光器，调谐激光器波长获得了基模、二阶模和相干混合模三种不同的横模状态，其中角向偏振模式和径向偏振模式的模式纯度分别为99.3%和99%。③采用二硫化钨体材料实现了重频和单脉冲能量可调的轴对称偏振调Q激光器，激光器工作于1547.5nm，脉冲重复频率可随泵浦强度在80 kHz到120 kHz间调节，最小脉宽可达958 ns，脉冲能量可达4.6 nJ。④结合高速光调制器实现全光纤轴对称偏振主动锁模激光器，得到了高纯度的径向和角向偏振激光超短脉冲，脉冲重复频率为12.6 MHz，脉宽为2 ns，在泵浦功率为350 mW时，得到的单脉冲能量为1.09 nJ，峰值功率为324 mW；利用非线性放大环形镜作为锁模元件，获得了稳定工作的轴对称偏振锁模激光器的方波脉冲输出，脉宽从2.8 ns到23 ns连续可调，得到了纯度为96%的径向偏振和角向偏振的超短脉冲激光；采用碳纳米管作为锁模元件，研制了自启动的全光纤轴对称偏振被动锁模激光器，脉冲重频为10.61 MHz，脉宽为22.73 ps，信噪比高达75 dB。研究成果极大地丰富了轴对称偏振光束的功能，并拓展了轴对称偏振光束在激光加工、粒子加速、激光光镊以及高分辨显微成像等领域的应用。