径向偏振、锁模光纤激光器研究

Characters)：Radially polarized light is knows for its annular intensity distribution and axissymmetrica electric field pointing to local radial direction.It is suitable for many important applications like in high-resolution microscopy and lithography, partical trapping and metal processing because of its unique features like sharp focusing and others. Direct emsssion of radially polarized light from a contineous-wave fiber laser has been attractive since the birth of first radially polarized fiber laser in 2006. Nevertheless, up to now, the investigation on the mode-locked and radially polarized fiber laser, which directly emits radially polarized and ultrashort pulse, still remained absent. In this investigation, we will, for the first time, associate the radial polarization control of fiberlaser with mode-locking technique. In our scheme, we will utilize a photonics crystal grating as intracavity polarization-selective component, and choose either an optically-driven-deposition graphene or a semiconductor saturable absorber mirror (SESAM) as the modelocker, and then build up an experimental platform of radially polarized,mode-locked and ytterbium-doped double-clad fiber laser for further investigation. This investigation would, for the first time, realize the oscillation of radially polarized and mode-locked fiber laser, and obtain ps or fs radially polarized pulse. Furthermore, by adjusting the laser parameters (including grating mirror's reflectivity, length and doping concentration of gain fiber, modulation depth of graphene or SESAM and etc), the effect of self phase modulation and mode coupling inside the gain fiber on the ultrashort and radially polarized pulse would be investigated both theorectically and experimentally.

径向偏振光具有环形光强分布且其电场呈指向半径方向的轴对称分布。它具有紧聚焦等特性，在高分辨显微、光刻、粒子捕获及金属加工领域可获重要应用。自2006年首次实现连续输出的径向偏振光纤激光器以来，利用光纤激光器输出高效、高功率径向偏振光成为研究热点，但将锁模和径向偏振技术同时用于光纤激光器以产生超短径向偏振脉冲尚属研究空白。 本研究将首次把光纤激光器的径向偏振控制和锁模技术结合起来，利用光子晶体光栅作为偏振选择器，分别选择光诱导沉积石墨烯和半导体可饱和吸收镜（SESAM）作为锁模元件，搭建径向偏振锁模掺镱光纤激光器平台并开展实验研究，率先实现径向偏振锁模光纤激光器振荡，获得皮（飞）秒径向偏振脉冲输出。同时通过对激光器参量（光栅镜反射率、增益光纤长度和浓度、石墨烯或SESAM 调制深度等）的优化，理论和实验探讨增益光纤中偏振色散、自相位调制和模间耦合对对径向偏振锁模脉冲的影响。

理论工作:.结合光纤的模式理论，根据速率方程对双包层光纤激光器进行了数值模拟，获得了小信号增益分布和激光器输出特性。分析了利用c切单轴晶体作为选模元件实现径向光选模的条件。建立了锁模、径向偏振光纤激光器理论模型，分析了增益光纤长度、输出镜透过率对输出激光束的偏振态的影响，以及不同SESAM参数（调制深度、弛豫时间、饱和强度等）、激光器腔结构（直线型腔和折叠型腔）及腔长等参数对形成调Q、调Q锁模、连续锁模脉冲的影响。..实验工作:.a).高功率连续切向偏振光纤激光器实验研究。提出了采用低成本，无波长敏感性的单轴晶体和单透镜成像系统组合的偏振选模机制，成功实现了连续输出的高效、高功率、切向偏振掺镱光纤激光器。激光器输出功率达3.94 W，斜效率约为71%，实验结果证明了偏振选模机制的有效性，获得了高光束质量和高功率效率的矢量光输出。.b).基于Cr4+:YAG可饱和吸收体的高功率被动调Q切向偏振光纤激光器实验研究。基于上述偏振选模机制，采用Cr4+:YAG可饱和吸收体，实现了高效、高功率的切向偏振被动调Q掺镱光纤激光器脉冲输出。该研究将汇聚光束直接作用于单面镀膜的Cr4+:YAG晶体，使得该晶体在被饱和的同时起到输出耦合镜的作用，简化了激光器结构并降低了成本。.c).基于SESAM高功率被动调Q径向偏振光纤激光器研究。基于上述偏振选模机制，分别采用线型腔和折叠腔结构，实现了基于SESAM器件的被动调Q、径向偏振的掺镱光纤激光器激光器。线型腔光纤激光器调Q脉冲功率达到3.6W，斜效率为68.7%。折叠腔光纤激光器的脉冲功率为637 mW，斜效率为14.1%，激光脉宽为300 ns，重复率为454.5 kHz。 .d).径向偏振、调Q锁模光纤激光器研究。基于上述偏振选模机制，开展了基于SESAM器件的被动调Q锁模、径向偏振的掺镱光纤激光器实验研究，获得了平均功率为202mW, 重频为66.67 MHz，脉宽为401 ps的径向偏振锁模脉冲。.以上研究结果达到了本项目的研究预期。