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高效地产生相互正交的各阶轨道角动量（Orbital Angular Momentum,OAM）模式具有重要的研究价值。目前全光纤系统中高效地产生高阶轨道角动量模式的方法主要是基于二氧化碳激光器加工的长周期光纤光栅（Long period fiber grating,LPFG）。然而产生高阶模式的光栅需要强的折射率调制与小的光栅周期，因此二氧化碳激光器高的功率和大的聚焦光斑不利于其刻写的重复性、成功率和延展性。为了解决这一问题，本文首次提出并制作了基于飞秒激光加工的三阶OAM模式转换器。本文在六模光纤上加工出了非对称的长周期光纤光栅，实验结果表明它在1550nm附近能将基模转换为三阶的角向线性偏振模式LP31模式，模式转换效率为98%，进一步地该模式可以被叠加转化为三阶OAM模式。与此同时，在1310nm附近，该光栅还能够产生角向一阶径向二阶的OAM模式。该方案证明了飞秒激光加工提供了一种可用于全光纤系统具有高重复刻写性的长周期光纤光栅来产生高阶OAM模式的思路。The generation of Orbital Angular Momentum (OAM) modes is of great importance in a variety of applications such as optical tweezers, quantum optics, and optical communication systems. Particularly, how can high-order OAM modes be generated efficiently in fibers with the advantage of low cost and compatible with fiber system. the Traditional method for first order to third order OAM is based on long period fiber grating (LPFG) fabricated by Carbon dioxide laser. However, high power and large focused spot of Carbon dioxide laser are unfavorable for stable and repeatable generation of higher-order OAM, which needs the LPFG with small grating pitch. In order to solve this problem, a third-order OAM mode converter based on femtosecond microfabrication is proposed and fabricated for the first time. With the advantage of 4.4μm focused spot size near the core, lower power and lower heat absorption efficiency, this method can be more stable and promising. Therefore, we firstly did mode filed analysis and simulate the intensity and phase profiles of the superposed mode field in LP odd-even mode with different scales and phases patterns to obtain OAM mode. Second, we use the coupled-mode theory to analysis and simulate the transmission spectrum of LPFG,which guide the setting of the grating parameter such as the grating pitch, the depth of modulation and the length of the grating. By experimental verification, an asymmetric modulated long-period fiber grating with a pitch setting to 194μm is fabricated on six-mode fiber. the fundamental mode can be converted to the third-order angular linear polarization mode LP31 mode with 98% mode conversion efficiency near 1550nm, and further converted to the OAM±3 modes by superposition of the odd and even LP31 mode with ±π∕2 phase difference. At the same time, this fiber gratings can also generate LP12 mode with 90% mode conversion efficiency near 1325nm. Then we can take the same approach to transform LP12 mode to OAM modes with angular first-order as well as radial second-order. The experiment is consistent with the simulation. Thus, this scheme provides an idea to generate high-order OAM modes in all-fiber systems using only one grating with high repeatability.
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Keywords:
- Orbital angular momentum /
- Fiber grating /
- Fiber optics components /
- Fiber optics communications
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