Long-period gratings based generator of all-fiber third-order orbital angular momentum modes assisted by femtosecond microfabrication

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.