We propose a method for the selective generation of high-order orbital angular momentum (OAM) modes by twisting a ring-core fiber (RCF). Theoretically, 22 OAM modes can be generated in the twisted RCF with effective mode separation. We experimentally demonstrate the excitation of OAM modes up to the fourth order. The positive or negative topological charge is determined by the clockwise or counterclockwise twist direction, indicating that the helical phase of the OAM can be controlled by the twist orientation. Compared with other fiber-based methods, the high-order OAM modes generated by our approach have the characteristics of low loss (minimum 0.3 dB), high mode purity (maximum 98%), and simple fabrication. This has potential applications in high-resolution imaging, high-capacity fiber-optics communication, vortex-optics sensing, and other fields.

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We fabricated a Yb:YAG crystal-derived silica fiber (YCDSF) by the melt-in-tube method with a CO2 laser-heated drawing tower and explored the influences of the heat treatment method on fluorescent properties in the YCDSF. After the heat treatment, the intensity of the emission peaks and the fluorescence lifetime of the YCDSFs improved. In particular, after 1350 °C of heat treatment, a series of sharp peaks appeared in the core layer, which may form a new crystalline phase. Moreover, its emission intensity at 1030 nm was significantly enhanced, over 2 times greater than before the heat treatment. Additionally, the fluorescence lifetime of Yb ions was also increased from 129 to 621 μs, indicating the changes in local environments around Yb ions. Then, schematic models were set up to show how the local environments around Yb ions are gradually changing. These results revealed that the assessed YCDSF is of excellent performance; after the heat treatment, it may be a potential material for realizing optical amplification, light sources, fiber lasers, and so on.

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