Near 400 nm Broadband Mode Converter Based on Cascaded Long Period Fiber Gratings

We demonstrated the ultra-broadband mode converters using cascaded long-period fiber gratings (c-LPFGs), which possess a maximum bandwidth of near 400 nm. The design and fabrication of c-LPFGs are fully investigated, exploring various period combinations and utilizing CO2 laser irradiated on specialty few mode fibers near the dispersion turning point. Theoretical simulations and experimental results demonstrate the efficiency of c-LPFGs in exciting ultra-broadband LP11 core modes and first order orbital angular momentum mode. The achieved experimental 10-dB bandwidth of 395.8 nm highlights broadband mode conversion capabilities. Additionally, the 10-dB bandwidth of the gratings reveals tunability through twist rate adjustments, providing a further enhancement of 13.7 nm with a twist rate of 36 rad/m. The proposed c-LPFGs showcase potential for practical applications in optical communication systems, making significant contributions to ultra-broadband high-order mode generation.