Broadband vector mode converters based on cascaded long-period fiber gratings written in slightly elliptical ring-core fiber

A broadband vector mode converter is proposed based on cascaded long-period fiber gratings(LPFGs) written in slightly elliptical ring-core fiber. In order to realize the phase shift between the cascaded LPFGs, an unirradiated optical fiber is introduced. The mode converter can directly convert fundamental mode into first-order vector modes (TE01, HEeven21, HEodd 21 and TM01 modes) with high efficiency without using assistive devices. The effect of cascaded grating period and the unirradiated optical fiber length on the performance of the mode converter is investigated. Through optimizing the structure parameters, the designed broadband vector mode converters provide a 20-dB bandwidth of 104 nm, 97.5 nm, 101 nm and 97 nm for respectively generating TE01, HEeven 21 , HEodd 21 and TM01 modes covering C+L band, which could be applied in broadband mode division multiplexing (MDM) transmission systems based on vector modes.