Vector Mode Conversion Based on Tilted Long-Period Fiber Grating Written in Ring Fiber

In this study, we propose a vector mode conversion method based on tilted long-period fiber grating written in ring fiber to realize mode conversion with respect to the mode division multiplexing system. In addition, we study the influences of the grating parameters, including the tilt angle modulated based on the refractive index, the amplitude function, the grating length, and the coupling coefficient, on mode coupling using the finite difference method and the coupled-mode theory. The obtained results indicate that the fundamental mode can be converted to high-order vector modes at specific wavelengths under the phase-matching constraint; further, the wavelength interval is greater than 150 nm. The tilt angle plays a major role during the mode coupling process, and the maximum conversion efficiency can be obtained when the tilt angle is approximately 84 degrees. The proposed converter has the advantages of larger mode interval, lower crosstalk, and higher conversion efficiency; therefore, it exhibits considerable potential for application in orbital angular momentum multiplexing and vector mode multiplexing when compared with those of the existing vector mode conversion methods.