为了实现模分复用系统中的模式转换,提出了基于环形光纤的倾斜长周期光纤光栅的矢量模式转换方法。利用有限差分法和耦合模理论,研究了折射率调制的倾斜角度、幅度函数、光栅长度及耦合系数对模式耦合的影响。结果表明,在相位匹配条件下,该光栅可在不同波长处实现基模到特定高阶矢量模式的转换,且波长间隔大于150 nm。倾斜角度在模式耦合中起着关键作用,在倾斜角度约为84°时转换效率可达到最大。与现有模式转换方法相比,这种模式转换器模式间隔较大,模式间串扰低、转换效率高,在轨道角动量复用和矢量模式复用中有潜在应用价值。

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°. 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.