Tunable Orbital Angular Momentum Mode Conversion in Asymmetric Long Period Fiber Gratings

In this letter, the asymmetric long period fiber gratings (aLPFGs) writing in hollow ring-core fiber is proposed to excite orbital angular momentum (OAM) modes. To reveal the characteristics of aLPFGs, the transmitted power of the modes supported in the ring core is calculated by coupled-mode theory analytically. The resonance wavelength for the OAM mode can be manipulated by both the grating period and the refractive index of liquid filled in the fiber. The good wavelength tunability and broadband performance can be guaranteed by choosing the grating period near the turning point of dual-wavelength resonance of the aLPFGs. The proposed fiber mode convertor based on aLPFGs is flexible and compact, making it a good candidate for tunable OAM mode excitation and optical fiber sensor.