Mode converter based on the long-period fiber gratings written in the two-mode fiber

Abstract

We demonstrate the fabrication of long-period fiber gratings (LPFGs) written in the two-mode fiber (TMF) by CO₂ laser. Both uniform and tilted LPFGs were fabricated to provide the light coupling between LP₀₁ mode and LP₁₁ mode with a coupling efficiency of more than 99%. The writing efficiency and the bandwidth of the LPFG mode converter can be adjusted by changing the tilt angle of the tilted TMF-LPFGs. The torsion sensitivity of conventional and tilted LPFG mode converters were measured to be 0.37 nm/(rad/m) and 0.50 nm/(rad/m), respectively. Two orthogonal vector modes (the HEeven 21and HEodd 21 modes) and corresponding orbital angular momentum state were successfully obtained at the resonance wavelength. The proposed LPFG mode converter could be used as not only a high efficiency wavelength tunable mode converter in the mode division multiplexing system but also a high sensitive torsion sensor in the field of optical sensing.

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“…At present, vortex beams in the fiber lasers have been demonstrated because of the promising applications in the quantum optics, 188 optical micromanipulation, 189 rotation detection, 190 WDM (modedivision multiplexing) systems, 191 and nonlinear fiber optics. 192 In the fiber systems, the vortex beams are generally realized by the modulating elements, including the mode selective couplers, 193,194 long period fiber gratings, 195,196 and microstructured fiber facets. 197 The mode-locked vortex beams through the mode fibers in the all-fiber lasers have been reported.…”

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