Bandwidth Tunable Ultra-Broadband OAM Generators Based on Chirped Long Period Fiber Gratings at Dispersion Turning Point

We demonstrated the fabrication of bandwidth tunable ultra-broadband mode converters based on chirped long period fiber grating (C-LPFG) and tilted chirped LPFG (TC-LPFG) in a two-mode fiber (TMF) by a CO
₂
laser. The proposed mode converters based on C-LPFG and TC-LPFG experimentally realized mode conversion from the fundamental mode (LP
₀₁
mode) to LP
₁₁
mode and ±1
^st
order orbital angular momentum with a 10-dB bandwidth of 409.1 nm and 405.3 nm, respectively. The maximum tunable range of 10-dB bandwidth the proposed TC-LPFG is 50 nm, which is twice wider than that of the C-LPFG, while maintaining the same twist rate ranging from -36 rad/m ∼36 rad/m applied on the gratings. Consequently, the 10-dB bandwidth of the TC-LPFG can cover the O + E + S + C + L bands, which excellently optimize the bandwidth tunable range. The proposed bandwidth tunable ultra-broadband mode converters hold great potential for applications in mode division multiplexing transmission systems and optical fiber sensing systems based on few-mode fibers.