All-fiber bandwidth tunable ultra-broadband mode converters based on long- period fiber gratings and helical long-period gratings

We demonstrated the fabrication of bandwidth tunable ultra-broadband mode converters based on CO2-laser inscribed long-period fiber gratings (LPFGs) and helical long-period gratings (HLPGs) in a two-mode fiber (TMF). The simulation and experimental results show that there is a dual-resonance coupling from LP01 to LP11 core mode at the dispersion turning point. The mode converters based on the TMF-LPFG and TMF-HLPG provide a 10-dB band-width of similar to 300 nm and similar to 297 nm, respectively, which covers O + E +S + C band. The 1st order orbital angular momentum (OAM) mode based on TMF-LPFG was generated by adjusting the polarization controllers (PCs), while the 1st order OAM mode can be generated directly by the TMF-HLPG. When the twist rate is varied from -36 rad/m similar to 36 rad/m, the tunable range of the 10-dB bandwidth is similar to 52 nm and similar to 91 nm for the LPFG and HLPG mode converters, respectively. The ultra-broadband mode converter can be adopted as a bandwidth tunable mode converter, which can be applied in ultra-broadband mode-division-multiplexing transmission systems and optical fiber sensing systems based on few-mode fibers. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.