Tunable Broadband Mode Converter Based on Long-Period Fiber Gratings at 2-μm Waveband

We demonstrate the fabrication of a broadband mode converter (MC) operating at 2 μm waveband based on long-period fiber gratings (LPFGs) written in the two-mode fiber (TMF) by CO
₂
laser. The proposed MC offers a 10-dB bandwidth of 58 nm, making it suitable for high-speed broad-band optical communication system at 2 μm waveband. The maximum conversion efficiency reaches 98.89% at wavelength of 2 μm. The torsion, strain, bending, and polarization characteristics of the broadband MC were measured experimentally. The torsion sensitivity of the LPFGs was measured to be 1.32 nm/(rad/m) at 2 μm waveband, which was almost three times higher than that of the grating with the same order mode coupling at 1.55 μm waveband. The proposed broadband MC can function as not only a promotion in fiber communication system but also a high sensitivity torsion sensor in fiber sensing system.