Dynamic monitoring of few-mode long-period fiber gratings using swept-wavelength interferometry

We present the dynamic monitoring of long-period fiber gratings (LPFGs) using swept-wavelength interferometry (SWI) for the first time, to our knowledge, including both grating fabrication and its environmental responses. One type of LPFGs designed for mode conversion is fabricated by applying pressure to a few-mode fiber. SWI tests provide mode-dependent losses (MDLs), insertion losses (ILs), and mode conversion efficiencies (MCEs) of the LPFG across the C-band. The LPFG exhibits a maximum MCE of 80.73% at 1550 nm, with an MDL of 0.08 dB and an IL of 0.62 dB. This approach enables the high-quality fabrication of LPFGs and is applicable to various fabrication techniques. Also, dynamic thermal responses of LPFGs are monitored using SWI, which reveals a highly linear relationship (R² = 0.99) between mode cross talk (XT) and temperature, providing a new approach for LPFG-based temperature sensing.