Femtosecond laser inscribed parallel long-period fiber gratings for multi-channel core mode conversion

We propose and demonstrate the inscription of parallel long-period gratings (LPGs) in a few-mode fiber (FMF) using femtosecond lasers. Mode conversion from the fundamental mode LP01 mode to high order core modes including LP11, LP21, LP31, LP02, and LP12 modes is achieved by controlling the inscription period of the gratings. Taking advantage of the highly focused femtosecond laser, LPGs with different off-axis offsets were fabricated, and the resonance wavelength and the inscription efficiency of the gratings versus the offset were investigated. Based on the off-axis writing technique and using the femtosecond laser source, we wrote parallel LPGs that contain multi-gratings in a single FMF and achieved a multi-channel core mode converter in a single FMF with flexibility in terms of the resonant wavelength and mode conversion among different modes. This approach offers a new option for implementing with high integration, and a multi-channel mode converter, which could find potential applications in the FMF multi-wavelength laser system, and wavelength/mode division multiplex communication system. Furthermore, this microstructured LPGs integrated into an optical fiber can be used as a multifunctional sensor.