Mode converter with C+L band coverage based on the femtosecond laser inscribed long period fiber grating

The mode converter (MC) is one of the key components for mode division multiplexing (MDM). Here, we experimentally demonstrate an all-fiber long period grating (LPG) based MC inscribed in the few-mode fiber (FMF) with the line-by-line femtosecond laser irradiation technique. Experimental characterization results agree well with the theoretical calculations, and a clear mode evolution from the LP₀₁ mode to the LP₁₁ mode is observed with the tuning of the operation wavelength. An average mode conversion efficiency of more than 90% and an average insertion loss of less than 5 dB, together with a polarization-dependent loss of less than 3 dB, are achieved over the C+L band with a good repeatability. The proposed MC based on the LPG inscribed into the FMF has the advantages of mode scalability, compact size, and wideband operation, which is desired for the wavelength division multiplexing (WDM) and MDM hybrid transmission.