An all-fiber mode converter assisted by coiled-fiber long-period grating

•An all-fiber mode converter is formed by periodically coiling one thin tapered SMF onto the thick straight tapered FMF.•Specific higher-order modes can be excited, since the coiled SMF can provide a strong periodical modulation to evanescent field of the straight FMF.•By adjusting coiling period number, coupling efficiency between fundamental mode in the SMF and specific higher-order modes in the FMF can be optimized.•This proposed structure provides flexibility in coupling efficiency optimization and compatibility with fiber communication systems.

An all-fiber mode converter assisted by coiled-fiber long-period grating (CFLPG), which is formed by periodically coiling one thin tapered single-mode fiber (SMF) onto one thick straight tapered few-mode fiber (FMF), is experimentally demonstrated. Owing to the strong periodic modulation of the coiled tapered SMF to evanescent field of the straight tapered FMF, specific higher-order modes in the FMF can be successfully excited from the fundamental mode of the SMF. Moreover, coupling efficiency from fundamental mode of the SMF to specific higher-order mode of the FMF can be optimized by adjusting the number of the coiling period, which is beneficial for the design and fabrication of mode converter. This proposed structure can be extended to even higher-order mode conversion, and has flexibility in coupling efficiency optimization and compatibility with current optical fiber communication systems.