Design and optimization of fundamental mode filters based on long-period fiber gratings

•A technique that applying a symmetric apodized LPFG that can selectively filter the fundamental mode and at the same time keep the high-order modes in the few-mode optical fiber core at low loss is proposed.•The proposed mode filter can be used to suppress the unwanted modes during the demultiplexing process for mode-division multiplexing (MDM) systems.•The technique of cascading LPFGs with different periods is presented to increase the operating bandwidth of the proposed mode filter.•The mode filtering characteristics of the cascading LPFGs are dependent strongly on the FMF segment with high-index coating.

A segment of long-period fiber grating (LPFG) that can selectively filter the fundamental mode in the few-mode optical fiber is proposed. By applying an appropriate chosen surrounding material and an apodized configuration of LPFG, high fundamental mode loss and low high-order core mode loss can be achieved simultaneously. In addition, we propose a method of cascading LPFGs with different periods to expand the bandwidth of the mode filter. Numerical simulation shows that the operating bandwidth of the cascade structure can be as large as 23 nm even if the refractive index of the surrounding liquid varies with the environment temperature.