Few-mode long-period fiber grating (FM-LPFG), as a highly-integrated mode converter with few-mode fibers, has the advantages of high conversion efficiency, low insertion loss, strong robustness, simple fabrication, and inherent compatibility with fiber systems. These characteristics make it a highly promising candidate for a wide range of applications in optical communication transmission systems, fiber lasers, and optical sensors. In this study, the research progress of few-mode long-period fiber grating mode converter and its applications are summarized. We believe that this study is valuable for all researchers interested in FM-LPFG based devices and systems.

It has been more than 40 years since long-period fiber gratings were proposed. In contrast to single-mode long-period fiber gratings, researchers mainly focus on the conversion between the fundamental mode and core modes for FM-LPFG,which is achieved by satisfying the phase-matching conditions between the core modes in few-mode fibers. High conversion efficiency, low insertion loss, and wide bandwidth are the key performance indicators of FM-LPFG. The precision and efficiency of FM-LPFG have been significantly improved with the continuous advancement of fabrication technology, which has further inspired researchers to explore FM-LPFG with higher orders, wider bandwidths, and more channels. Significant progress and development have been achieved in these aspects. Researchers have proposed various techniques to fabricate FM-LPFGs, thereby achieving the generation of first- to fourth-order modes with high conversion efficiencies and low insertion losses, first- to third-order modes with a 15 dB bandwidth of more than 100 nm, and first- to fourth-order modes with multiple channels. These outstanding advances not only provide new perspectives and ideas for the research of FM-LPFG but also lay a solid foundation for the promotion of the development and application of related technologies.

This study summarizes the research progress of a few-mode long-period fiber grating mode converter and its applications. First, the mode theory of few-mode fibers and the mode-coupling mechanism of long-period fiber gratings are introduced. Next, the characteristics and methods of the principal manufacturing techniques for few-mode long-period fiber gratings are described in detail. Subsequently, we focus on the progress in the development of higher-order, broadband, and multichannel few-mode long-period fiber gratings. A detailed summary is provided to introduce the applications of few-mode long-period fiber gratings in the fields of few-mode fiber lasers, sensing, and optical communication transmission systems. As the demand for high-capacity data transmission continues to grow, the development of new structures as well as high-performance and multifunctional few-mode long-period fiber gratings is expected to become an important direction of development for the new generation of fiber passive components.