Advances on Mode-Coupling Theories, Fabrication Techniques, and Applications of the Helical Long-Period Fiber Gratings: A Review

In this paper, we have briefly review the developing history and recent advances made with regard to helical long-period fiber gratings (HLPGs) in three aspects, i.e., the mode-coupling theories, the fabrication techniques, and the applications. It is shown that, due to the intrinsic helicity characteristics, which are especially suitable to control the loss, polarization, and orbit-angular-momentum (OAM) states of the light in optical fiber, HLPGs have recently attracted great research interest and have found various applications, such as the mode-converters, the torsion sensors, the band-rejection filters, wave plates, linear- and circular-light polarizers, and OAM mode generators, etc. It is believed that HLPGs and the HLPGs-based devices would find further applications to not only the fields of optical sensors and optical communication, but also other fields such as ultrahigh precision measurement, quantum optics, and biochemistry, etc.