Abstract
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.
Highlights
Helical fiber gratings, called the chiral fiber gratings or the periodically twisted fibers, refer to the fibers where there exists a periodic screw-type deformation in the core or the equivalent screw-type index-modulation along the fiber direction
In terms of the period magnitude of the twisted fiber, helical fiber gratings mainly can be divided into two types: the first one is the helical long-period fiber grating (HLPG), whose period lies in the range between 100 μm to several millimeters
We focus on the HLPGs hereafter
Summary
Before 2015, most of the works about the mode couplings in HLPG were concentrated on the solutions of the scalar waveguide equation using the conventional perturbation analysis method. From the Equations (10) and (11), it is easy to infer that for a single-helix HLPG, i.e., l = 1, due to either the eccentric core of the originally utilized fiber [18], or the imperfect cylindrical symmetry of the HLPG produced during the twisted processing [14], there exist some higher-order harmonics in azimuthal direction of the index-modulation, which implicitly means that even in a single-helix HLPG, the mode couplings between the fundamental mode and the higher azimuthal modes, e.g., the LP21 and LP31 modes may occur as long as the Fourier coefficients S2 and S3 shown in Equation (8) are nonzero ones and the HLPG is written in a fiber where such two modes are supported.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
