Abstract
In this paper, comprehensive remarks are given that focus on the main fabrications and wide applications of helical long-period fiber gratings (HLPGs). Firstly, the techniques of fabricating HLPGs by CO2 laser, hydrogen–oxygen flame heating, and arc discharge are summarized. Furthermore, the applications of HLPGs are investigated, i.e., orbital angular momentum (OAM) mode converters, all-fiber band-rejection filters, and sensors for measuring physical perturbation of torsion, strain, temperature, curvature, and surrounding refractive index (SRI). Furthermore, several long-period fiber gratings (LPFGs) of near-HLPG structures with periodic refractive index change along the azimuthal direction are introduced. Lastly, the prospects and key challenges for HLPGs are discussed.
Highlights
The long-period fiber grating (LPFG) is an optical passive device, which can couple the core mode to the cladding modes transmitted in the same direction
Since the chiral long-period fiber grating (CLPG) is formed by continuously twisting glass fiber when passing through the miniature heat zone, whereby a periodic structure is generated in the fiber with a helical path, the CLPG is known as a helical long-period fiber grating (HLPG)
Because of this excellent property, HLPGs implemented in various types of fiber, such as single-mode fiber (SMF) [26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60], two-mode fiber (TMF) [61,62,63,64,65], few-mode fiber (FMF) [66,67,68], double-clade mode fiber (DCF) [69,70] photonic crystal fiber (PCF) [71,72,73,74,75,76,77,78], multicore fiber (MCF) [79,80,81] polarization-maintaining fiber (PMF) [82], and dual-hole elliptical core fiber (DEF) [83], have been proposed and demonstrated
Summary
The long-period fiber grating (LPFG) is an optical passive device, which can couple the core mode to the cladding modes transmitted in the same direction. Unlike traditional LPFGs induced by UV irradiation, HLPGs have the characteristic of periodic spiral refractive index modulation along the axis direction and do not require the fiber materials to have photosensitive properties Because of this excellent property, HLPGs implemented in various types of fiber, such as single-mode fiber (SMF) [26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60], two-mode fiber (TMF) [61,62,63,64,65], few-mode fiber (FMF) [66,67,68], double-clade mode fiber (DCF) [69,70] photonic crystal fiber (PCF) [71,72,73,74,75,76,77,78], multicore fiber (MCF) [79,80,81] polarization-maintaining fiber (PMF) [82], and dual-hole elliptical core fiber (DEF) [83], have been proposed and demonstrated.
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