Abstract
This paper presents a review of our work concerning the recent progress in fabrications and applications of heating-induced long period fiber gratings (LPFGs). Firstly, three kinds of heating fabrication techniques based on CO2 laser, hydrogen–oxygen flame and arc discharge are demonstrated to fabricate LPFGs, i.e., standard LPFGs (SLPFGs) and helical LPFGs (HLPFGs), in different types of optical fibers such as conventional fibers, photonic crystal fibers, and photonic bandgap fibers. Secondly, the all-fiber orbital angular momentum (OAM) mode converters based on heating-induced SLPFGs and HLPFGs in different types of fibers are studied to increase the transmission capacity. Finally, the heating-induced SLPFGs and HLPFGs are investigated to develop various LPFG-based strain, pressure, torsion and biochemical sensors.
Highlights
The optical fiber gratings play a vital role in the field of optical communications and fiber optical sensing
Compared with standard LPFG (SLPFG), helical LPFGs (HLPFGs) refers to a fiber where there exists a periodical helical structure such as a screw index-modulation along the fiber axis [9,10,11,12,47]
Xi et al investigated a strain sensor with a sensitivity of –1.18 pm/με based on HLPFG in the photonic crystal fiber (PCF), and the analysis showed that the tension-induced shift in resonance wavelength is determined both by the photoelastic effect and the change in twist rate [19]
Summary
The optical fiber gratings play a vital role in the field of optical communications and fiber optical sensing. Vengsarkar et al wrote the first standard LPFG (SLPFG) with periodic refractive index change in the conventional glass fiber core using an ultraviolet (UV) laser radiation in 1996 [8]. Among above-mentioned fabrication techniques, CO2 laser, arc discharge, and hydrogen–oxygen flame all heat the fiber into a fused status, the periodic refractive index modulation in the fiber is reserved due to the possible mechanisms, i.e., residual stress relaxation, Sensors 2019, 19, 4473; doi:10.3390/s19204473 www.mdpi.com/journal/sensors.
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