![]() ![]() Liu and Chiang 29 presented recent development on the CO 2-laser-induced LPFGs. This technique could be, hence, used to write LPFGs in almost all types of fibers including pure-silica photonic crystal fibers (PCFs). 16–29 Moreover, the CO 2 laser irradiation process can be controlled to generate complicated grating profiles via the well-known point-to-point technique without any expensive masks. Compared with the UV-laser exposure technique, the CO 2 laser irradiation technique is much more flexible and low cost because no photosensitivity and any other pretreated process are required to write a grating in the glass fibers. 16,17 reported what is believed to be the first example of gratings written by the CO 2 laser irradiation technique in a conventional glass fiber in 1998. Numerous LPFG-based devices have also been developed to realize their sensing and communication applications. Various fabrication methods, such as ultraviolet (UV) laser exposure, 5–15 CO 2 laser irradiation, 16–29 electric-arc discharge, 30–37 femtosecond laser exposure, 38–43 mechanical microbends, 44–51 etched corrugations, 52–57 and ion beam implantation, 58–60 have been demonstrated to write LPFGs in different types of optical fibers. 5 wrote the first FBG and LPFG in conventional glass fibers in 19, respectively, the fabrications and applications of in-fiber gratings have achieved rapid developments. There are two types of in-fiber gratings: fiber Bragg gratings (FBGs) with periodicities of the order of the optical wavelength 1–4 and long period fiber gratings (LPFGs) with periodicities of several hundred wavelengths. Optical fiber gratings already play a vital role in the field of optical communications and sensors. Finally, communication applications of the CO 2-laser-induced LPFGs are investigated to develop various LPFG-based band-rejection filters, gain equalizers, polarizers, and couplers. Fifthly, sensing applications of the CO 2-laser-induced LPFGs are investigated to develop various LPFG-based temperature, strain, bend, torsion, pressure, and biochemical sensors. Fourthly, several pretreament and post-treatment techniques are proposed to enhance the efficiency of grating fabrications. Third, asymmetrical mode coupling, resulting from single-side laser irradiation, is discussed to understand unique optical properties of the CO 2-laser-induced LPFGs. Second, possible mechanisms, e.g., residual stress relaxation, glass structure changes, and physical deformation, of refractive index modulations in the CO 2-laser-induced LPFGs are analyzed. First, various fabrication techniques based on CO 2 laser irradiations are demonstrated to write LPFGs in different types of optical fibers such as conventional glass fibers, solid-core photonic crystal fibers, and air-core photonic bandgap fibers. This paper presents a systematic review of long period fiber gratings (LPFGs) written by the CO 2 laser irradiation technique. ![]()
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