Research Of Novel Long Period Fiber Gratings Inscribed By CO₂ Laser And Its Sensing Characteristics

Long period fiber gratings(LPFGs) have been extensively explored and widely used in the fields of optical fiber sensors and telecommunications. Compared with other inscription methods, CO2 laser irradiation has advantages of low cost, more flexible, high reliable, and can be used to inscribe LPFG in almost all types of fiber, which already aroused widespread concern. In this paper we proposed a promising CO2 laser irradiation system based on an improved 2-D scanning technique. We used such a system to inscribe novel LPFGs in various types of optical fiber, such as thin-core fiber(TCF) and photonic crystal fiber(PCF). The spectral characteristics and sensing characteristics of these novel LPFGs are studied in detail, and some innovative results were achieved. The main work is summarized as follows:1. The mechanism for the refractive index modulation of CO2-laser-induced LPFGs is analyzed as three aspects: residual stress relaxation, glass densification and physical deformation. The transmission spectrum characteristics and some important parameters, i.e. core and cladding mode effective refractive index of LPFG is analyzed and simulated by use of coupled mode theory. The mode distributions of optical fiber waveguide before and after CO2 laser exposure are simulated by means of finite element analysis, and the CO2-laser-induced uneven distribution of the refractive index within the cross-section of fiber result in asymmetric mode coupling in LPFG.2. A promising CO2 laser irradiation based on an improved two-dimensional scanning technique was proposed and demonstrated, including CO2 laser optical system and computer-control motorized translation stages. Such a system could be used to inscribe LPFGs with good reproducibility, stability and success rate. High-quality LPFGs without physical deformation are inscribed in conventional SMF by used of this system. The spectral characteristics, i.e. transmission spectrum evolution, phase matching curves, mode field distribution, polarization dependent loss, etc and the sensing characteristics, i.e. temperature response, strain response, surrounding refractive index response, etc are experimently investigated, which lay an important foundation for following study of novel LPFGs.3. It is demonstrated, for the first time to our best knowledge, that asymmetric thin-core long period fiber gratings(T-LPFGs) with periodic grooves are inscribed in TCF by use of focused high frequency CO2 laser pulses. This proposed T-LPFG exhibits a high attenuation dip of 25 d B and a narrow 3d B-bandwidth of only 8.7 nm, which much narrower than that in conventional SMF. Such a T-LPFG possesses a high polarization dependent loss of more than 20 d B, while that only 3.6 d B for conventional LPFG with the same attenuation dip. Moreover, the response of T-LPFG to axial strain, temperature and surrounding refractive index(RI) were investigated. The experimental result shows that periodic grooves and the stretch-induced periodic microbends can effectively enhance the average strain sensitivity of T-LPFG to-8.3pm/με. Meanwhile, T-LPFG exhibits a maximum sensitivity of 1419.4nm/RIU at RI range from 1.42 to 1.44, which much higher than that of conventional LPFG. The temperature sensitivity of T-LPFG is similar with that of conventional LPFG, and the measurement error due to thermal effects of the fiber materials is only ~8% in refractive index measurement.4. It is demonstrated, for the first time to our best knowledge, a type of novel inflated long period fiber gratings(I-LPFGs) was inscribed in a solid core photonic crystal fiber by use of a pressure-assisted CO2 laser beam scanning technique to inflate periodically air holes along the fiber axis. Such periodic inflations enhanced the sensitivity of the I-LPFG-based strain sensor to-5.62pm/με. Hence, such an I-LPFG could be used to develop a promising high-sensitivity strain sensor. Moreover, such an I-LPFG exhibits a very high gas pressure sensitivity of 1.68 nm/MPa, which is one order of magnitude higher than that, i.e. 0.12 nm/Mpa, of the LPFG without periodic inflations. So the I-LPFG could be used to develop a promising gas pressure sensor, and the achieved pressure measurement range is up to 10 MPa. The I-LPFG has a very low temperature sensitivity of 2.8 pm/°C due to the pure silica material for low temperature test from 30oC to 100oC. After high temperature annealing, the I-LPFG achieved a good repeatability and stability of temperature response with a sensitivity of 11.92 pm/°C for high temperature test from 100oC to 800oC. The enhancement of temperature sensitivity maybe is due to the changes of silica glass structure and frozen-in stress in high temperature environment.