Research On Sensing Characteristics Of Long-period Fiber Grating Coated With High Index Nano-film

As a promising optical fiber-based device, long period fiber grating（LPFG） possess many important advantages such as small configuration, light weight, immunity to electromagnetic interference and demonstrate excellent performance in the optical fiber communications and optical fiber sensors. LPFG are intrinsically sensitive to many physical parameters such as temperature, strain, bending, and torsion. In addition, due to the sensitivity of the LPFG to the surrounding refractive index（SRI）, considerable attentions have been paid on the LPFG-based refractometers. LFPGs are suitable to be used in the high temperature, corrosive, and dangerous environment, for example, the petrochemical industry, the biochemistry, and environmental monitoring. Much attention has been paid on how to improve the sensitivity, expand the field of the application, and solve the temperature cross-sensitivity recently. In this work, the LPFG coated with high index nano-film are proposed to fulfill the SRI, temperature sensing with high sensitivity. A new technology, atomic layer deposition（ALD） technology is utilized for the deposition of Al₂O₃ and Ti O₂ nano-film. The LPFG pairs are employed to eliminate the temperature cross-sensitivity. The main content are as following:1. The theoretical model of the hybrid modes in four-layer cylindrical waveguide is proposed for numerical analysis of the LPFG with high index nano-film coating. The influences of the parameters of the nano-film（thickness and refractive index） and the SRI on the resonance wavelength of LPFG are investigated. The phenomenon of the reorganization of cladding modes is analyzed in detail.2. ALD technology is utilized for the deposition of Al₂O₃、Ti O₂ nano-film on the surface of the LPFG. Higher index sensitivity can be achieved in the transition region of the coated LPFG. For the LPFG coated by Ti O₂ nano-film with a thickness of 35 nm, the high index sensitivity of-5000 nm/RIU are obtained, which is 5 times higher than that of the bare LPFG. For the LPFG coated by Al₂O₃ nano-film with a thickness of 100 nm, the high index sensitivity of-3000 nm/RIU are obtained, which is about 3 times higher than that of the bare LPFG. The cladding modes transition is observed experimentally using an infrared camera with increasing surrounding index. The grating contrast of the over-coupled LPFG and conventional LPFG are measured. The over-coupled gratings are found to have a higher contrast in the transition region. It is meaningful for the practical application. The sensing characteristics of the high index nano-film coated LPFG written in the tapered fibers are investigated experimentally. The SRI sensitivity is improved by one order of magnitude, compared with the conventional LFPG with the same parameter（the period, the order of the cladding mode）. The SRI sensitivity can be further improved by combinating of the tapering and high index nano-film coating of the LPFG.3. We present a new method to improve the temperature sensitivity of the LPFG. The LFPG with Al₂O₃ nano-film is packaged by the polymer material or the ultraviolet adhesive. Thank to the high SRI sensitivity of the coated LPFG（CLPFG） and the thermo-optic effects of the package, the temperature sensitivity of the packaged and coated LPFG（PCLPFG） could be improved remarkably. Experimental results show that in the range of 20 ℃-100 ℃, the temperature sensitivity up to 0.77 nm/°C is achieved for the HE1,10 mode of the LPFG coated with 200 nm nano-film and then packaged by the silicone rubber, which is almost one order of magnitude larger than that of the bare LPFG. In the range of 20 ℃-100 ℃, the temperature sensitivity up to 1.26 nm/°C is achieved for the HE1,10 mode of the LPFG coated with 100 nm nano-film and then packaged by the ultraviolet adhesive, which is almost 15 times larger than that of the bare LPFG. According to the similar principle, we immersed the CLPFG in the magnetic fluid（MF）, whose refractive index change with the external magnetic field. A LFPG-based magnetic field sensor with high sensitivity is obtained. The sensitivity up to 120 pm/Oe is achieved for the HE1,10 mode of the LPFG coated with 250 nm Al₂O₃ nano-film, which is almost 13 times larger than that of the bare LPFG.4. The LPFG pair（including LFPG1 and LPFG2） with different parameter（the period, length and cladding mode order of the LPFG） is utilized to measure the temperature and the SRI with high sensitivity simultaneously. We fabricate the LPFG1 working around the turning point and deposite 100 nm Al₂O₃ nano-film on it, the SRI sensitivity of-4800 nm/RIU are obtained in the transition region. The LPFG1 is tuned by the SRI and the temperature according to the principle of the linear superposition. The LPFG2 with 100 nm Al₂O₃ nano-film and the package of the ultraviolet adhesive is insensitive to SRI but sensitive to the temperature. It possesses the temperature sensitivity of 3.2 nm/°C in the range of 20 °C-40 °C. LPFG2 is used as a temperature compensating element. By tracking the attenuation bands of the LPFG1 and LPFG2, simultaneous measurement of the temperature and the SRI could be fulfilled. The temperature resolution of the device reachs up to 0.0063 and the SRI resolution reach℃ s up to 64.17 10^-6 RIU.