Sensing Properties Of Long-period-fiber-gratings Based On Photonic Crystal Fiber

Long period fiber grating have been widely used in optical sensors,communication and other fields due to its no backscatter, highly sensitive to the external environment and so on. The feature of PCF's?Photonic Crystal Fiber?porous structure makes it superior to SMF. In order to change the propagationproperties of the PCF, we can change the arrangement of air holes, size,hole pitch in the cladding. PCF-LPFG?Photonic Crystal Fiber-long period grating?combines the merit of both the PCF and LPFG, which is achieved by the periodic refractive index modulation in the PCF. The air holes in the cladding can provide the channels for gas, liquid and particles. Comparing with ordinary LPFG,PCF-LPFG has the advantages of flexibility, craftdiversification and multi-parameter measurement. PCF-LPFG has singularcharacteristic such as insensitive to temperature, which can well solve the problem of cross sensitivity between temperature and other physical parameter. There are a number of ways to fabricate LPFG, for example, UV, femtosecond laser, mechanical method.Compared with other inscription methods, CO₂ laser irradiationhas advantages oflow cost, flexibility, high reliableand can be used to inscribe LPFG in almost all types of fiber, which has given rise to widespread concern. In this paper wedemonstrated a promising CO₂ laser irradiation system based on an improved 2-D scanning technique. We employed such a system to inscribe LPFGs in SMF and PCF. The major work is summarized as follows:?1?We presented five methods of inscribing LPFG, and the mechanism for the refractive index modulation of CO₂-laser-induced LPFGs is analyzed as three aspects: residualstressrelaxation, glass densification and physical deformation.Thetransmission spectrum characteristicis analyzed and simulated by use of coupled mode theory.?2?Apromising CO₂ laser irradiation based on an improved two-dimensional scanning technique wasdemonstrated, including CO₂ laser optical system. This system could beavailable to inscribe LPFGs with good reproducibility, stability. We employed such a system to inscribe the LPFG in SMF and solid core PCF. The extinction ratio and insertion lossof LPFG and PCF-LPFG can respectively achieve30 dB, 25 dB, less than 0.3dB and 0.7dB. We tested the characteristics of LPFG and PCF-LPFG,i.e.transmission spectrum evolution, temperatureresponse, strain response, surrounding refractive index response, etc. PCF-LPFG is insensitive to temperature, the sensitivity is only4pm/°C. When the refractive index is less than1.46, drift direction is opposite with the increase of the refractive index. And PCF-LPFG exhibits a very high strain sensitivity of 3.5pm/??, which is one order of magnitude higher than LPFG, i.e. 0.35pm/??.The resonant wavelength and loss peak of PCF-LPFG's pressure sensitivity are low, respectively 0.12nm/MPa,0.25 dB/MPa.?3?In addition, we also made the work on the F-P interferometer?FPI?.The FPI has attracted attention due to its simple structure, reliability, and high sensitivity. In this paper, two FPIs are both fabricated by silica tube and SMF. The highest pressure sensitivity can achieve 1000nm/kPa and the temperature sensitivity can achieve23.4nm/°C.