Parallel And Helical Fiber Grating Fabrication Methods And Sensing Characteristics

Since the first fiber Bragg grating(FBG)was fabricated in 40 years ago,FBG has been widely used in the field of fiber sensing and fiber communication.Based on the research of scholars at home and abroad,the author of this paper proposes two new fabrication technologies of fiber gratings: parallel integrated fiber Bragg gratings(PI-FBGs)and helical long-period fiber Bragg gratings(HLPFGs).The fabrication technology and sensing application of PI-FBGs and HLPFGs were studied systematically and deeply.Some innovative achievements have been made,and the main work contents are summarized as follows:1.The research status of FBG is reviewed.The coupling principle of FBG's core mode and LPFG's cladding mode is introduced.Then the coupled mode equations of FBG and LPFG are derived by combining the coupled mode theory.Finally,the simplified FBG coupled mode equation is solved,and the relationship between FBG reflectance and grating period and refractive index modulation is discussed.2.Conventional grating-inscription technology usually inscribes only one fiber Bragg grating(FBG)in the core of an optical fiber.A novel femtosecond laser point-by-point technology was demonstrated to parallel-inscribe multiple FBGs,i.e.so-called parallel-integrated FBGs(PI-FBGs),with either different or the same reflection wavelengths in the core of a standard single-mode fiber.Single-or multi-wavelength PI-FBGs exhibited one or multiple peaks in the reflection spectrum,respectively.The length of PI-FBGs can be shortened to hundreds of micrometers to increase the spatial resolution of the FBG-based sensors.Our ultra-short PI-FBGs with a length of 500 ?m could be used to realize ultra-high-temperature sensing with a high spatial resolution of less than 1 mm,a linear sensitivity of 15.0 pm/°C,and a large measurement range of up to 1100 °C,which overcomes the shortcomings of conventional FBG-based sensors with a low spatial resolution of more than 1 cm and a small measurement range of up to 300 °C.Moreover,the grating parameters,such as reflectivity,reflection wavelength,polarization-dependent loss(PDL),and full width at half maximum,can be improved to achieve desired PI-FBGs by changing the spatial distribution and grating number of the PI-FBGs.The PI-FBGs are insensitive to bending,which reduces bendinduced measurement errors.Therefore,our PI-FBGs will find potential applications in the fields of optical fiber sensors,communications,and lasers.3.We demonstrate a high-efficiency grating fabrication system,which can be used to inscribe a high-quality helical long period fiber grating(HLPFG)on single-mode fiber by means of hydrogen-oxygen flame.Such the HLPFG can be produced in enormous quantities with a uniform grating parameters and good reproducibility of grating inscription.Possible mechanisms for refractive index modulation in the HLPFG can be attributed to residual stress concentration by solidifying the periodic twisting stress under a fused status of optical fiber.Moreover,the HLPFG exhibits an excellence performance of high temperature sensing with a high sensitivity of ~132.8 pm/°C and a measuring range from room temperature to 900 °C.Comparing to the traditional LPFG fabricated by CO2 laser or arc discharge technique,the HLPFG has a low the bending and tensile strain sensitivity of 1.94 nm/(1/m)and 1.41 pm/??,respectively.So the proposed HLPFG could have a great potential in special applications as optical high-temperature sensors.