Research On Light Field Modulation And Sensing Property Of Curved Grids In Long-Period Fiber Grating

Along with the progress of informatization in modern society,the requirements for information perception,transmission and processing from every fields are daily raised,and the requirement for the property of sensing device is also increasing constantly.With the advantages of small volume,fast response,anti-jamming,easy integration,etc.,optical fiber sensors have played an important role in the miniaturized,networked and integrated sensing system.The property of fiber device is closely related to its structure.As a kind of important fiber sensing device,optical fiber grating can modulate the light wave within a special wavelength range,and its characteristics mainly depend on the structure and period of grating grids.In comparison with traditional grids with planar structure,the curved grids have more flexible design of shape and larger scope of refractive index modulation,which provides more effective modulation to the light wave transmitting through optical fiber.Thus,the fiber grating constituted by curved grids show better sensing property,and the difficulty of fabrication also reduces along with the advance of fiber processing technique.Long-period fiber grating（LPFG）has a period large enough to excite different types of cladding modes,and it is also able to accommodate curved grids with multiple structures to form new sensing devices with various structures and remarkable property.In this dissertation,based on the research experience and condition of the group,with the support of National Natural Science Foundation of China and other projects,a systematic research on the LPFGs constituted by curved grids are carried out theoretically and experimentally,including the structure design,spectral characteristics,parameter response,device fabrication and sensing property in detail.The main research contents and innovate results are as follows:（1）Based on the refractive index distribution and light transmission principle of optical fiber,the structural features,fabricated techniques and application fields of LPFG are systematically summarized and compared.The advantages of curved grids are analyzed on light field modulation and sensing property compared with planar grids,and the design,development and application of novel LPFG constituted by curved grid also have a strong foundation.（2）Based on the structural feature of curved grids and the fabricated technique of laser scanning,the model of curved grid and curved fiber grating are proposed and built.By using simulation software,the laser-scanning caused heating effect for optical fiber is calculated,and the three-dimensional range and temperature distribution on fiber are analyzed with the heating area corresponding to curved scanning path.The transmission spectra of curved LPFGs with different structural parameters are simulated,and the result mainly matches the experimental spectra of curved LPFG with the same structural parameters.（3）Based on the traditional chirped grating structure and structural feature of curved grids,a kind of novel LPFG with chirped curvature radius of grid is proposed and fabricated.The impact of mode coupling and sensing characteristics caused by radius-chirped structure are analyzed,and the transmission spectrum and directional bend sensing property of LPFG device are tested.It is shown that different cladding modes are excited by the radius-chirped structure,and directional shifting of loss dip in transmission spectrum is caused by fiber bending.The bending sensitivity demodulated by the wavelength of oppositely shifted dips reaches 12.67 nm/m^-1,and the influence of temperature can be compensated.（4）Based on the working principle of fiber grating and fiber interferometer,a kind of novel LPFG with bidirectional chirped curvature radius of grid is proposed and fabricated.The working principle of bidirectional chirped structure is analyzed,and the transmission spectrum and axial strain sensing property of LPFG device are tested.It is shown that bidirectional chirped structure not only has the ability of coupling light wave with special wavelength like LPFG,but also causes dual beam interference in fiber.Axial strain leads to obvious intensity response of spectral dips.The strain sensitivity demodulated by the oppositely changed dip intensity reaches 0.0103 d B/με,and the influence of temperature can be compensated.（5）Based on the structure of traditional fiber grating and shape feature of curved grid,a kind of novel LPFG with alternate curved direction of grid is proposed and fabricated.The impact of light transmitting condition caused by grid curved direction is analyzed,and the transmission spectrum and directional bending response of LPFG device are tested.It is shown that alternate curved direction can affect the optical path between two adjacent grids in fiber cladding and cause the splitting of spectral dip,and the bending status of fiber also impact the shifting direction and splitting status of dips.The bending sensitivity demodulated by wavelength shifting reaches 15.69 nm/m^-1,and the influence of temperature can be removed by sensitivity matrix of two dips.