Sensing Characterization Of Thermally Induced Long Period Fiber Gratings

Optical fiber has the advantages of the anti-electromagnetic interference,corrosion resistance,small volume,and the optical fiber sensors play a very important role in the filed of sensing.As an important all-fiber device,long-period fiber gratings(LPFG)play an important role in many fields,such as high sensitivity refractive index sensor and temperature sensor.So far,the study of symmetric LPFG is very mature,however,the theoretical research and fabrication methods of the asymmetric LPFG are very few because of the mode coupling conditions between the symmetric mode and asymmetric mode achieving difficult.At present,the fabrication method of asymmetric LPFG is mainly the CO2 laser pulse method,and the LPFG fabricated by using this metod is asymmetric due to the fiber core mode coupling to the asymmetric cladding mode,however,there is not a mature theoretical model to explain it because of the complex forming mechanism.Combining LPFG in the few mode fiber which only allow two modes spreading in the fiber core with the structure of single-mode Fiber-few-mode fiber-Multi-mode Fiber(SFS),an asymmetric structure of LPFG has been proposed in this thesis.This proposed structure is named as "SFS-LPFG" for short.This structure not only avoids the complexity of symmetric fiber core mode coupling to asymmetric cladding modes,also overcome the interface between multiple modes in few-mode fiber.To fabricate SFS-LPFG,this thesis proposed a kind of thermal induced method which is low cost,simple but effective.The main contents in this thesis are listed as follows:Firstly,several current fabrication metods and the application status of LPFG are introduced and summarized.Secondly,the principle and theory of uniform LPFG are discussed.Based on the LPFG transfer matrix method,the theoretical model of asymmetric LPFG in few-mode fiber is established.Thirdly,the fabrication system of thermal induced LPFG is designed and set up,and the steps and technique in the process of fabricating LPFG is studied.Then,the single-mode LPFG and asymmetric SFS-LPFG are fabricated by using this fabrication system.Fourth,the sensing characteristics of thermally induced single-mode LPFG are investigated theoretically and experimentally,temperature measurement experiment and strain detection experiment of thermally induced single-mode LPFG are conducted.In the temperature range of 23 ??110 ?,the sensitivity is 73.8pm/?.The strain sensitivity is 0.319pm/??.The experiment indicated that thermally induced single-mode LPFG is sensitive to external refractive index and twist ratio.Fifth,the sensing characteristics of thermally induced asymmetric SFS-LPFG are investigated theoretically and experimentally,including temperature,strain and refractive index sensing characteristics.The temperature sensitivity and strain sensitivity are 58.86pm/? and-5.4pm/?? respectively.In addition,the experiment demonstrated that the thermally induced asymmetric SFS-LPFG is not sensitive to external refractive index.The proposed thermally induced LPFG fabrication system has the advantages of low cost,easy to control,effective,which can be used to fabricate thermally induced single-mode LPFG and thermally induced asymmetric SFS-LPFG.In addition,the proposed structure of SFS-LPFG in this thesis is not sensitive to the external refractive index due to the principle of mode coupling between two fiber core modes,which can be used for temperature or strain sensing measurement in different refractive index liquid.