Researches And Applications Of Long-Period Fiber Gratings Based On Few-Mode Fibers

With the rapid development of Internet technology,the demand of bandwidth resources is more and more urgent.When time,frequency,polarization and orthogonality are fully utilized,only the spatial dimension still has the huge research value and the utilization space.The space division multiplexing(SDM)technology is coming.As an important transmission medium of SDM,few-mode fibers(FMF)can transmit multiple modes simultaneously in a single fiber.By using the characteristic of FMF,fiber devices based on FMF will have unique performance.Long-period fiber gratings(LPFG)are widely used in optical fiber communication and optical fiber sensors systems due to their small size,light weight,and immunity to electromagnetic interference.The few-mode fiber long period fiber gratings(FMF-LPFG)can realize the coupling between multiple modes and has the transmission characteristics of multiple resonance peaks,which enriches the research content of LPFG and provides more potential applications for optical fiber communication and sensing systems.The main research content of the thesis includes:(1)Several fabrication methods of LPFG were studied,and the advantages and disadvantages of various methods were analyzed.A fabrication platform based on special optical fiber fusion machine was built,and the fabrication of FMF-LPFG was completed by using the arc discharge micro-tapered method,which has the advantages of cost saving,simple platform,high flexibility and easy operation.(2)The high-quality FMF-LPFG was successfully fabricated based on arc discharge micro-tapered method.There are two resonant dips on the transmission spectrum of the FMF-LPFG,which are induced by the coupling between the fundamental mode LP₀₁ and the low-order cladding mode LP₀₃(dip 1),and the coupling between the fundamental mode LP₀₁ and the high-order core mode LP₁₁(dip 2).What's more,the factors that affect the performance of FMF-LPFG during the fabrication were analyzed in detail.(3)The application of FMF-LPFG in sensing is studied.Because two resonant dips have different temperature sensitivity and strain sensitivity,discriminative determination of temperature and strain is realized and the relative measurement error is less than 3%.And the radiation sensitivity of FMF-LPFG was also studied,and it was found that it has potential for simultaneous measurement of radiation and temperature.