Research On The Mode Control Mechanisms And Applications Of Hollow Core Photonic Bandgap Fibers

Hollow-core optical bandgap fiber（HC-PBF）is a kind of fiber that uses the photonic bandgap effect formed by the axially extending periodic photonic crystal structure to confine light waves of specific wavelength/frequency in the air core for low-loss conduction.Compared with traditional solid core fibers,it has excellent optical properties such as low nonlinearity,low time delay,high temperature stability and so on.It has great application potential in low-delay optical communication,high-energy laser transmission,fiber optic gyroscope and light-material interaction,which has attracted wide attention of many researchers.However,the development of HC-PBF is still immature,and there are still many key scientific and technical problems to be solved.HC-PBFs are limited by the structure of the core.Generally,they are few-mode or even multi-mode fibers.The coupling crosstalk between modes limits their application.In addition,the surface mode introduced in the cladding structure may be coupled with the core mode when the phase matching condition is met,resulting in a sharp increase in core mode loss and a decrease in available bandwidth.Therefore,it is of great scientific value and practical significance to study the mode control mechanism,technology and application of HC-PBFs.The research work in this paper is combined with the contents and objectives of the National Natural Science Foundation and the national key research and development program.Based on the goal and content of the project,in this thesis,we have performed systematic as well as in-depth theoretical and experimental investigations on the mode characteristics,mode control,surface mode coupling rules in HC-PBF and its application,and pure single-vector mode HC-PBFs.The research work and acquired original outcome are as follows:1.The high-order mode characteristics of the 7-cell HC-PBF are studied in detail theoretically and experimentally,and the phenomenon of HE₂₁ odd-even mode separation（non-degenerate）in the prepared 7-cell HC-PBF is found experimentally,which has the potential to realize few-mode transmission.A model that can truly reflect the dynamic changes during the fiber drawing process is constructed,and the effect of changes in key structure parameters around the fiber core on the mode polarization characteristics is studied.The mechanism of mode separation found in experiments is explained.Based on this phenomenon,a 5-mode and 6-mode HC-PBF structure without mode crosstalk is proposed;and the bend-insensitive properties of the few-mode HC-PBF are experimentally confirmed.2.A novel structure of 37-cell HC-PBF with angular mode selection is proposed.By modulating the lattice duty cycle in 12 specific directions of the cladding,more than40 modes can be completely filtered out,achieving a bandwidth of 50 nm（<1d B/km）and a total loss as low as 0.127 d B/km,single TE₀₁ transmission,the loss ratio of TE₀₁mode and other lowest loss modes in this fiber is as high as 150000,and the fiber also has low bending loss（1.6 d B/km@1cm）and operating wavelength scalability（via size expansion）and other excellent characteristics.3.The coupling rules of surface modes in hollow-core photonic band gap fibers are clarified.Accurate manipulation of surface modules and a thorough and clear understanding of them have been achieved in theory.The surface mode coupling effect is applied to mode control and fiber sensing,and the bandwidth exceeds 100 nm in a7-cell hollow-core photonic bandgap fiber,covering the C+L band of pure TE₀₁ single vector mode transmission,and the minimum loss reaches 10.2 d B/km.Using the stability of the hollow photonic bandgap fiber and the sensitivity of the surface modes,the unique temperature and strain sensing characteristics of the surface mode resonance coupled resonance peak are observed:the resonance peak is not sensitive to temperature at 150℃,but is sensitive to transmission intensity above 150℃with a sensitivity coefficient of-0.26 d B/℃.