Study Of The Mode Characteristic And MZ Interferometric Sensor On Few-mode Photonic Crystal Fiber

Photonic crystal fibers?PCFs?have attracted widespread attention from researchers ever since last century due to its excellent optical properties,the variety of raw-material selection,and high degree of freedom in structural design.Among various PCFs,the few-mode photonic crystal fibers?FMPCF?possess vital merits,such as easy to control the guiding modes,strong mode confinement ability,and large mode area.They are easy to realize mode selective excitation and mode coupling,which brings increasing applications in mode division multiplexing and mode interference sensing.In this paper,a step-index FMPCF is designed and fabricated,and the relationship between the number of transmitted mode and the structural parameters of the fiber is analyzed.Furthermore,the transmission characteristics and mode characteristics of FMPCF are studied.Mach-Zehnder?MZ?interferometric sensor based on FMPCF is studied and applied to stress and temperature dual-parameter sensing.The main results are as following:?1?Using the finite element method?FEM?of COMSOL Multiphysics,the effects of different cladding parameters on the number of transmission modes of three-ring step-index guiding PCF are analyzed,later the FMPCF was successfully fabricated by stacking method according to the design structure.The main drawing process is introduced.The transmission characteristics and guided modes of the prepared FMPCFs were analyzed,and the transmission characteristics among different FMPCFs were compared.Furthermore,the mode field shape and transmission range of FMPCFs were measured.?2?The mode selective excitation process of LP01 and LP11 mode is discussed by using the offset coupling excitation mechanism of single mode fiber?SMF?and FMPCF.The interference between the two modes is analyzed on the basis of two-beam interference theory,and the influence of excitation conditions on the contrast and intensity of interference spectra is further discussed.?3?The mode leakage process of curved FMPCFs and the bending response characteristics of three kinds FMPCFs are analyzed.It is concluded that the higher air-filling fraction?AFF?brings lower sensitivity of FMPCF to bending.The FMPCF with lowest AFF was used to study the energy leakage of FMPCF under different bending radius.?4?Based on the principle of inter-mode interference,a cascaded MZ interference structure of SMF-FMPCF-SMF is constructed.The energy distribution and leakage in the process of MZ interference is studied.The MZ interferometric sensor based on a FMPCF is fabricated,with a structure of SMF-FMPCF-SMF.The preparation methods of the sensor are summarized.The dual-parameter measurement characteristics of the sensor are studied in the temperature range of 20-60°C and the stress range of 0 to541.2??.The temperature sensitivity of the MZ interferometer is 10.78 pm/°C,with linear coefficient of 99.72%,and the strain sensitivity is 2.9 pm/??,with linear coefficient of 99.68%.At the same time,the temperature and stress sensing response characteristics of the interference of LP₀₁/LP₁₁ and LP₁₁/LP₁₁ in the interferometric are further analyzed.