Fabrication And Characteristics Of The Mode Coupling Devices Based On The Long Period Fiber Gratings

As a new type mode coupling component,Long period fiber grating（LPFG）can realize mode coupling from fundamental core mode to high-order modes,which can be used in fiber sensing and optical communication fields.The LPFG fabricated in single-mode fiber（SMF）can realize mode coupling between fundamental core mode and high-order cladding modes,while LPFG fabricated in few-mode fiber（FMF）mainly realizes mode coupling between fundamental core mode and high-order core mode.The coupling efficiency of different cladding modes in SMF is varying,and the mode coupling of LPFG with larger period is more complex.The coupling of the high-order core mode in FMF have a low efficiency.In addition,the conventional LPFG usually has a smaller bandwidth.In this dissertation,different types of LPFGs were fabricated in boron-germanium co-doped SMF,circular-core FMF,and elliptic-core FMF,respectively,and the corresponding mode couplings were investigated theoretically and experimentally.The main research contents and results of the dissertation are as follows:1.The spectra of LPFG were investigated theoretically and experimentally with different parameters,including core and cladding diameters,refractive indices,grating period,and duty ratio of LPFG.When the duty ratio of the grating is 0.2 and grating period is changed in the ranges of400μm～600μm,the wavelength shift of the resonance dip with first diffraction order cladding mode for LP₀₃～LP₀₇ mode and high diffraction order cladding mode for LP₀₉ and LP_0,10mode are theoretically studied.The superposition of cladding modes with different diffraction orders and the wavelength shift of resonance dips at dispersion turning point are analyzed.The LPFGs with different periods were fabricated in boron-germanium co-doped SMF to realize the coupling of first diffraction order LP₀₂～LP₀₆ modes and high diffraction order LP₀₇ modes with dual-dips at dispersion turning point.The resonance dip of high diffraction order LP₀₇ mode has a temperature sensitivity of 3.3 nm/℃ and the highest refractive index sensitivity of 11175.0nm/RIU.2.The mode characteristic of the elliptic-core FMF was investigated with simulation and experiment.The LP_11a,LP_11b,and LP_21a modes are experimentally generated in elliptic-core FMF by LPFG with coupling efficiencies of 99.8%,99.6%,and 95.7%,respectively.The LP₀₂ mode and LP_31a modes are generated within the doped layer of elliptic-core FMF by the LPFGs with smaller grating period.3.The coupling efficiency between different core modes is analyzed when the elliptic-core FMF is bent along the major or the minor axis.The results show that the conversion of LP_11a-LP_21a mode and LP_11b-LP_21b modes can be realized by the elliptic-core FMF with micro-bending along the major axis efficiently.In the experiment,the LP_11a and LP_11b modes are generated from LP₀₁ mode by the LPFG,and then coupled to LP_21a and LP_21b modes,respectively,by the cascaded micro-bending of the elliptic-core fiber.4.A kind of cascaded FMF-LPFGs with different periods is proposed.The transmission spectra of cascaded LPFGs with different periods and grating spacing are investigated by simulation and experiment.In the simulation,the broadband mode conversion can be realized in the three wavebands of 1.0μm,1.55μm,and 2.0μm with 10-dB bandwidth of 91.8 nm,213.4nm,413.8 nm,respectively,which is much higher than the bandwidth of conventional LPFG.In the experiment,the cascaded LPFGs fabricated in the two-mode fiber successfully realized the broadband conversion of LP₁₁ mode and first-order orbit angular momentum mode at 1.55μm and 2μm wavebands,respectively.The maximum 10-dB bandwidth was 193.6 nm and 447.5 nm,respectively.The wavelength range of the mode conversion could be adjusted in the waveband of1.0μm～2.2μm.5.Based on the idea of cascaded LPFGs in FMF,we simulated the transmission spectra of the cascaded LPFGs in a specialty FMF at its dispersion turning point.According to the simulation results,we fabricated the cascaded LPFGs and the mode conversion with a bandwidth of 395.8nm can be realized experimentally.By twisting the cascaded LPFGs,the bandwidth can be further increased up to 409.5 nm.Compared with conventional LPFG inscribed in the same fiber,the bandwidth of cascaded LPFGs is increased by nearly 100 nm.