Research On Key Technologies Of High-Order Transverse Mode Fiber Lasers

Fiber lasers have attracted great attention due to their merits of excellent beam quality,small size,strong immunity to interference,and good compatibility with optical fibers.As a new type of fiber laser,the high-order transverse mode fiber lasers have significant meanings in the fields of optical sensing,optical communications,optical tweezers,the excitation of plasma,and material processing due to their ability to output high-order modes with special intensity distribution,phase distribution,and polarization distribution.As the order of the output modes,the slope efficiency of fiber laser,the number of output wavelengths,the tunable range of wavelengths,and the linewidth of the laser are the key technical parameters that characterize the performance of high-order transverse-mode fiber laser.High-quality fiber lasers with high-order transverse-mode output can provide important technical support for the technological development in the above-mentioned fields,and it becomes a new research hotpot.A series of in-depth theoretical and experimental researches on the order of output modes,the slope efficiency of fiber lasers and the number of output wavelengths,the tunable range of wavelengths,laser linewidths are carried out.The main research contents and innovations are as follows:1.Aiming at the problem of low output mode orders,limited by the conversion rate and insertion loss of the mode conversion devices of the traditional high-order transverse mode fiber lasers,a scheme that can directly output a specific high-order transverse mode from the fiber laser is proposed.By controlling the distribution range of the erbium ions in the erbium-doped fiber（EDF）,the proposed fiber laser can oscillate in the specific high-order vector mode.Based on this method,the fiber laser that can directly output high-order vector modes(HE₂₁,TE₀₁,HE₃₁,EH₁₁ and HE₁₂is proposed.The output mode purity and the slope efficiency can reach 99.99% and 61.25%,respectively.Then another EDF laser with an azimuthally polarized beam(TE₀₁ mode)output is proposed.The fiber laser oscillates directly in the TE₀₁ mode.The mode purity and the slope efficiency can reach 99.99% and 66.15%,respectively.The proposed scheme has guiding significance in realizing the high-purity vector mode output and increasing the output power of fiber lasers.2.A narrow linewidth cylindrical vector beam fiber laser based on a composite cavity structure and a mode selection filter is proposed.The compound ring cavity structure is composed of a main ring cavity and two passive subring cavities so that the single longitudinal mode operation is achieved.A mode selective filter is composed of a fiber Bragg grating and a mechanical long period fiber grating（LPFG）,and it is used to realize the mode switch and wavelength switch.By controlling the polarization state of the light in the laser cavity,the tilt angle of the LPFG and the pressure applied to the mechanical LPFG,both the radially and azimuthally polarized CVBs are obtained at 1543.35 nm（1544.38 nm）.Meanwhile,the signal-to-noise ratio（SNR）is higher than 60 d B.The 20 d B linewidth of the proposed fiber laser for each wavelength is approximately 5.7 k Hz and 6.8 k Hz.The proposed fiber laser can be used in the fields of lidar,dense wavelength division multiplexing systems,and fiber sensing.3.As the number of the output wavelengths of high-order scalar mode fiber lasers is small,two multi-wavelength LP₁₁ mode fiber lasers are proposed.A mode selective coupler（MSC）is fabricated,and it is used to output LP₁₁ mode for the two multiwavelength LP₁₁ mode fiber lasers.Based on a Mach-Zehnder filter（MZF）,the proposed fiber laser can achieve LP₁₁ mode output at single-,dual-,triple-,and quarterwavelength,the SNR is higher than 36 d B and mode purity of the LP₁₁ mode is higher than 95%.Moreover,based on the dual-Sagnac comb filter（DSCF）,another multiwavelength LP₁₁ mode fiber laser is proposed.The proposed fiber laser can output up to sextuple-wavelength with the SNR higher than 40 d B.The mode purity of LP₁₁ mode is higher than 95%.Both the proposed fiber lasers can find applications in the fields of wavelength-mode division co-multiplexing systems to further increase communication capacity.4.As the number and the tunable range of output wavelengths of the orbital angular momentum fiber laser is small,a multi-wavelength orbital angular momentum fiber laser based on the MSC,MZF and Sagnac loop filter is proposed.The MSC is used to realize the conversion from the fundamental mode to the LP₁₁ mode,and the polarization controller is added to the output port of the few-mode fiber of the MSC to generate the orbital angular momentum mode.The working principle of the filter is studied,the MZF is used to provide comb filter spectrum,and the Sagnac loop filter is used to increase the tunable range of wavelength.The proposed fiber laser can achieve single-,dual-,triple-and quadruple-wavelength outputs of OAM modes.The tunable range of the output wavelengths is up to 22.64 nm.Besides,the multi-wavelength fiber laser is able to emit OAM modes with its topological charge of ±1.The proposed fiber laser has the advantages of wide wavelength tunability,good stability,high SNR（higher than 34 d B）,and high mode purity（95%）.The proposed fiber laser can provide high-quality light sources for optical communication systems and optical sensing systems.5.An FP（Fabry-Perot,FP）filter for high-order transverse mode fiber lasers is proposed.Two tilted fiber Bragg gratings（TFBGs）function as the reflecting mirror in the FP cavity as well as the mode converters.Therefore,compared with the traditional MZF,Sagnac loop filter,traditional FP filter and mode converter,the proposed FP filter can realize the conversion of vector modes at specific wavelengths with narrow-bandwidth characteristic.The influence of the tilt angle of the TFBG on the transmission characteristics of the FP filter is studied.When the tilt angle is 0°,the filter exhibits wavelength selection with narrow-bandwidth characteristics.When the tilt angle is 2°,the filter exhibits wavelength and mode selection characteristics,as well as mode conversion function and narrow-bandwidth characteristics.At the same time,the influence of other parameters on the transmission characteristics of the FP filter is also studied.The proposed FP filter can find applications in the fields of fiber lasers and mode/wavelength division multiplexing systems.