Research On Mode Manipulation Technique Of Fiber Laser

Fiber lasers have gained a solid reputation in optical communication,remote sensing,biomedical and industrial process,due to their comparable advantages in compact structure,excellent beam quality and convenient thermal management.In most occasions,mode control technologies are employed to suppress the generation of high-order modes,aiming to high power near diffraction-limited fiber laser.However,high-order modes are required in the fields of mode division multiplexing technique,structured beams and spatio-temporal modulation in multimode nonlinear,etc.In this thesis,we investigate the mode manipulation technique of fiber laser,and the main work is summarized as following:1.The theoretical research on step-index fiber mode profiles and fiber grating are investigated.Firstly,based on Helmholtz equation,the theoretical expression of vector mode and LP mode are derived,and the propagation profiles of LP mode are discussed.Then,based on the coupled mode theory,the simulation models of fiber Bragg grating?FBG?and long-period fiber grating?LPFG?are established.The effects of maximum refractive index modulation coefficient and grating length on output spectrum are discussed.The results indicate that the changes of maximum refractive index modulation coefficient and grating length will not change the central wavelengths of corresponding modes in FBG and central wavelength of mode conversion in LPFG,which provide theoretical guidance for selective transverse mode output and mode conversion.2.The fiber oscillator with selective transverse mode generation based on FBG is explored.We design and fabricate a pair of high-reflection FBG and low-reflection FBG in which only the reflective peaks of LP₁₁ mode are coincided and the reflective peaks of LP₀₁ mode are well separated.The first cladding pumped all-fiber oscillator configuration with selective transverse mode generation is demonstrated.Operating in LP₁₁ mode,the slope efficiency is?38%with maximum output power of 4.2 W and?90.2%mode purity.Our work suggests the potential of realizing higher power with selective transverse mode operation based on mode-selective FBG pair.Based on a pair of high-reflection FBG and low-reflection FBG in which the reflective peaks of LP₁₁and LP₀₁ mode are coincided,the first double-wavelength hybrid mode all-fiber oscillator is demonstrated.3.The fiber amplifier with high-order mode output based on LPFG is investigated.we design and fabricate a LPFG mode converter in which LP₀₁1 mode is converted to high-purity LP₁₁ mode.The first continue-wave power scaling of a tandem pumped LP₁₁ mode fiber amplifier,with maximum output power of 69.11 W and slope efficiency of?73.8%is demonstrated.In addition,the similar power extraction abilities of LP₁₁ and LP₀₁ mode are also demosatrated.And an all-fiber MOPA configuration with kW-order output power is built,the first LP₁₁ mode MOPA amplifier with maximum output power of 1270 W is demonstrated.4.The phase compensation of high-order mode fiber laser is explored.The near-field and far-field intensity distribution of LP₁₁ and CLP₁₁ mode are discussed,and for the first time far-field brightnesses are compared through PIB value.The notably enhanced beam brightness of CLP₁₁ mode with a near-Gaussian intensity distribution in far-field similar to LP₁₁ mode is demonstrated.5.The beam incoherent combining of high-order mode is investigated.The experimental scheme of all-fiber flattop beam through incoherent combining of LP₁₁mode annular beam and LP₀₁ mode Gaussian beam is designed.The first flattop beam with minimum flattop degree C=0.0343 through combining the all-fiber LP₁₁ mode oscillator and all-fiber LP₀₁ mode oscillator is demonstrated.6.A novel hybrid genetic global optimization algorithm,named GA-SPGD is first proposed to employ in numerical mode decomposition.Firstly,GA is applied to search the rough global optimization position based on near-and far-field intensity distribution with high accuracy.Upon those initial values,SPGD algorithm is afterwards used to find the exact optimization values based on near-field intensity distribution with fast convergence speed.The GA-SPGD algorithm combines the global searching profile of genetic algorithm?GA?and fast convergence feature of stochastic parallel gradient descent?SPGD?algorithm,and the feasibility and reliability are validated with numerical simulations.7.For the first time,the intermodal cross-phase modulation induced noise-seeded modulation instability taking place in the normal dispersion regime of step-index multimode fiber is investigated.Based on linear-stability analysis,the onset condition and gain spectrum are derived.For the case of total power being distributed equally in certain bi-modal combinations,the peak sideband frequency scales as square root of total power and peak gain increases linearly with total power.For the case of total power being distributed unequally,there exists an optimal power ratio to obtain peak gain,which does not depend on total power significantly.When the mode group velocity mismatch is considered,the range and peak values of gain spectra are notably increased.