Fiber Laser Spectrum Synthesis Based On Double Gratings

As a new generation of high-power laser source,fiber laser has significant advantages such as good beam quality and high electro-optic conversion efficiency.However,the output brightness of single fiber is limited by factors such as nonlinear effects and mode instability.The beam combining technology is an effective method to break through the single-fiber power limitation and obtain high-power laser output.Compared with other beam combining methods,the spectrum beam combination(SBC)technique has the characteristics of relatively simple system and good scalability which can improve the total output power while maintaining high beam quality and high combining efficiency.The output power of fiber laser source based on SBC is expected to achieve high brightness in a short period of time,and can be applied in many fields such as national defense,industrial processing,and basic scientific research.Recent progress of the geometric splicing,coherent beam combining(CBC)and SBC is reviewed.The requirement of fiber lasers,the beam quality,efficiency and the scalability of these combining methods are compared.Furthermore,advantages and disadvantages of several SBC schemes are analyzed.The research shows that the dual-grating SBC(DGSBC)has excellent development potential and obvious advantages in system robustness and compactness.In this thesis,a notion of polarization multiplexing and high density combining DGSBC is proposed to solve the problems of existent DGSBC,and comprehensive theoretical and experimental study of is demonstrated.The physical model of DGSBC is build,and the factors affecting the beam quality of DGSBC such as spectrum of fiber lasers,the aperture of laser beams and the mismatch of the gratings are analyzed.Based on this theory high efficiency,high beam quality and high power DGSBC system could be designed and build.The theoretical framework of diffraction transmission of the fundamental mode Gaussian beam in DGSBC system based on matrix optics is established,and the analytical expression of beam quality is obtained.The main factors affecting the beam quality of DGSBC are analyzed.The results show that the degradation of the beam quality is contributed by the enlargement of both near-field and far-field of the laser which is caused by the dispersion of the laser in high density DGSBC.When the combining parameters are fixed,the beam quality of DGSBC is determined by the second-order moment spectral linewidth of the laser,and has no relationship with the grating's dispersion capacity.The degradation of beam quality caused by the mismatch between two gratings is analyzed,and the typical tolerances are obtained.The angular spectrum theory analysis of the propagation of high-order linear polarization modes of high-power fiber laser in DGSBC system is carried out.The combining light field distribution is obtained by numerical calculation,so that the beam quality of DGSBC with multi-size and multi-modes laser can be calculated.The experimental systems based on the polarization multiplexing and high density combining notion are build,and the theory is validated by a serial of DGSBC examinations.DGSBC of five kilowatt non-polarization-maintain all fiber laser amplifiers using fiber Bragg-grating front-end is achieved.This DGSBC system features a combining efficiency of 91.2%,the beam quality of M2=2.96×1.52 and the output power of 5.02 kW.Based on this design the combining channel is extended to 20 channels,and 20 kilowatt with the beam quality of ?=2.29 is achieved.The feasibility of building high efficiency,high beam quality and high power DGSBC system is verified by the experimental results.The factors affecting the scalability of DGSBC,such as the ability of enduring high power laser,the efficiency of the diffraction grating and the scalability of the narrow-spectrum fiber laser are studied,and the output capability of the DGSBC system is evaluated.It is proposed to use a combination of spectrum,coherent and polarization combining methods to further improve the brightness of fiber laser system.The nature and limitations of the existing combining methods with filled aperture are summarized.The ideal using the output of CBC in filled aperture as the input of the SBC is proposed to increase the combining channels with controllable difficulty and reliability.Design method of diffractive optical elements for CBC in filled aperture is studied,and a CBC theme based on a multi-beam combiner composed of partition coating is presented to reduce the technical difficulty of combining device and phase control.Through the theoretical analysis combined with experimental research,the basic laws of diffraction propagation of fiber laser in DGSBC system are clarified,and the feasibility of achieving high efficiency,high beam quality and high output power using polarization multiplexing and high density combining DGSBC system is verified.The technical approach provides a reference for the design of practical SBC systems.