Study On Time-domain Coherent Beam Combination Of Ultrashort Pulse Fiber Laser Based On Active Phase Control Technology

High-power ultrashort pulse lasers play an important role in the fields of basic scientific research,industrial manufacturing,and biomedicine.In recent years,with the continuous innovation of fiber laser technology,the average power of ultrashort pulse lasers based on a single fiber has reached the k W level.However,in high-power ultrashort pulses,the nonlinear effect and thermal effect of the fiber are significant,which makes it difficult for the ultrashort pulse laser based on a single fiber to obtain high-power laser pulse output under the condition of ensuring the beam quality.The coherent synthesis technology of ultrashort pulse fiber laser is one of the technologies that can break through the power limit of a single fiber.This technology can solve the nonlinear polarization of the fiber in the high-power ultrashort pulse and the mode instability caused by the fiber thermal effect.In the coherent synthesis of ultrashort pulse fiber laser,in order to ensure the synthesis efficiency and beam quality of the output pulse beam,it is necessary to control the phase of each sub-beam in the synthesis.Nowadays,with the rapid increase of the number of synthesis channels in the ultrashort pulse fiber laser coherent synthesis,higher requirements are put forward for the control of the sub-beam phase.At present,the phase control technology of the ultrashort pulse fiber laser coherent synthesis has become a research hotspot.This thesis mainly studies the phase control technology in the time-domain coherent synthesis of ultrashort pulse fiber lasers.The main work of the thesis is as follows:The first part,the research background and significance of the subject were described.This article summarized the ultrashort pulse coherent synthesis and its active phase control technology,and combed the research status of the ultrashort pulse coherent synthesis technology at home and abroad,and the various active phase control technologies in the ultrashort pulse coherent synthesis were compared and analyzed in detail.Determined the research content.In the second part,the active phase control technology applied in the coherent synthesis of ultrashort pulses was mathematically derived.First,the mathematical model of the stochastic gradient descent algorithm was derived,and then the mathematical model of the Q-learning algorithm applied in time-domain coherent synthesis was derived based on the reinforcement learning theory.In the third part,a 2-channel ultrashort pulse fiber laser time-domain coherent synthesis experimental device was built to verify the feasibility of the algorithm.The experimental device includes: a mode-locked fiber oscillator,a time domain beam splitting device,a polarization maintaining fiber amplifier,a time domain beam combining device and a phase control system.First,the SPGD algorithm was used to phase lock the experimental device,and a laser output with a power of 260 m V and a pulse width of 34.5ps was obtained.The synthesis efficiency of the experimental device is 85%.Then a stable laser output was obtained by using the Q-learning algorithm to phase lock the experimental device.Then,compared and analyzed the phase-locked laser output with the SPGD algorithm.The analysis results showed that the stability of the Q-learning algorithm in the experimental device is better than the SPGD algorithm.After that,a coherent synthesis dynamic phase noise model was established based on white noise filtering,and the model was used to analyze the phase-locking performance of the Q-learning algorithm in multi-channel coherent synthesis,and compared with the SPGD algorithm.The comparison results showed that the stability of the Q-learning algorithm is better than that of the SPGD algorithm,and it is easier to adjust.Finally,an NSI algorithm based on swarm intelligence was introduced,and the two-channel ultrashort pulse fiber laser time-domain coherent synthesis experimental device was phase-locked using this algorithm,and finally a stable laser output was obtained.Then,a coherent synthesis dynamic phase noise model was established by the superposition of sine waves.The NSI algorithm was simulated and analyzed using this model.The analysis showed that the convergence of the algorithm is mainly related to the population size,and the appropriate population size makes the algorithm very stable.At the same time,101-channel static coherent synthesis simulation was carried out on the NSI algorithm and the SPGD algorithm.The simulation results showed that the convergence speed of the NSI algorithm is better than that of the SPGD algorithm.