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As passively mode-locked fiber lasers based on Nonlinear Polarization Rotation(NPR)have the problem of low stability and difficulty to self-start into a mode-locking state during operation,an online automatic mode-lock circuit was designed to adjust the laser in real time to get a stable modelocked output.In the design,the signal from NPR passively mode-locked laser was transformed to an electrical pulse signal with a high speed photoelectric detector,and it was amplified and shaped linearly to be received by the Micro Computer Unit(MCU)to count.Then the MCU distinguished the state of the laser based on quick mode-lock judge algorithm and adjusted the electric controlled polarization controller(PC)on the fiber laser automatically.The experiment results show that for the NPR modelocked fiber laser with a repetition rate of 6.238MHz,the automatic mode-lock circuit can distinguish the losing lock in 6ms,and can find and get into the mode-lock state in 10.24sautomatically.ForNPR passive mode-locked fiber lasers with repetition rate lower than 16 MHz,this automatic modelock circuit can get into a stable mode-locked state quickly or restart immediately if the mode-lock state is lost for some reasons,which reaches the design requirements and shows advantages of simple structures,low costs,low powers,and high reliability.

According to internal pressure effect and without internal pressure effect,the temperature field,thermal stress,strain and the damage of the combustion chamber case under the laser irradiation,are calculated and analyzed respectively,based on the finite element analysis software ANSYS.Analysis results show that,the distribution of the shell temperature field and beam power is identical,and the maximum temperature is at the center of spot.The maximum stress is not in the center,but is located in the flare edg.If the shell absorption of laser power density is lager than 1 000 W/cm2,the stress of the shell will be greater than limited stress and the shell will soften.In the presence of internal gas pressure condition,shell stress produces a local focus,along the circumferential surface by laser spot center axis region crack probably.Compared with no internal pressure shell,the stress and deformation produced by the shell with internal pressure are greater.In order to achieve the same damaging effects,in the circumstances of exist internal pressure,the laser radiation intensity can be appropriately reduced.