| In this paper, the technology of high power pulsed Nd:YAG laser is studied anddiscussed. Based on the optimized design of pulsed Nd:YAG module unit, thealignment accuracy of Nd:YAG rods in the resonator has been analyzed theoretically,and experiment of multi-rod series connection are carried out. Through six rodsseries connection resonator and a MOPA structure of four rods oscillator and tworods amplifier, the pulsed laser with an average power of more than3000W havebeen obtained. And the differences of the two structures are compared and analyzedAccording to the application requirements of high power pulsed Nd:YAG laser,several technical schemes are analyzed, then the design of pulsed Nd:YAG moduleunit is optimized, and two schemes of six rods resonator and a MOPA structure offour rods oscillator and two rods amplifier are given.The distribution of pump energy and thermal stress of the module unit are simulated,and the optimum operating parameters are studied experimentally. An output laserwith an average power of586W, a peak power of3.45kW, a maximum single pulseenergy of34.9J and a beam parameters product of22.5mm×mrad has been obtained.The electro-optical conversion efficiency is4.04%, and the instability of the outputpower is1.1%.The thermal depolarization effect of Nd:YAG rods of different cut directions aretheoretically and experimentally studied. The results show that, for [111]-cutNd:YAG rod, the thermal depolarization does not change with the polarizationdirection. However, thermal depolarization of [100]-cut direction Nd:YAG rodvaries with the polarization direction, the minimum depolarization can be obtainedby adjusting the direction of linear polarization, the thermal depolarization of thisdirection is less than that of [111]-cut Nd:YAG rod.Experiments of laser percussion drilling are carried out on nickel-based superalloyusing pulsed Nd:YAG module unit. The influence of combination of pulses, focalposition and the number of pulses are experimentally studied. Through applyingpulse combination with increasing pulse energy, placing the focal position1.1~1.7mm above the top surface of the sample, and adjusting the number of pulses,non-tapered holes with diameters of480μm and510μm are drilled on nickel-based superalloy samples with thickness of1.5mm and3mm. Error of diameter in repeatdrilling is about30μm, and the hole taper is <1%.The range of stable region and the influence of rod spacing and thermal focal lengthfor six rods series connection resonator are theoretical analyzed. Considering theneed of multi-rods series connection pulsed lasers, a method is presented, and4×4matrices are used to analyze the deviation of resonator optical axis, the mode volumeand the output direction affected by offset of rods:(1) Through calculating thedeviations of optical axis of every arrangement of offset rods, the required alignmentaccuracy of Nd:YAG rods in six rods series connection resonator and thearrangements producing the maximum deviation of optical axis are given;(2)Through calculating the loss of mode volume, the rod positions which producing themaximum loss are given;(3) The changes of the output direction varying with theoffset and the thermal focal length of rods are simulated.In experiment, through adjusting module unit, the offset angles of all rods are≤0.037°, six rods series connection pulsed Nd:YAG laser is achieved. Thermal lensfocal length of rods is compensated by adjusting pump power. With input electricpower of87kW and a duty cycle of17%, a maximum average output power of3043W, a peak power of17.75kW, a maximum single pulse energy of66J and a beamparameters product of26.3mm×mrad have been obtained. The electro-opticalconversion efficiency is3.5%.The deviations of optical axis affected by offset rods of the MOPA structure of fourrods oscillator and two rods amplifier are analyzed theoretically, and the requiredalignment accuracy of Nd:YAG rods in the MOPA laser is given. The influence ofrod spacing and thermal focal length are analyzed. The results show that, when therod of amplifier is placed symmetrically and the thermal focal length is same withthe rod of oscillator, the mode volume can be effectively used, which can improvethe amplification level of energy extraction efficiency, it is helpful for high poweroutput.A MOPA system of four rods oscillator and two rods amplifier has been used in theexperiment, and the offset angle of all rods is adjusted to≤0.042°. With inputelectric power of87kW and a duty cycle of17%, a maximum average output powerof3011W, a peak power of17.7kW, a maximum single pulse energy of67J and abeam parameters product of25.2mm×mrad have been obtained. The electro-optical conversion efficiency is3.46%, and instability of the laser is less than2%.Comparing with six rods series connection pulsed laser, MOPA pulsed system hasthe following advantages:(1) MOPA system reduces the number of oscillator, thematch of thermal focal length is more easily achieved;(2) The rod tolerance ofMOPA system is bigger, the requirement of precision of series connection is moreeasily achieved;(3) With the same offset angle, the deviation of optical axis of theMOPA system is less than that of six rods resonator. It is helpful to improve thesecurity performance of multi-rod pulsed laser, and obtain high average outputpower and high pulse energy.
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