| Being one of the most important techniques in the field of combustion and flow research,planar laser induced fluorescence(PLIF)imaging can selectively excite free radicals or tracer particles in the flow field,and obtain the spatial distribution information of the flow field by imaging the fluorescence emitted by those free radicals or tracer particles.At present,commercial PLIF systems typically have a frame rate of?10 Hz,and it is not capable of diagnosing high-speed combustion or flow fields with the timescale of milliseconds,sub-milliseconds,or shorter.Therefore,the PLIF technique is developing toward the goal of high frame rates.To meet the needs of high-speed PLIF diagnostics,pulse lasers are required to have the output capability of high repetition rate and high energy.However,due to the impact of thermal effects and the limitation of average power,it is generally difficult to obtain laser pulses with both high repetition rate and high energy.The bottleneck in laser performance has limited the development of high-speed PLIF diagnostics.As a new type of pulsed lasers,burst-mode lasers employ intermittent pumping to output a burst in each pumping period,and each burst contains multiple single pulses.The burst-mode laser is an alternative approach to solve the contradiction of repetition rate and pulse energy,and provides laser pulses with both high repetition rate and high energy in a burst.The intermittent output just meets the requirement of applications such as high-speed PLIF diagnostics and laser-induced plasma ignition.Therefore,burst-mode lasers have become an optimum laser source for high-speed PLIF imaging diagnostics.However,current burst-mode lasers have the disadvantages of lacking flexible generation schemes,very-low seed pulse energy,and very-large and complex amplification systems.These disadvantages limit the practicality of burst-mode lasers.To solve the problem of low seed pulse energy(nJ-?J)generated by extra-cavity modulation,which then leads to complex and huge amplification systems,in this thesis we propose a novel scheme based on pulsed pumping combined with intra-cavity modulation to obtain burst-mode lasers.With the burst-mode scheme being analyzed theoretically,experiments with the diode end-pumped Nd:Gd0.69Y0.3TaO4 laser and side-pumped Nd:YAG laser are conducted to verify the novel method for burst-mode laser generation.In the 879-nm diode-pumped cavity-dumped Nd:Gd0.69Y0.3TaO4 laser operation,burst-mode lasers with the single-pulse repetition rate of 20 kHz,single-pulse energy of 0.51 mJ,and single-pulse width of 3.4 ns have been obtained.In the diode side-pumped cavity-dumped Nd:YAG laser operation,burst-mode lasers with the single-pulse repetition rate of 100 kHz,single-pulse energy of 4.4 mJ,and single-pulse width of 2.6 ns have been obtained.The above experiments verify that the new burst-mode laser scheme based on pulse pumping combined with intra-cavity modulation is feasible and efficient.Burst-mode lasers with single pulse energy at millijoule-level can be obtained by the oscillator,which solves the problem of low seed pulse energy caused by the extra-cavity modulation scheme.To obtain burst-mode lasers with high repetition rate and high energy,it is necessary to amplify the laser output of the master oscillator.However,the sectional intensity distribution of the laser beam is likely to deteriorate,and laser-induced damage can occur through the process of amplification,due to thermal effects and uneven gain distribution in the gain medium.To address this issue,the method of gain distribution management has been introduced and demonstrated in this thesis to improve the sectional intensity distribution of the laser beam.Firstly,a theoretical model of the gain distribution in side-pumped gain mediums has been built based on the unsaturated pump absorption theory,the ray tracing algorithm,and the optical configuration of side-pumping modules.And based on the model,a finite-element-analysis(FEA)software has been developed to simulate and analyze the pumping power distribution inside the gain medium.The software can be used to optimize the design of the diode side-pumping modules,so as to achieve effective management of the gain distribution in the gain medium.Secondly,with the help of the simulation software,laser diode side-pumping modules with a "near-flat-top"gain distribution and a "ring-shaped" gain distribution have been designed successively,and experiments have proved the effectiveness of the newly designed gain modules on reducing thermal effects and improving the sectional intensity distribution of the laser beam.Compared with the commercial gain module,the"near-flat-top" gain module has the thermal focal length of the gain medium increased by?50%under the same working conditions.Finally,with the help of the gain management technique,a burst-mode laser MOPA(master oscillator power amplifier)system with two "near-flat-top" gain modules and one "ring-shaped-gain"thermal compensating module employed has been developed.The single-pulse repetition rate,burst energy,single-pulse energy,and single-pulse width reach 100 kHz,2.0 J,10.2 mJ,and 3.3 ns,respectively,with?200 single pulses per burst.The output beam shows a near flat-top intensity distribution on its cross-section,and no spot with excessive intensity is found on the beam section.The laser works stably,and no laser-induced damage has been found through long-time operations.The laser is particularly suitable as a pump source for dye lasers and used in high-speed PLIF imaging systems. |
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