Study Of QCW Longitudinally Pumped Passively Q-switched Microchip Laser Used For Laser-induced Plasma Ignition Of Internal Gasoline Engines

Laser-induced spark ignition method for internal combustion engines is an ideal alternative to traditional spark ignition method.Laser diode quasi-continuous-pumped passively Q-switched Nd:YAG/Cr:YAG laser based on microchip technology has many advantages,such as small size,compact structure,stable operation,long lifetime,all-solid-state and high conversion efficiency.It can produce laser pulses with high peak power and short pulse width.Therefore,it is an ideal light source for laser-induced spark ignition system of internal combustion engine.In this paper,a passively Q-switched microchip laser with high pulse energy and peak power which is suitable for laser-induced spark ignition of internal combustion engines is studied.The factors affecting the performances of Q-switched pulses are studied theoretically and experimentally,and a composite microchip laser with MW class peak power is built.Firstly,the research background,research significance and the development status of microchip laser technology for laser ignition in domestic and abroad are introduced in this paper.Three key technologies for building high performance microchip lasers are introduced,including the choosing of gain media,Q-switching technology and pumping technology.A microchip laser test system and a data acquisition system are built.Secondly,the influence of the initial transmission of saturable absorber and the reflectivity of output mirror on the performances of output pulses of Q-switched laser is analyzed by using the passively Q-switched rate equation.The calculation results show that the single pulse energy and the peak power increase,and the pulse width shortens with the decrease of the initial transmittance of saturable absorber and the reflectivity of output mirror.The influences of the initial transmission on the parameters are non-linear while the influences of output mirror reflectivity on the parameters are approximately linear in a certain range.Under the given parameters,when the reflectivity of output mirror increases from 20% to 80%,the single pulse energy and peak power decrease approximately linearly.Then,a Nd:YAG/Cr:YAG microchip laser system is built.The effects of pumping parameters and resonator parameters on the performance of Nd:YAG/Cr:YAG microchip laser are studied respectively.The experimental results show that the output pulse energy and the optical-optical conversion efficiency both increase with the increase of pump power.When the initial transmittance decreases from 60% to 15%,the single pulse energyincreases from 1.13 mJ to 2.94 mJ,and the pulse width decreases from 5.93 ns to 1.5 ns.When the reflectivity of output mirror increases from 50% to 80%,the single pulse energy decreases from 2.50 mJ to 1.13 mJ,and the pulse width increases from 2.53 ns to 3.65 ns.The experimental results of the influence of cavity parameters on Q-switched pulse performance are basically in good agreement with the theoretical calculation results.Finally,a high performance Nd:YAG/Cr:YAG microchip laser is developed based on composite crystal structure and integrated pumping module.The single pulse energy of the prototype A is 3.5 mJ,corresponding to a pulse width of 2.1 ns and a peak power of 1.67 MW.The single pulse energy,pulse width and peak power of the prototype B are 1.4 mJ,1.0 ns and 1.4 MW respectively.