| Ultrashort pulse lasers have a wide range of applications in radar detection,micromachining,and biomedical fields.Among the many solutions for generating ultrashort pulse lasers,passively Q-switched microchip lasers pumped by laser diodes(LD)have become a hot research topic because of their simple structure,low cost,and single longitudinal mode output.When LD is used as a solid-state laser pump source,the divergence angle of the fast and slow axis of the output beam is different,and the LD output beam needs to be shaped.In addition,the energy distribution of the laser output beam is Gaussian type,and the uniformity is poor,which greatly limits the application range of the laser,and it is necessary to design a homogenization device to obtain a flat-top beam.Therefore,in this paper,a low-cavity method combined with passive Q-switched scheme is used to design an LD-pumped passive Q-switched microchip laser with high pump absorption efficiency and flat-top beam,which realizes the single-longitudinal mode pulse output in the order of picoseconds.In this paper,the physical and optical properties of Nd:YAG and Cr:YAG crystals and passive Q-switched theory are studied,a passive Q-switched rate equation system applicable to picosecond pulses is established,and expressions of single pulse energy,pulse width and peak power are derived.In Matlab,the effects of neodymium ion doping concentration,Cr:YAG initial transmittance and output mirror reflectivity on the output pulse parameters were explored.Then,in order to improve the beam quality of the output laser,the pump photoshaping system is designed in Zemax,so that the pump absorption efficiency of the working substance is greatly improved,and the thermal effect of the crystal can be reduced,so as to obtain a high beam quality laser output.On the basis of theoretical analysis,the experimental research of Nd:YAG/Cr:YAG passive Q-switched microchip laser with LD end-face pumping was carried out.Firstly,the Nd:YAG/Cr:YAG bonded crystal with a neodymium ion doping concentration of 1.1at.%was pumped with continuous LD pump light,and a single longitudinal mode output with a pulse width of 525.9ps was obtained,and at a pump power of 5W,the laser realized a multi-longitudinal mode output with a repetition frequency of 18.1k Hz,a single pulse energy of 10.8μJ and a pulse width of 353.9ps.Then,the Nd:YAG/Cr:YAG bonded crystals with doping concentrations of 1.1 at.%and 1.5 at.%were pumped with pulsed light of 100μs pulse width and 2 m J single pulse energy,and single longitudinal mode pulse outputs with pulse widths of 309.29 ps and 280.89 ps were obtained,respectively.When the bonded crystal with neodymium ion doping concentration of 1.5at.%pumped light is 3.5m J,the pulse output of 10Hz,pulse width is 248.91ps,and the single pulse energy is 0.18μJ,the energy stability of the laser is better than 0.01%,and the M~2factors of the output beam in the x and y directions are 1.27 and 1.29,respectively.The experimental results show that LD-pumped passive Q-switched microchip laser can achieve pulse output with single longitudinal mode,narrow pulse width and high beam quality.Finally,an aspheric homogenization lens is designed in Zemax,and the Gaussian beam is shaped into a Gaussian flat-top beam,but the divergence angle of the beam after shaping is large,in order to obtain a parallel beam,an aspheric collimating lens is designed to collimate the homogenized beam and obtain a parallel homogenized beam. |
PDF Full Text Request