Research On All-solid-state Raman Mode-locked Lasers

Mode-locked technology can make the output lasers exhibit shorter pulse width,higher peak power and wider spectrum compared with Q-switched lasers.With the development of semiconductor technology,all-solid-state mode-locked lasers pumped by laser diodes have been widely used in ultra-fine processing,laser ranging,biomedicine and other fields.Stimulated Raman Scattering(SRS),as a third-order nonlinear optical effect,can not only extend the spectral range,but also make the output Stokes beams have narrower pulse width and higher brightness relative to the fundamental beams.Besides,the beam cleanup effect of stimulated Raman scattering can greatly improve the beam quality of the output Stokes lasers without the additional beam shaping elements.Hence,the Raman mode-locked laser,which combines SRS and mode-locked technology,has important research value for realizing laser output with high beam quality,high brightness,high peak power and narrow pulse width.In this paper,a series of results are achieved and as follows:1.The passively Q-switched mode-locked Nd:YVO₄ based on Cr⁴⁺:YAG is studied.According to the energy level structure of the Cr⁴⁺:YAG crystal and the principle of the mode-locked,Nd:YVO₄ is utilized as the laser crystal.The thermal effect of the laser crystal is analyzed and calculated.The resonant cavity is well designed and reasonably optimized.The maximum average output power of 1064 nm Q-switched lasers are 4.06 W,3.92 W and 5.31 W in straight cavity,V-type cavity and Z-type cavity,respectively,with corresponding conversion efficiencies of 30.15%,26.45%and 26.61%.The corresponding slope efficiency is 38.22%,29.81%and 28.37%respectively,and the mode-locked pulse width is 240.52 ps,676.65 ps and 882.47 ps,respectively.2.A passively continuous wave mode-locked Nd:YVO₄ laser based on SESAM is set up.A W-shaped resonant cavity composed of five mirrors is designed.The beam spot distribution in the resonant cavity is calculated by MATLAB to reduce the astigmatism caused by the folding mirrors.When the incident pump power is 14.95 W,a continuous wave mode-locked laser with the maximum average output power of 3.99W is obtained.The optical efficiency is 26.73%with the slope efficiency of 29.88%.The mode-locked pulse width is 214.84 ps with the pulse repetition frequency(PRF)of89.01 MHz.The beam quality factors M² in the horizontal and vertical directions are1.65 and 1.72,respectively.3.The passively Q-switched mode-locked Nd:YVO₄/YVO₄ 1176 nm internal Raman laser and the device of a passively Q-switched mode-locked Nd:YVO₄/YVO₄1176 nm Raman laser with a composite cavity based on Cr⁴⁺:YAG are studied respectively.The internal Raman Q-switched mode-locked laser adopts the device of concave-flat cavity.The output mirrors with a transmission rate of 2%and 5%at 1176nm are used.The 1176 nm Raman Q-switched mode-locked average output power of401 m W and 478 m W are obtained when the incident pump power is 14.95 W.The corresponding optical efficiency is 2.67%and 3.19%,respectively,and the slope efficiency is 4.11%and 5.06%,respectively.Based on the internal Raman laser,a compound cavity is adopted,so that the length of fundamental and Raman lasers cavity can be independently adjusted to achieve better mode-matching.Besides,the fundamental and the Raman lasers are also resonated separately.The 1176 nm Raman Q-switched mode-locked laser output with an average power of 1.11 W is obtained at an incident pump power of 15.81 W.The mode-locked pulse width is 278.89 ps,and the PRF is?899 MHz.It provides a good foundation for the next realization of passively Q-switched mode-locked Raman laser based on multi-mirrors compound folded-cavity.