重频可调高功率绿光皮秒激光器

Tunable Repetition-rate High Power Picosecond Green Laser

采用多级串联的主振荡放大技术,将SESAM锁模的种子光通过多级串联端泵Nd∶YVO_(4)晶体放大,产生平均功率为148.7 W、重复频率500 kHz~4 MHz、脉冲宽度8 ps的1064 nm基频光,同时开展了LBO晶体高效率二倍频的研究。通过优化相位匹配,实现了平均功率为95 W的532 nm皮秒激光输出,光-光转换效率达65%,光束质量因子M_(X)^(2)=1.27/M_(Y)^(2)=1.42,6 h功率抖动均方根低于0.8%。该绿光皮秒激光系统具有平均功率高、转换效率高、光束质量好、稳定性高、重频可调等优点,可以被作为皮秒激光加工系统的光源,被应用于科学研究和工业加工领域。

In this paper,an 1064 nm,148.7 W,8 ps laser is obtained through a multi-stage end-pumped Nd:YVO_(4) solid-state laser amplification system.Further a 1mm-diameter spot is coupled to a LBO crystal for frequency doubling,and a 95 W,6.43 ps laser output is obtained at 532 nm,with a tunable Pulse Repetition Frequency(PRF)of 500 kHz~4 MHz.The Nd∶YVO_(4) Master Oscillator Power Amplifier(MOPA)consists of a high-power oscillator stage,fiber pre-amplifier and four end-pump amplifier stages.The seed is an all-fiber laser mode-locked by SESAM with a pulse duration of 7.8 ps and PRF of 20 MHz.The Acoustic Optical Modulator(AOM)is used for reducing the PRF to realize a higher peak power.The end-pump power amplifier is a four-stage free-space bulk amplifier based on neodymiumdoped yttrium vanadate(Nd∶YVO_(4))crystals for high-efficiency frequency conversion.It has a high polarization-dependent gain spectrum due to the birefringence of the uni-axial YVO_(4) crystal.Each amplifier is comprised of a 0.3%doped a-cut Nd∶YVO_(4) crystal having dimensions of 4 mm×4 mm×15 mm and is mounted on a water-cooled heat sink.All amplifiers are end-pumped by a fiber-coupled 878.6 nm(400μm core,NA=0.22)Laser Diode(LD),instead of conventional 808 nm LD,to reduce the degradation of the beam quality by thermal effects.The first amplifier stage is a double-pass configuration and increases the signal average power to 13 W for a 65 W pump light,while the second amplifier stage is a single-pass and scales up to 56 W of average power for a 115 W pump light.To match the amplified light with the pump light of the third stage,a lens of 80 mm focal length is added between the second stage and the third stage.Finally,56 W,2 MHz infrared(1064 nm)8.08 ps output with M2<1.5 is obtained.Then,the 56 W infrared picosecond laser is used as the source of the pre-amplification,and after shaping,it is focused to the third and fourth-stage amplifying module.This module uses two 115 W LD pumps with a center wavelength of 878.6 nm to pump two Nd∶YVO_(4) crystals.The seeding picosecond laser passes through the third crystal and is then deflected by a dichroic mirror(DM3)to the fourth crystal for further power amplification.An output power of 148.7 W is achieved,and the beam quality factor of the amplified pulses is M_(X)^(2)=1.72 and M_(Y)^(2)=2.18.along horizontal and vertical directions,respectively.After shaping the output,a 1 mm-diameter spot is coupled to LBO crystal for frequency doubling.The highest output power is 95 W at 2 MHz PRF,corresponding to the best second harmonic conversion efficiency of 65%.The conversion efficiency is expected to be further improved with the fundamental frequency optical power,the beam quality factor is M_(X)^(2)=1.27,M_(Y)^(2)=1.42.At the highest average power output,the stability of the system is observed for more than 6 hours,and the power fluctuation RMS is lower than 0.8%.In addition to achieve the highest frequency-doubling conversion efficiency,the effect of the fine regulation of the temperature of the LBO crystal on the optimal critical phase matching condition of the crystal is also studied.The laser system has the advantages of simple optical path,high average power,good beam quality,tunable repetition frequency and high stability.It is an ideal fundamental frequency light source for high-power ultraviolet and deep ultraviolet lasers,and has important applications in the fields of industrial processing and scientific research.It is expected to achieve more efficient and can be utilized for high-quality processing fields such as battery welding,and hard and brittle materials processing.