Reserch On Good Beam Quality High Peak-power Solid-state Laser Amplifier Technology

In recent years,thanks to the strong support and policy guidance of the Chinese government in the industrial upgrading and intelligent manufacturing field,high peak power,high beam quality infrared,green or even ultraviolet lasers have become the research focus in the field of laser.The solid-state laser based on the main oscillator power amplifier structure has been favored in the field of laser micro processing and fine machining due to its simple and compact structure,good stability and high cost performance.This paper focuses on the laser application market,mainly studies the solid-state laser amplification technology,in order to obtain the laser output with high beam quality,high peak power.This paper is mainly divided into two aspects:firstly,starting with the crystal thermal effect that restricts the beam quality,the theoretical analysis and simulation of the thermal induced spherical aberration affecting the beam quality in the resonant cavity are carried out.the spherical aberration compensation theory is studied in the same media and different media;the second part is based on the theory of spherical aberration compensation,we design a hybrid fiber solid-state amplifier system with good beam quality and high peak power,and the nonlinear conversion is analyzed.In this paper,mature commercial laser gain medium Nd:YVO4 and Nd:YAG are the main research objects,and both of them are good laser amplification media.However,the beam quality deterioration caused by thermal distortion becomes an important factor that restricts the acquisition of higher power.By studying the resonators with different structures,it is found that the intensity distribution and phase distribution of the Gauss beam in the cavity will change with the propagation process.Both of them work together and show the change of beam quality.In the symmetric cavity structure,the beam quality does not change during the propagation,while in the asymmetric cavity the beam quality undergoes ameliorating to deteriorating cyclic process.The intracavity beam quality evolution in the cavity can be imitated by multistage amplifier outside the cavity to realize the effective management of the beam quality.This paper analyzes the process from simulation,theory and experiment,and it is concluded that the positive spherical aberration introduced by the crystal thermal effect is the main factor affecting the beam quality.However,during the Gauss beam propagation,spherical aberration will change constantly.After it passes the waist,the spherical aberration changes from positive to negative.When the negative spherical aberration whose absolute value is close to the positive spherical aberration goes through the crystal with the positive spherical aberration,the spherical aberration can be compensated to realize the light beam quality improvement.This process can be achieved through a single pass of two stage or a double pass of one stage.This theory is not only applicable to main oscillation multistage amplification process of the same media,but also can be used for different gain media.The beam when a beam of high beam quality through a gain medium double pass amplification,can ensure that the beam quality is basically unchanged,which continued into the next stage double pass amplification.This helps us to make full use of fiber laser to obtain the small signal,the output characteristics of narrow pulse width,frequency spectrum and adjustable,and the advantages of Nd:YVO4 crystal in small signal input to obtain high gain and high output power of Nd:YAG crystal is easy to obtain in high fill power extraction conditions,fully integrate the three.A laser amplification structure with mode locking fiber laser as seed source,two stage Nd:YVO4 double pass amplification as preamplifier and one stage Nd:YAG double pass amplification as main amplifier is designed.The pulse width of the fiber source is 4.5 ps,the output frequency can be adjusted from 30 kHz to 30 MHz,single pulse energy is?1 nJ,the line can be adjusted ± 0.2 nm near the central line of 1064.4 nm.For two-stage Nd:YVO4 double-pass amplification,high gain is ensured by reasonably designing the size and fill factor of each amplification stage pump.For single-stage Nd:YAG double-pass amplification,we increase the pump size and fill factor to improve the extraction efficiency while reducing the peak power density of the crystal surface in order to prevent damage to the crystal.Finally,the laser with an average output power of 31.8 W is obtained under the condition of repetition frequency of 30 kHz and center spectrum line of 1064.38 nm.The peak power of single pulse is more than 100 MW,and the beam quality is 1.26.The gain of the whole amplifier system is more than 60 dB.Extra-cavity second and fourth harmonic generation are performed with the beam from the above MOPA system,respectively.Due to the high peak power and high beam quality of the above system,25.5 W output power of 532 nm green laser is avhieved corresponding to a nonlinear conversion efficiency of 80%.And we are able to generate 8.9 W UV laser corresponding to a nonlinear conversion efficiency of 35%.