Research On In-band Pumping Intro-cavity Q-switched Narrow Linewidth 2.1?m Laser

2.1um laser is not only located in the atmospheric window band,but also a good pumping source for mid-infrared?3⁵um?lasers.It is widely used in the fields of radar,laser medical,photoelectric countermeasure and biology.Intracavity 2.1um laser has compact structure and can make use of the technical advantages of high intracavity power,which is more in line with the application requirements in the technical field.Therefore,it is more important to carry out the research of intracavity 2.1um laser.However,there are two problems of the intercavity structure.First,the serious thermal effect of the crystal under the traditional 808nm pumping mode,which results in low output power and poor beam quality at 2.1?m.The second is the high gain in the cavity under strong pumping and the influence of factors such as the wavelength degeneracy point results in a wider linewidth of the 2.1?m.In this paper,based on the above two problems,the theoretical and experimental research is carried out by using the 880nm in-band pumping combined with the intracavity reflection Bragg grating to narrow the 2.1?m laser linewidth.Theoretically,we analyze the theory which based on PPMgLN-OPO firstly.Based on the three-wave coupling equation and quasi-phase matching principle,the wavelength characteristics of PPMgLN crystal,the threshold of PPMgLN-OPO and the super-threshold multiple are analyzed.Then we analyze the cause of the line width broadening of 2.1?m laser,and simulated the spectrum and angular spectrum selectivity of VBG,optimizing the parameters of VBG.Finally,starting from the energy level characteristics of Nd:YVO₄crystal,we compare and analyze the thermal effects and thermal focal length changes of the traditional 808nm pumping and 880nm in-band pumping method.On this basis,the structure and parameters of the OPO resonator are optimized.Experimentally,the self-built LD in-band pumping acousto-optic Q-switched Nd:YVO₄ laser is used as the pumping source for the intercavity PPMgLN-OPO.First,we analyze the spectral characteristics of the 2.1?m laser output by adjusting the temperature of the PPMgLN crystal,and we obtain the 2.1?m degenerate laser output at 48°C.Under this temperature,we compare the output characteristics of 2.1?m laser with different transmittances at different repetition rates.When the output mirror transmittance is 50%,the repetitive frequency is 80KHz,and the crystal absorption power is 40.33W,we obtain the 2.1?m laser output with an output power of 5.87W,and the line width is 73.65nm.Finally,we introduce VBG to obtain the 2.1?m narrow linewidth laser with an output power of 4.68W and line width of 1.89nm.