Objective Mid-infrared lasers with a narrow pulse and high peak power in 3-5 μm band have important research value in practical applications in laser communication,lidar,and atmospheric environment detection.Optical parametric oscillators(OPOs)are attracting increasing attention as a common method for fabricating lasers in this band.Currently,the OPO nonlinear crystals used to generate mid-infrared lasers mainly include ZnGeP2(ZGP),AgGaSe2(AGS),and period-polarized LiNbO3(PPLN).Among these,ZGP and AGS crystals have high growth costs and strong absorption of lasers less than 2 μm,which is not suitable for pumping with a 1 μm laser,and PPLN crystals are prone to photorefractive damage at room temperature.Period-polarized LiNbO3 doped with MgO(MgO∶PPLN)is the preferred nonlinear crystal for quasi-phase matching,owing to its large nonlinear coefficient,high damage threshold,and wide light transmission range.However,owing to the inherent properties of MgO∶PPLN crystals,the mid-infrared laser obtained using an acousto-optic Q-switched MgO∶PPLN intracavity optical parametric oscillator has a wide pulse width and low peak power,which makes it difficult to meet market demands.To solve these problems,a 3.8 μm intracavity optical parametric oscillator(IOPO)with an MgO∶PPLN crystal based on a double Q switching of an acousto-optic modulator and a Cr4+∶YAG saturable absorber is reported in this study.Methods In this study,the pulse width of the mid-infrared laser is significantly compressed,and the peak power and beam quality of the mid-infrared laser are improved.First,based on OPO theory,the polarization period and temperature of the crystal required for the output of the 3.8 μm mid-infrared laser by the MgO∶PPLN intracavity optical parametric oscillator are determined.Second,an experiment using an acousto-optic Q-switched MgO∶PPLN intracavity optical parametric oscillator is performed,and the output characteristics of the idler light are measured.Then,we analyze the influence of different small-signal transmissions on the output characteristics of a double Q-switched MgO∶PPLN intracavity optical parametric oscillator to find the best small-signal transmittance of Cr4+∶YAG passive Q crystals.Next,we perform an experiment using a double Q-switched MgO∶PPLN intracavity optical parametric oscillator and measure its idler light output characteristics,including the output power,pulse width,single pulse energy,peak power,beam quality,and spectral wavelength.Finally,the experimental results of two types of intracavity optical parametric oscillators pumped by an acousto-optic Q-switched laser and a double Q-switched laser are compared.Results and Discussions In this study,experiments on two types of intracavity optical parametric oscillators pumped by an acousto-optic Q-switched laser and a double Q-switched laser are carried out.When the pump power is 18.41 W and the repetition frequency is 40 kHz,the 3.8 μm idler light with a pulse width of 9.50 ns and peak power of 1.29 kW is obtained for acousto-optic Q-switching(Fig.3).Meanwhile,the beam quality factor(M2)of the idler light in horizontal direction and beam quality factor(M2)of the idler light in vertical direction in acousto-optic Q-switching is 3.11 and 3.44,respectively(Fig.4).When the pump power is 18.41 W,repetition frequency is 40 kHz,and small-signal transmittance of the inserted Cr4+∶YAG passive Q crystal is 92.67％,the 3.8 μm idler light with a pulse width of 6.43 ns and peak power of 1.48 kW is obtained for double Q-switching(Fig.6).The experimental results show that,compared with an acousto-optic Q-switched OPO,the 3.8 μm idler light pulse width of a double Q-switched IOPO is compressed by 32.3％,and the peak power is increased by 14.7％(Fig.9).Simultaneously,the beam quality of the idler light in double Q-switching is improved,and the M2x and M2y is 1.98 and 2.05,respectively(Fig.8).Conclusions In this study,a mid-infrared intracavity optical parametric oscillator with a MgO∶PPLN crystal based on a double Q-switched pumping is reported.The experimental results show that when using an acousto-optic Q-switching and a Cr4+∶YAG passive Q crystal with a small-signal transmittance of 92.67％,the pulse width of 3.8 μm idler light can be effectively compressed,and the peak power can be increased in the single resonant intracavity optical parametric oscillator.Compared with an acousto-optic Q-switched IOPO,the 3.8 μm idler light pulse width of a double Q-switched IOPO is compressed by 32.3％,and the peak power is increased by 14.7％.When the pump power is 18.41 W,repetition frequency is 40 kHz,and small-signal transmittance of the inserted Cr4+∶YAG passive Q crystal is 92.67％,the 3.8 μm idler light with a pulse width of 6.43 ns and peak power of 1.48 kW is obtained for double Q-switching.Simultaneously,the beam quality of the idler light is improved,and the M2x and M2y is 1.98 and 2.05,respectively.The results of this study provide new technical support for the development of high-quality mid-infrared light sources with narrow pulse widths and peak powers.
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