2μmTm,Ho∶YLF激光抽运ZnGeP_2光参量振荡技术研究

Technical Study of ZnGeP_2 Optical Parametric Oscillator Pumped by a 2 μm Tm,Ho∶YLF Laser

ZnGeP2晶体具有宽的透明范围(0.7～12μm),较大的非线性系数(d36=75pm/V),最高损伤阈值能量密 度为10J/cm2,较高的热导率(0.18W/(m·K)),因而非常适合作为高功率中红外光参量振荡器(OPO)晶体。理 论上分析了ZnGeP2光参量振荡器相位匹配特性,实现3～5μm连续调谐范围输出的Ⅰ类相位匹配角在52.5～ 55.2°之间。实验上,以15W光纤耦合激光二极管(LD)抽运的2.05μm高重复频率声光调QTm,Ho∶YLF激光 器作为抽运源,其最大平均功率4W,脉冲宽度小于40ns,脉冲重复频率100Hz～10kHz可调。为降低准三能级 系统激光器阈值,提高激光脉冲能量抽取效率,Tm,Ho∶YLF晶体采用液氮制冷方式,工作在77K温度条件下。 非线性频率转换晶体ZnGeP2长15mm,55.7°切割,光参量振荡器谐振腔为平平腔,腔长约20mm。在3.6W的抽 运功率下,脉冲重复频率10kHz,实现了4.1μm附近中红外激光输出,参量光脉冲宽度为20ns,平均输出功率为 0.7W,光 光转换效率为20%,抽运光阈值功率为0.65W。

ZnGeP2 crystal with high nonlinear coefficient (d36 = 75 pm/V), broad infrared transmission region (0.7-12 μm) and very high thermal conductivity (0.18 W/(m&middotK)) is very suitable for high power mid-infrared radiation generation. Phase matching characteristics of ZnGeP2 optical parametric oscillator (OPO) are analyzed theoretically, the requirement of type I phase matching angle of 52.5-55.2° must be met to achieve continuously tuning of 3-5 μm. Experimentally, the acousto-optic Q-switched Tm,Ho:YLF laser end pumped by fiber-coupled diode is employed as the pumping source of OPO with the maximum average power of 4 W, pulse duration of less than 40 ns, and pulse repetition rate of 100-10000 Hz. For higher extraction efficiency and low threshold operation to the quasi-three level laser system, the Tm,Ho:YLF crystal is cooled by liquid nitrogen reserved in a small detector type dewar, and works at the temperature of 77 K. The 15-mm-long, 55.7° cut ZnGeP2 as a frequency converting crystal was placed in a piano-piano cavity with resonator length of 20 mm. Under a pumping power of 3.6 W and pulse repetition rate of 10 kHz, the mid-infrared coherent radiation of OPO near 4.1 μm was achieved with output power of 0.7 W, pulse width of 20 ns, optical-optical conversion efficiency of 20% and threshold pump power of 0.65 W.