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非线性频率转换是利用非线性晶体的二阶非线性效应,通过非线性晶体中电磁场的相互作用来实现的。目前常用的非线性频率转换技术主要包括光参量产生(optical parametric generation, OPG)、差频混合(difference frequency mixing,DFM)、差频产生(difference frequency generation, DFG)、光参量振荡(OPO)和光参量放大(OPA)等。基于非线性频率转换方法,利用不同非线性晶体可以实现紫外(200 nm)到远红外(12 µm)宽光谱范围可调谐输出,激光器结构简单,且晶体本身并不参与能量的交换,因此没有量子亏损,产热很少。所以,非线性频率转换技术是目前获取中长波红外激光的主要方法。
非线性频率转换对非线性晶体的要求主要包括:双折射、机械强度、非线性系数、热导率、损伤阈值、透过光谱等。由于二次非线性效应是光光转换过程,要求非线性晶体对泵浦光的吸收系数尽可能小,减少损耗。损伤阈值越高,则晶体承载的最大能量密度越高,这有利于产生大能量、高功率的激光。非线性系数反映了频率转换的难易程度,其值越大则泵浦阈值更低,频率转换越容易。热导率、吸收系数与热透镜效应直接相关,更大的热导率以及更小的吸收系数的非线性晶体是产生高功率、大能量激光的关键。
目前用于中长波红外输出的有发展前景以及输出效果较好的部分非线性晶体的特性如表1所示[22]。
Crystal Nonlinear coefficient/pm·V−1 Transparency range/µm Thermal conductivity/W·m−1·K−1 Damage threshold/MW·cm−2 ZnGeP2 d36=75 0.7-12 35 55.6 (1.064 µm, 10 ns) BaGa4Se7 d11=24.3
d13=20.40.47-18 0.74∥a
0.64∥b 0.56∥c557 (1.064 µm, 5 ns) KTiOPO4 d15=1.9, d24=3.6
d33=16.90.35-4.0 2∥a
3∥b
3.3∥c500 (1.064 µm, 10 ns) PPKTP d33=16.8 0.28-4.5 2∥a
3∥b
3.3∥c900 (1.064 µm, 5 ns) KTiOAsO4 d15=4.2, d24=2.8
d33=16.20.35-5.2 1.8∥a
1.9∥b
2.1∥c>500 (1.064 µm, 10 ns) LiNbO3 d22=2.1, d31=4.3, d33=27.2 0.35-4.5 5.6 120 (1.064 µm, 10 ns) PPLN d33=27.2 0.33-5.5 5 200 (1.064 µm, 10 ns) MgO:PPLN d13=14.8 0.36-5 4.4 600 (1064 nm,9 ns) AgGaS2 d36=12.6 0.47-13 1.4∥c
1.5⊥c34 (1.064 µm, 15 ns) AgGaSe2 d36=39.5 0.76-18 1.0∥c
1.1⊥c13 (2.0 µm, 30 ns) LiGaS2 d31=5.8 0.32-11.6 6~8 >240 (1.064 µm, 14 ns) LiInSe2 d31=11.78 0.46-14 6.74∥x
8.54⊥z40 (1.064 µm, 10 ns) CdSe d31=18 0.75-25 6.9∥c
6.2⊥c56 (2.09 µm, 46 ns) GaSe d22=54 0.62-20 16.2 30 (1.064 µm, 10 ns) CdSiP2 d36=84.5 0.52-9 13.6 41 (1.064 µm, 8 ns) OP-GaP d14=70.6 0.5-12 110 >104 (2.09 µm, 12 ns) OP-GaAs d14=94 0.86-18 55 >38 (2.09 µm, 50 ns) Table 1. Physical properties of some mid/long-wave infrared nonlinear crystals
Research progress of 2 µm Ho single-doped solid laser and application of ZnGeP2 on middle-long-wave infrared (Invited)
doi: 10.3788/IRLA20201056
- Received Date: 2020-09-13
- Rev Recd Date: 2020-10-27
- Available Online: 2021-01-14
- Publish Date: 2020-12-24
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Key words:
- 2 µm /
- middle-long-wave infrared /
- nonlinear optics /
- solid laser
Abstract: 2 µm, 3-5 µm and 8-12 µm infrared lasers are located in the atmospheric transmission window, which have broad applications in laser medicine, laser imaging, environmental monitoring, lidar, chemical remote sensing and infrared countermeasure. Based on the optical nonlinear frequency conversion technology and nonlinear optical crystals, it has obvious advantages in achieving middle-long-wave infrared solid lasers, such as compact and simple structure, wide tunable wavelength range and high output power. Using ZnGeP2 crystal with 2 µm Ho single-doped solid laser as pump especially has an outstanding performance in middle-long-wave infrared field. In the aspect of average output power, it has reached the level of 102 W@3-5 µm, 12.6 W@8.2 µm and 3.5 W@9.8 µm. Moreover, they all have a beam quality factor M2 less than 3 and the corresponding optical-to-optical conversion efficiency of 3-5 µm is about 60%. This paper reviewed the research progress of 2 µm Ho single-doped solid laser and application of ZnGeP2 on middle-long-wave infrared in detail.