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摘要: 为了控制重频放大器的热致波前畸变，设计并加工了均匀冷却的背面水冷激活镜激光放大器，对放大器的热畸变特性开展了实验研究，实验发现在泵浦功率密度较高即重复频率达到10 Hz，平均功率密度达到200 W/cm2时，放大器的热畸变既影响远场分布又对近场产生显著的调制。近场的调制会给放大器带来较大的损伤风险。为了消除热畸变对近场的调制，首先对泵浦强度分布进行了匀化，然后对介质进行了边缘热平衡控制，消除了热畸变引起的近场调制。通过对上述因素的控制，采用水冷激活镜构型的四程放大器实现了在10 Hz频率下良好运行。在没有进行主动补偿的情况下，实现了远场焦斑优于5倍衍射极限的输出。Abstract: To control the thermal wavefront distortion of repetition frequency laser, we′ve developed a water-cooled active-mirror laser amplifier, which was uniformly cooled from the rear of crystal disk. The numerical anslysis and experimental study on the characterstics of the amplifier’s thermal distortion were carried out. It was found that the thermal distorition devoted a significiant modulation to the near field of the laser when the average pump power density was as high as 200 W/cm2 with the operation frequency of 10 Hz. Near-field modulation would bring a risk to damage the amplifier. To eliminate the modulation of thermal distortion in the near field, two approaches were taken. Firstly, the pump intensity distribution was homogenized, then the edge thermal balance control was carried out. The near field modulation from thermal wavefront distortion was eliminated by these means, a four-pass amplifier with water-cooled laser heads ran well at 10 Hz. The focal spot of output laser was smaller than 5 diffraction limits without any compensation.
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Key words:
- laser amplifiers /
- diode-pumped lasers /
- thermal effects /
- water-cooled active mirror
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Figure 1. Diagrammatic sketch of the laser head and mechanical structure of the water viscolizer
Figure 4. (a) near-field distribution of the pump, (b) near field of output laser with 1 Hz pumping and (c) near field of output laser with 10 Hz pumping
Figure 5. (a) near-field distribution of the pump after optimization and (b) near field of output laser
Figure 6. Pumping distribution and near field after one-pass amplification for a ϕ35 mm disk
Figure 7. (a) Cr4+:YAG ceramic cladded Yb:YAG crystal, (b) near field after four-pass amplification
Table 1. Diode pumped active mirror lasers of several institutions
institution pump power denisity/(W·cm−2) energy of single laser pulse/ J output frequency/ Hz cooling condition LULI, France 32 14 2 room temperature 55 30 10 cryogenic temperature Osaka University, Japan[8] 55 1 100 cryogenic temperature Industrial Development Center,Japan[9] 37.5 11.5 10 cryogenic temperature Colorado State University, USA[10] 504 1.5 500 cryogenic temperature Tsinghua University, China[11] 12 12 10 room temperature Laser Fusion Research Center
of CAEP, China20 8.5[12] 1 room temperature 
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