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非链式脉冲DF激光器脉冲能量受工作气体气压、气体配比和电压等因素影响。在前期自引发放电的工作基础,文中选取总气压为8 kPa,混合气体分压比SF6∶D2=10∶1,研究了轴流DF激光器的放电特性,在充电电压在16~20 kV的范围内均可得到稳定的辉光放电,采用Nikon公司的COOLPIX相机拍摄的放电照片如图2所示。
采用Gentec-eo公司的QE50LP-H-MB型能量计对输出的单脉冲能量进行测试,获得的脉冲能量与电光转换效率随充电电压的变化关系如图3所示。首先单脉冲输出能量随充电电压呈近似线性上升,在充电电压20 kV时,获得的最大单脉冲输出能量800 mJ。电光转换效率随充电电压的升高呈明显的先上升后下降趋势,且在19 kV时,电光效率达到3.4%,与能量随充电电压变化趋势相符。高电压时过大的注入能量引起的工作气体过度电离导致的强烈的消激发及过高的温升可能是电光转换效率下降的主要原因。文中的电光转换效率高于之前报道的3.12%[10]。
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非链式脉冲DF化学激光器的工作物质一般为SF6和D2或碳氘化合物的混合物。其工作机理为:主电极被放电击穿后,电极间的高能电子(电子能量大于5 ev)碰撞六氟化硫产出F原子,化学活性极强的F原子与D2发生化学反应生成激发态的DF分子,进而受激跃迁形成震荡输出。由上述机理可知:工作物质的消耗是不可逆的,且放电产物的消激发作用严重影响电光转换效率。因此如何快速更新工作气体是实现轴流非链式脉冲DF激光器重复频率运转的核心问题。
放电区风速直接决定了工作气体更新速度,放电区清洗系数是表征工作气体更新能力的量化参数,它由下式给出:
$$ k = \upsilon \cdot S/fV $$ (1) 式中:υ为流经放电区的风速;S为放电区截面积;f为激光器重复频率;V为放电区体积。放电区的风速υ可采用多普勒测试仪或皮托管等设备测试,但测试过程较为复杂。文中依据风机额定流量(0.016 m3/s)对风速进行估算,在激光器重复频率20 Hz时,计算的清洗系数约为1.5,它表征在两次放电时间隙有1.5倍的放电气体被抽走,保障了每次放电之前工作区气体的更新。总气压为8 kPa,混合气体分压比SF6∶D2=10∶1,充电电压为19 kV条件下,开展轴流非链式脉冲DF激光器重复频率运转,重复频率20 Hz时,采用Gentec-eo公司的UP55N-300F-H12型功率计对输出的平均功率进行测试,获得的激光器平均功率为13.1 W,如图4所示。受风机性能影响,未开展更高重复频率运转实验。
在20 Hz时,采用Vigo公司的PVM-10.6-T08型HgCdTe光电探测器测试的激光脉冲波形如图5所示。图5(a)为重复频率激光波形,激光脉冲能量存在随机的波动,但整体稳定性较好,没有脉冲丢失现象,激光脉冲幅值差优于±5%。放电区工作气体涡流引起的非均匀放电及折射率变化,吸附于器壁的残余放电产物的消极发作用等因素是导致脉冲能量抖动的主要因素。图5(b)为单个脉冲的波形,其全波半高宽约为120 ns,由此估算该轴流DF激光器输出脉冲峰值功率为6.67 MW。
Miniaturized axial flow non-chain pulsed deuterium fluoride laser
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摘要: 采用轴流循环流动方式更新非链式脉冲氟化氘(DF)激光器工作介质,搭建了一台小型化自引发放电DF激光器实验装置,开展了轴流DF激光器输出性能实验研究。单脉冲工作时,在工作气体配比SF6∶D2=10∶1,总气压8 kPa时,实现单脉冲能量800 mJ激光输出,全波半高宽约120 ns,其性能与横流放电非链式脉冲DF激光器相似。重复频率放电时,实现了DF激光器重复频率20 Hz稳定运转,得到的最大输出功率为13.1 W,重频脉冲幅值差优于±5%,并展望了轴流式DF激光器高重频工作的前景。文中提出的轴流式非链式脉冲DF激光器为小型化、工程化中红外光源提供了新的技术途径。Abstract: A compact non-chain pulse deuterium fluoride (DF) laser renewing the working gas by axial-flow has been established. The characteristics of miniaturized axial flow non-chain pulse DF laser were studied experimentally. When the ratio of working gas SF6∶D2 = 10∶1 and total pressure=8 kP, the output energy of DF laser was about 800 mJ and full width at half maximum was about 120 ns at single pulse mode, which was similar to transverse flow DF laser. When working at repetition rate of 20 Hz, a maximum output power of 13.1 W was reached in this laser, whose amplitude difference of laser pulses was less than 5%. Then, the possibility of high repetition rate operation of axial flow DF laser was prospected. An axial flow non-chain pulse DF laser was proposed in this paper, which provides a new technology approach for the miniaturization and engineering application of the mid-infrared laser.
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Key words:
- non-chain DF laser /
- axial-flow /
- discharge bump /
- repetitively working /
- miniaturization
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