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为了得到高能量的倍频激光脉冲,笔者在不同的驱动电流下,对LD侧泵浦模块的激光放大能力进行理论分析。将一个能量为10.68 mJ的小信号通过LD泵浦模块进行放大,通过输出能量与输入能量的比值得到小信号增益G0,其小信号单程增益表达式为
${G}_{0}={\rm exp}({g}_{0}\ell)$ 。当小信号注入时,输出激光能量随LD泵浦模块驱动电流变化曲线和LD模块的小信号增益(SSG)曲线如图3所示。图 3 输出激光能量随LD泵浦模块驱动电流变化曲线和LD模块的小信号增益(SSG)曲线
Figure 3. Change curves of output laser energy with the driving current of the LD pump module and the small signal gain (SSG)
通过半导体泵浦模块对激光介质进行泵浦时,增益介质的激光上能级储存较高的粒子能量。同时激光通过放大器,增益介质的激光上能级粒子产生受激辐射跃迁,辐射出能量为hν的光子,从而实现输出激光能量放大。激光器采用尺寸为 Φ18 mm×146 mm的Nd: YAG晶体作为激光增益介质,半导体激光放大器的储能密度[17]为:
$$ {E}_{\mathrm{s}\mathrm{t}}=\gamma {{g}}_{0}{E}_{\mathrm{s}} $$ (1) 式中:Est为增益介质单位体积内储存的能量;Nd: YAG晶体属于四能级系统,反转减少因子γ=1;Es为饱和增益密度,其表达式为:
$$ {E}_{\mathrm{s}}=h\nu /\sigma $$ (2) 式中:σ为Nd: YAG晶体的受激发射截面,σ=4.58×10−19 cm−2[18-19];普朗克常数h=6.626×10−34 J·s;ν为泵浦光的频率,其表达式:
$\mathrm{\nu }={c}/{\mathrm{\lambda }}$ ,其中光速c=2.998×108 m·s−1,LD侧泵浦模块辐射的泵浦光波长λ为803 nm。在不同LD泵浦模块的驱动电流下,增益介质的激光上能级粒子储能不同。对于体积为V的增益介质,其储能E的表达式为:
$$ E={E}_{\mathrm{s}\mathrm{t}}\times V $$ (3) 由公式(1)~(3)和小信号单程增益表达式可以得到增益介质储能E。增益介质储能随LD侧泵浦模块驱动电流变化曲线如图4(a)所示。激光通过放大器时将增益介质储能转化为激光输出能量,该能量随着LD侧泵浦模块驱动电流的增大而增大。
图 4 (a)增益介质储能E随LD泵浦模块驱动电流变化;(b)输入激光脉冲为3.5 J时,增益介质增益G随LD泵浦模块驱动电流的变化
Figure 4. (a) Energy storage E of gain medium varies with the driving current of LD pump module; (b) G of the gain medium varies with the driving current of the LD pump module at input laser pulse of 3.5 J
介质增益G与输入脉冲能量密度Ein、饱和能量密度Es和小信号单程增益G0的关系如下[20]:
$$ G=\frac{{E}_{\mathrm{s}}}{{E}_{\mathrm{i}\mathrm{n}}}{\rm ln}\left\{1+\left[{\rm exp}\left(\frac{{E}_{\mathrm{i}\mathrm{n}}}{{E}_{\mathrm{s}}}\right)-1\right]{G}_{0}\right\} $$ (4) 当输入激光脉冲能量为3.5 J时,增益介质增益G随LD侧泵浦模块驱动电流的变化如图4(b)所示。通过理论计算,两个LD侧泵浦模块对激光能量进行放大,驱动电流为66 A,输入激光脉冲能量为3.5 J,输出激光能量为5.68 J。采用的驱动电流为66 A,重复频率100 Hz时,实际测得输出激光能量为5.5 J@1064 nm,与理论计算结果相符。
当激光重复频率为100 Hz时,激光器输出基频光和倍频光的脉冲宽度波形如图5所示,激光脉冲宽度分别为11.90 ns和9.92 ns。如图5所示的脉宽波形图中,中间附近有两个尖峰,是由于激光起振后激光纵模调制导致的,可通过对谐振腔注入单纵模种子源进行改善。在EAST TS系统中,采集和探测系统的探测精度只有纳秒量级,此激光的波形对亚纳秒级的测量有影响,故不影响目前对等离子体电子温度和密度的诊断。
图 5 激光器出口处输出基频光(a)和倍频光(b)的脉冲波形图
Figure 5. Pulse waveform of the laser output fundamental frequency light (a) and multiple frequency light (b) at the laser exit
如图6所示,基频光与倍频光的光强分布(二维、三维),激光器的激光光斑模式为VRM模近平顶分布。由于Nd: YAG 晶体棒对激光的衍射,图6所示的二维激光脉冲光强分布中存在衍射环,激光倍频效率与脉冲的能量密度成正比,倍频光的衍射环更加明显。激光器出口处基频光的光斑直径为14.51 mm,几何发散角为0.341 mrad。倍频光的光斑直径为18.71 mm,几何发散角为0.219 mrad。
Design of high-energy and high-frequency Nd: YAG laser used in EAST TS diagnostic
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摘要: 设计了一台可输出1064 nm和532 nm激光脉冲的高能高频激光器,分别用于EAST汤姆逊散射诊断系统对芯部区域和边界区域等离子体电子温度和密度的诊断。该激光器采用电光调Q、卡塞格林非稳腔以及氙灯泵浦脉冲放大器实现频率为100 Hz的3.5 J@1064 nm激光输出。通过两级半导体侧泵浦模块对基频光能量放大,输出激光能量5.5 J@1 064 nm。通过理论计算和分析,确定泵浦模块的放大能力,并与实验结果进行对照。采用LBO晶体对基频光进行倍频,输出能量为3 J@532 nm的脉冲激光,倍频效率为55%。输出基频光光斑直径约为14.51 mm,脉冲宽度11.90 ns,倍频光光斑直径约为17.81 mm,脉冲宽度9.92 ns,激光脉冲呈超高斯平顶分布。重复频率从1~100 Hz可调,汤姆逊散射诊断的空间分辨率达10 ms,为芯部和边界输运垒等微观物理问题的研究提供了条件。Abstract: A high-energy and high-frequency laser with 1 064 nm and 532 nm laser pulses is designed to diagnose the plasma electron temperature and density in the core and boundary regions of EAST Thomson scattering diagnostic system, respectively. We use electro-optic Q-switching, Cassegrain unstable cavity and xenon lamp pumped pulse amplifier to output a frequency of 100 Hz and an energy of 3.5 J@1064 nm laser pulse. The fundamental frequency optical energy is amplified by two-stage semiconductor side pump module to output laser energy of 5.5 J@1064 nm. Through theoretical calculation and analysis, the amplification capacity of the pump module is determined and compared with the experimental results. LBO crystal is used to double the fundamental frequency light, and the output energy is 3 J@532 nm pulse laser, the frequency doubling efficiency is 55%. The diameter of the output fundamental frequency light spot is about 14.51 mm, the pulse width is 11.90 ns, the diameter of the frequency doubling light spot is about 17.81 mm, the pulse width is 9.92 ns, and the laser pulse is Gaussian flat top distribution. The repetition rate is adjustable from 1 Hz to 100 Hz, and the spatial resolution of Thomson scattering diagnostic is up to 10 ms, which provides conditions for the study of microphysical problems such as core and boundary transport barrier.
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Key words:
- solid-state laser /
- Thomson scattering diagnosis /
- Nd: YAG /
- frequency doubling
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