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根据Byer等人针对泵浦光、参量光在空域以及时域上呈现高斯型的泵浦单谐振光学参量振荡器(SRO)提出的理论模型,可计算单谐振OPO的阈值[16]:
$$ {J_{th}} = \frac{{2.25\tau }}{{k{g_{sj}}l_{eff}^2}}{\left[ {\frac{L}{{2\tau c}}\ln \left( {\frac{{{P_{{n}}}}}{{{P_{{\textit{0}}}}}}} \right) + 2\alpha l + \ln \left( {\frac{1}{{\sqrt R }}} \right) + \ln 2} \right]^2} $$ (1) 式中:耦合常数
$k = \dfrac{{2{\omega _{sj}}{\omega _{ij}}d_{Qj}^2}}{{{n_p}{n_{sj}}{n_{ij}}{c^3}{\varepsilon _0}}}$ ;耦合系数${g_{sj}} = \dfrac{{w_p^2}}{{w_{{sj}}^2 + w_p^2}}$ ;wp、wsj表示泵浦光、信号光的所对应光斑的半径;τ表示入射光脉宽;准相位匹配时相互作用长度leff近似等于MgO:PPLN具体总长;L为谐振腔总长;Pn、P0是信号光及其要发生噪声的极值,同时彼此具有${{{P_n}} \mathord{\left/ {\vphantom {{{P_n}} {{P_\textit{0}}}}} \right. } {{P_{\textit{0}}}}} \leqslant 1{0^{14}}$ ;α为晶体吸收信号光时所对应的系数;R表示输出镜在谐振信号光波段的反射率。从公式(1)可知,泵浦光斑和参量光斑的差距越小模式匹配越好,以此来优化OPO阈值,图1为聚焦镜F到泵浦源距离分别为125、145、160 mm时,总腔长为80 mm情况下,模拟出聚焦镜在四个不同位置时,参量光的光束变化情况。由于聚焦镜F位置距离输入镜的长度逐渐变小,OPO腔中光斑的匹配效果也随之变差,将导致效率极大的下降,所以通过模拟分析可知,当聚焦透镜F与光纤激光器距离为125 mm时,OPO腔内光斑模式匹配效果最好。
图2为宽线宽光纤激光器和窄线宽光纤激光器输出高斯光束示意图,输出高斯光束中心波长为1064 nm,阴影部分为靠近中心波长的其他波长,谱线较宽的情况下其他波长占用了1064 nm贡献了较大一部分功率,也就是1064 nm的总功率,致使1064 nm泵浦光泵浦OPO达到阈值时功率提高,所以使用具有窄线宽的光纤激光器泵浦MgO:PPLN,可以有效增强相位匹配的效果,从而提升参量光的转换效率。而且当闲频光的实际波长变大时,泵浦光的线宽展宽对参量光线宽影响呈正比,所以采用窄线宽光纤激光器泵浦光学参量振荡器有实质性意义。
Study on mid-infrared optical parametric oscillator based on MgO:PPLN pumped by narrow linewidth 1064 nm fiber laser
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摘要: 提出了一种基于1064 nm掺镱光纤激光器泵浦MgO:PPLN的3.83 μm中红外光学参量振荡器。基于单谐振光学参量振荡器的阈值理论和线宽压窄前后的光束能量集中性理论,分析了不同泵浦光束聚焦深度下,谐振腔内光束分布情况以及线宽调制前后能量的不同集中程度对阈值和光-光转换效率所产生的影响。通过采用单个光纤布拉格光栅的方式压窄了泵浦光线宽,对比分析了在不同占空比下,泵浦光线宽压窄前后对中红外光学参量振荡器输出特性的影响。当泵浦功率为18 W,脉冲激光占空比为0.2%,脉宽为100 ns,泵浦光线宽为2.5 nm时,MgO:PPLN中红外光学参量振荡器获得功率为1.42 W的3.83 μm激光输出,光-光转换效率为7.9%。将线宽压窄到0.1 nm后,脉宽为2 ns,MgO:PPLN中红外光学参量振荡器获得最高功率为1.98 W的3.83 μm激光输出,光-光转换效率为11%,光束质量M2=1.89;同时相比于线宽压窄前激光输出效率提高了39.2%。Abstract: A 3.83 μm mid-infrared optical parametric oscillator based on 1064 nm Yb-doped fiber laser pumped MgO:PPLN is proposed. Based on the threshold theory of single resonant optical parametric oscillator and the beam energy concentration theory before and after linewidth narrowing, the effects of the beam distribution in the resonant cavity and the different energy concentration levels before and after linewidth modulation on the threshold and optical-optical conversion efficiency under different pump beam focusing depths are analyzed. By using a single fiber Bragg grating to narrow the pump light width, the influence of the pump light width on the output characteristics of the mid-infrared optical parametric oscillator is analyzed under different duty ratios. When the pump power is 18 W, the pulse laser duty cycle is 0.2%, the pulse width is 100 ns, and the pump light width is 2.5 nm, the MgO:PPLN mid-infrared optical parametric oscillator obtains 3.83 μm laser output with a power of 1.42 W, and the optical-optical conversion efficiency is 7.9%. When the linewidth is narrowed to 0.1 nm and the pulse width is 2 ns, the infrared optical parametric oscillator in MgO:PPLN achieves 3.83 μm laser output with the highest power of 1.98 W. The optical-optical conversion efficiency is 11% and the beam quality M2=1.89. At the same time, the laser output efficiency is increased by 39.2% compared with that before linewidth narrowing.
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Key words:
- optical parametric oscillatoin /
- narrow linewidth /
- fiber laser /
- mid-infrared /
- duty cycle
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