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1064 nm基频光的性能对OPO输出有重要影响。实验中采用泰克DPO4104 B示波器及DET10光电探测器、分别测量了30~50 kHz基频光的重复频率与脉冲宽度,其中30 kHz和50 kHz重复频率和单脉冲波形测试结果如图2所示。
图2所示的基频光的脉宽和重复频率测试结果表明,1064 nm基频光在30 kHz重复频率下输出脉冲宽度为3.2 ns,相比于50 kHz重复频率下的3.9 ns更窄。
采用Ophir-sp620型光束质量分析仪测试了基频光在30 kHz输出时的光束质量,测试结果如图3所示。
$ M_x^2 $ =1.62$ M_y^2 $ =1.53。 -
采用上述1064 nm 激光器泵浦MgO: PPLN光参量振荡器。将MgO: PPLN晶体温度控制在80 ℃,闲频光输出功率与基频光重复频率及功率的关系如图4(a)所示。当30 kHz的1064 nm激光抽运功率达到11.83 W时,得到输出功率1.744 W的3.93 μm闲频光。为防止光参量晶体的损伤,未进一步提高基频光功率。根据不同泵浦功率下对应的闲频光输出结果,计算了不同频率基频光与闲频光的功率转化曲线,计算结果如图4(b)所示。图中横坐标为基频光功率,纵坐标为泵浦光至闲频光的转换效率,当基频光功率接近12 W时,三种重复频率的转换效率都大于10%。当基频光功率相同时,30 kHz频率相对于40、50 kHz有更高的转换效率,最高光光转换效率14.71%。同等注入功率条件下,闲频光的输出功率和光-光转换效率随着重频增加而降低。
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实验中,通过示波器及VIGO中波光电探测器,对3.93 μm的闲频光在不同重复频率下的脉冲宽度进行了测试,测试结果如图5所示。
图 5 3.93 μm 闲频光在不同重复频率下的脉冲宽度
Figure 5. The pulse duration of 3.93 μm idler laser at different repetition rates
当激光器工作在30 kHz时,获得了脉冲宽度为1.197 ns的3.93 μm激光输出,相对于40 kHz和50 kHz工作状态下脉冲宽度更窄,主要是因为基频光在30 kHz时脉冲宽度更窄。闲频光相对于基频光脉冲宽度出现明显压缩,主要是因为光参量振荡器在参量光产生的过程中会产生晶体吸收损耗以及振荡腔的腔损耗,低能量的部分被抑制。
由以上实验可看出,同等注入功率条件下,基频光以30 kHz工作比40、50 kHz具有更高的转换效率,这主要是因为同等注入功率条件下,30 kHz的基频光具有更高的单脉冲能量和峰值功率。
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光参量转换过程需满足能量守恒和动量匹配条件,能量为hνp的基频光光子转化为能量为hvs的信号光光子和能量为hvi的闲频光光子[18],在这个过程中三波波长同时满足公式(1)和(2):
$$ \frac{1}{{{\lambda _p}}} = \frac{1}{{{\lambda _s}}} + \frac{1}{{{\lambda _i}}} $$ (1) $$ 2\pi \left[ {\frac{{{n_p}({\lambda _p},T)}}{{{\lambda _p}}} - \frac{{{n_s}({\lambda _s},T)}}{{{\lambda _s}}} - \frac{{{n_i}({\lambda _i},T)}}{{{\lambda _i}}} - \frac{1}{\varLambda }} \right] = \Delta {k_{opo}} $$ (2) 式中:p、s、i分别代表基频光、信号光和闲频光;λ为波长;n为折射率;Λ为晶体极化周期; Δkopo为最佳相位失配量,准相位匹配条件下Δkopo=0。通过以上两个方程式,可以看出当泵浦光波长一定时,改变温度可以实现闲频光和信号光波长的调谐。结合实际应用需求,实验中控制MgO: PPLN晶体温度在50~200 ℃温度变化,使用Aecopitx公司的CH-2000型中红外光谱仪测试了光参量振荡器的调谐波长,测试结果如图6所示。
图6 表明,当MgO: PPLN的温度在50~200 ℃范围改变时,其波长调谐范围为3770 ~3961 nm。进一步将MgO: PPLN OPO温度调谐实验测量结果与理论曲线的对比,对比结果如图7所示。光谱实验测量结果与理论曲线符合较好。
图 7 MgO: PPLN温度调谐理论曲线与实验结果对比
Figure 7. Comparison between experimental results and theoretical curve of temperature tuned MgO: PPLN
实验中采用刀口法测量了3.93 μm闲频光的光束质量,测试结果为
$ M_x^2 $ =2.12,$ M_y^2 $ =1.98。实验结果表明,经过OPO腔后,光束质量有一定程度的下降,在后续的研究中应优化OPO腔参数、增加选模措施,进一步提升光束质量。
High repetition rate, high peak power mid-infrared optical parametric oscillator based on MgO: PPLN
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摘要: 报道了一种基于周期性极化掺氧化镁铌酸锂晶体(MgO: PPLN)的高峰值功率、纳秒宽调谐中红外光参量振荡器(Optical parametric oscillation, OPO)。采用重复频率30~50 kHz、脉冲宽度小于4 ns的高光束质量1064 nm基频光泵浦基于极化周期为29 μm MgO: PPLN的OPO,当MgO: PPLN温度为80 ℃时获得30 kHz、脉宽1.19 ns、峰值功率48.45 kW的3.93 μm激光输出。在MgO: PPLN 温度区间为50~200 ℃时,中红外调谐输出波长为3.77~3.96 μm。根据实验结果分析讨论了不同重复频率下转换效率的特点、温度调谐特性与理论分析的一致性。
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关键词:
- 光参量振荡器 /
- 周期性极化掺氧化镁铌酸锂晶体 /
- 中红外激光器 /
- 高峰值功率
Abstract: A widely tunable mid-infrared optical parametric oscillator (OPO) with high peak power, nanosecond pulses output, which was based on MgO-doped periodically poled LiNbO3 (MgO: PPLN) was reported in this paper. The MgO: PPLN OPO with a domain period of 29 μm was pumped by a fundamental laser of 1064 nm with high beam quality, and the fundamental laser operated at the repetition rates from 30 kHz to 50 kHz with the pulses duration less than 4 ns. The 48.45 kW peak power, 1.19 ns, and 30 kHz pulses at 3.93 μm were obtained, as the temperature of MgO: PPLN was at 80 ℃. The tunable range of mid-infrared wavelength was from 3.77 μm to 3.96 μm, as the temperature of MgO: PPLN was from 50 ℃ to 200 ℃. According to the experimental results, the characteristics of conversion efficiency and temperature tuning characteristics under different repetition rates were analyzed and discussed. -
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