kHz、mJ量级的可调谐MgO∶PPLN中红外光参量振荡器
Nanosecond mid-infrared lasers with central wavelengths of 3?5 μm are used in various applications, such as environmental monitoring, LIDAR , and electro-optical countermeasures, owing to the high atmospheric transmission at this band. Thus, developing high-energy mid-infrared laser sources with high pulse repetition frequencies is crucial for practical applications because such lasers exhibit superior jamming effects and can effectively target an opponent’s anti-jamming mechanism. An optical parametric oscillator (OPO) is suitable for efficiently realizing a tunable, high-pulse-repetition-frequency laser owing to its compactness, wide tuning range, and high conversion efficiency. The output characteristics of an OPO are primarily determined by the optical properties of the constituent nonlinear crystals. Among the various nonlinear crystals used in OPOs, periodically poled magnesium-oxide-doped lithium niobate (MgO∶PPLN) has attracted considerable attention because of its large effective nonlinear coefficient, high damage threshold, and flexible phase matching ability. However, to date, a pulse energy of the order of hundred microjoules has been realized for PPLN-OPOs with high pulse-repetition frequencies. In this study, a kilohertz- and millijoule-level mid-infrared OPO based on an MgO∶PPLN crystal is developed. The OPO, which is pumped by a 1064-nm nanosecond pulsed laser based on multi-period MgO∶PPLN with a double-pass single-resonance structure and flat-concave cavity, delivers a high output energy of up to 1.041 mJ at 4.08 μm with a pulse repetition frequency of 1 kHz. Subsequently, the output energy of the high-pulse-repetition-frequency mid-infrared OPO based on a PPLN crystal is increased to mJ range, which is suitable for mid-infrared electro-optical countermeasure applications.
The output energy of a mid-infrared laser with a high pulse-repetition frequency is significantly increased. First, the OPO is pumped by a high-performance, high-pulse-repetition-frequency 1064-nm laser with a double-pass single-resonance structure, which improves the utilization of the pump light and reduces the pumping threshold. Second, we evaluate the output characteristics for different OPO cavity types and compare them with the theoretical calculation results to determine the optimal conversion efficiency under these pumping conditions. With the combined periodic and temperature modulations, the OPO maintains a flat and high output energy over a wide range of 3.49?4.48 μm, wherein the energy exceeds 0.9 mJ.
In the experiment, a pump power higher than 6.2 W easily damages the surface of the crystal, indicating that maximizing the pump power to 6.2 W will result in a high output energy. We determine the relationship between conversion efficiency and suprathreshold multiplicity. The maximum conversion efficiency is obtained when the suprathreshold multiplier reaches 6.6 (Fig.3). Further, the pumping threshold changes significantly with changes in the cavity type because a flat-concave cavity improves the mode matching of the three interacting waves in the OPO cavity, resulting in a low OPO threshold. However, as the radius of curvature (R) decreases, the input mirror affects the pump light dispersion, which in turn reduces the intracavity pump power density, resulting in a decrease in the conversion efficiency of the parametric process. Figure 4(b) shows that using a flat concave mirror with R=300 mm as the input mirror results in a low threshold of 0.9 W. In our case, the suprathreshold multiplier is the closest to the optimum value of 6.6, and thus, the highest output power is obtained for this cavity. Consequently, we maximize the use of pumping energy below the damage threshold, thereby further increasing the output energy.
We demonstrate tunable mid-infrared lasers based on MgO∶PPLN crystals with a pulse repetition frequency of the order of kilohertz and an output energy of the order of millijoules. At a pump power of 6.2 W, the single-pulse energy of the 4.08-μm mid-infrared laser is 1.041 mJ with a pulse repetition frequency of 1 kHz. The optical-to-optical conversion and slant efficiencies are 16.8% and 19.3%, respectively, and the mid-infrared laser pulse width is approximately 9.53 ns. The fluctuation in the average power measured within 30 min is 0.24% (root mean square, RMS). The final output is highly stable, and the observed small fluctuations are caused by the drifts in the central wavelength of the laser diode. Combined with periodic and temperature modulations, the OPO delivers a flat and high output energy signal over a wide range of 3.49?4.48 μm. The tunable range covers 3.49?4.48 μm within the polarization period range of 27.5?29.6 μm at 25?200 ℃.
1 引言
3~5 μm的中红外波段是重要的大气窗口,具有良好的大气穿透特性,适用于远距离传输,已经被广泛应用于大气遥感、环境监测、激光雷达和光电对抗等领域[1-3]。此外,中红外波段包含丰富的分子官能团的吸收峰,可用于生物医学领域的分子特异性识别[4]。3~5 μm波段的高重复频率激光源是3~5 μm光源发展的重要方向之一,在光电对抗领域,高重复频率激光器在短时间内能够发射多脉冲激光,这一特点提高了对探测器实施干扰的成功概率[5]。同时,纳秒量级的中红外激光的单脉冲能量越高,其致盲、致昡的效果越好。因此,研制3~5 μm可调谐的高重复频率、大能量的中红外激光器具有现实意义。
非线性光学频率变换技术是实现可调谐、高重复频率中红外输出的有效方式,差频产生(DFG)、光学参量放大器(OPA)、光学参量振荡器(OPO)等是非线性光学频率变换中实现中红外输出的最常见方式[6-9]。其中,OPO具有结构紧凑、调谐范围宽、转换效率高的优势,所以OPO已经被广泛用于宽调谐中红外激光产生[10]。OPO的输出特性在很大程度上由非线性晶体的光学特性决定,常用的中红外非线性晶体主要有磷锗锌(
基于PPLN晶体的高重复频率中红外激光器的研究开展较早。2007年,Tillman等[13]采用重复频率为1 kHz、脉宽为3 ns的1047 nm激光器泵浦MgO∶PPLN-OPO,得到了24 mW的3.226 μm闲频光输出,单脉冲能量为24 μJ。2010年,Wu等[14]设计了双通单谐振半外腔的MgO∶PPLN-OPO结构,有效缓解了非线性晶体的热效应,采用重复频率为52 kHz、平均功率为48.2 W的1064 nm泵浦源,在3.82 μm处得到了9.23 W的闲频光输出,单脉冲能量提升至177 μJ。2015年,李海速等[15]采用主振荡功率放大器(MOPA)结构的重复频率为10 kHz的大功率1064 nm激光器泵浦MgO∶PPLN-OPO,在3.81 μm处得到了5.4 W的闲频光输出,单脉冲能量为0.54 mJ。2019年,Xu等[16]设计了准连续激光器泵浦的基于MgO∶PPLN晶体的MOPA,实现了重复频率为2 kHz、平均功率为966 mW的3.73 μm闲频光激光输出,单脉冲能量为483 μJ。目前基于PPLN晶体的纳秒OPO同时实现高能量、高重复频率中红外激光输出还具有一定难度,现有基于PPLN晶体的kHz量级高重复频率中红外激光器输出的单脉冲能量多为百μJ量级,通过优化泵浦源以及优化OPO结构,能够进一步提升中红外输出能量。
本文采用实验室自制的脉冲能量为10 mJ、脉宽为12 ns、重复频率为1 kHz的高峰值功率的1064 nm激光器泵浦基于MgO∶PPLN的OPO,近MW量级峰值功率的输出泵浦光有利于提高OPO输出的转换效率。实验实现了单脉冲能量mJ量级的高能量、高功率中红外激光输出。采用泵浦双通单谐振的OPO结构提高泵浦利用率、降低阈值,并通过OPO腔型优化,最终实现了输出能量和转换效率的提升。当泵浦光功率为6.2 W时,在4.08 μm处得到了重复频率为1 kHz,单脉冲能量为1.041 mJ的闲频光输出。此时的光-光转换效率达到了16.8%,斜效率为19.3%。中红外激光的脉宽为9.53 ns,对应的峰值功率为109.2 kW,通过设计晶体极化周期实现了调谐范围覆盖3.49~4.48 μm的可调谐中红外输出。脉宽中红外激光水平方向的光束质量因子(
2 实验装置
实验采用自制的重复频率为1 kHz的纳秒量级1064 nm激光器作为泵浦源,泵浦基于MgO∶PPLN晶体的双通单谐振OPO结构,实验装置如
MgO∶PPLN晶体尺寸为24 mm×10 mm×2 mm,掺杂MgO的摩尔分数为5%,晶体两端均镀有增透膜(1064 nm、1300~1600 nm和3500~4500 nm增透),晶体具有5个极化周期(27.5、27.8、28.4、29.0、29.6 μm)。OPO谐振腔选用平凹腔型,腔长60 mm,前腔镜M1为平凹镜,曲率半径为300 mm,表面镀有1064 nm泵浦光增透、1300~1600 nm信号光以及3000~5000 nm闲频光增反的介质膜。后腔镜M2镀有1064 nm泵浦光增反、1300~1600 nm信号光增反以及3000~5000 nm闲频光增透的介质膜。为避免漏出的泵浦光以及少部分透过的信号光对输出功率测量造成干扰,采用镀有1064 nm介质膜(1300~1600 nm全反,3000~5000 nm透过)的滤光片进行滤光。
3 实验结果与讨论
3.1 OPO调谐特性
实验中采用极化周期调谐结合温度调谐的方式实现可调谐输出,晶体放置在温度调节范围为25~200 ℃、调节精度为±0.1 ℃的温控炉内,晶体的5个极化周期为27.5、27.8、28.4、29.0 、29.6 μm。利用三波耦合方程对极化周期-温度调谐波长范围进行理论模拟。在光学参量振荡过程中,三波需要满足能量守恒和动量守恒:
式中:
式中:
Sellmeier方程的各项参数如
表 1. Sellmeier方程的参数
Table 1. Parameter values in Sellmeier equation
|
图 2. OPO的调谐特性。(a)信号光光谱;(b)闲频光调谐波长范围
Fig. 2. Tuning characteristics of OPO. (a) Spectra of signal light; (b) tuning wavelength range of idler light
3.2 OPO输出特性
在实验过程中,采用功率计测量激光平均功率。当泵浦光功率增加到高于6.2 W时,晶体表面会发生损伤现象,因此实验中将最高泵浦光功率固定为6.2 W,并通过调节半波片控制注入OPO的泵浦光功率。当利用高斯脉冲泵浦时,将高斯光束光场横截面分成若干等光强的均匀平面波(高斯光束光斑半径为x),并利用积分表示,此时高斯光束的转换效率[19]为
式中:
图 4. 闲频光的输出功率。(a)不同腔型的闲频光输出功率;(b) 4.08 μm闲频光的输出特性
Fig. 4. Output powers of idler light. (a) Output powers of idler light for different cavity types; (b) output characteristic of 4.08 μm idler light
在120 ℃温度、28.4 μm极化周期下,闲频光的中心波长为4.08 μm,此时在6.2 W的泵浦功率下得到了1.041 W的最高输出功率,单脉冲能量为1.041 mJ,光-光转换效率达到16.8%,斜效率为19.3%。实验采用响应时间为2 ns的中红外二极管探测器测量了闲频光输出的时域波形,采用示波器采集并显示,得到的结果如
图 5. 闲频光的时域特性。(a)闲频光的时域序列;(b)闲频光波形
Fig. 5. Time-domain characteristics of idler light. (a) Time-domain sequence of idler light; (b) pulse waveform of idler light
通过温度-极化周期结合的调谐方式,测量并绘制调谐输出曲线,如
实验中采用84/16刀口法分别测量了泵浦光、闲频光的光斑分布,如
图 7. 光斑分布。(a)泵浦光光斑分布;(b)闲频光光斑分布
Fig. 7. Spot distributions. (a) Spot distribution of pump light; (b) spot distribution of idler light
4 结论
报道了一种基于MgO∶PPLN晶体的kHz、mJ量级的可调谐中红外光参量振荡器。OPO的泵浦源采用实验室自制的重复频率为1 kHz、脉宽为~10 ns的1064 nm高性能激光器。采用极化周期调谐和温度调谐方式实现了覆盖3.49~4.48 μm范围的可调谐中红外输出。采用泵浦双通单谐振的OPO结构提高泵浦光利用率,降低阈值,通过优化OPO腔型,采用曲率半径R=300 mm的平凹镜作为输入镜,实现了输出能量和转换效率的提升。当泵浦光功率为6.2 W时,在4.08 μm处得到了最高输出功率1.041 W,单脉冲能量为1.041 mJ,脉宽为9.53 ns,对应峰值功率为109.2 kW,光-光转换效率达到16.8%,根据结果估计在3.49~4.18 μm范围内都能够保持~100 kW的连续调谐、高峰值功率输出。闲频光输出的光束质量因子分别为Mx2=8.92,My2=7.89。保持最高功率输出状态,中红外激光器运行30 min的输出功率RMS为0.24%。报道的基于MgO∶PPLN的OPO实现了kHz、mJ量级的可调谐中红外输出,同时能够长时间稳定输出,具有应用于光电对抗等领域的潜力。
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Article Outline
刘帅, 陈锴, 孙悦, 闫超, 乔鸿展, 常继英, 李吉宁, 王与烨, 钟凯, 徐德刚, 姚建铨. kHz、mJ量级的可调谐MgO∶PPLN中红外光参量振荡器[J]. 中国激光, 2024, 51(7): 0701019. Shuai Liu, Kai Chen, Yue Sun, Chao Yan, Hongzhan Qiao, Jiying Chang, Jining Li, Yuye Wang, Kai Zhong, Degang Xu, Jianquan Yao. kHz, mJ Level Tunable MgO∶PPLN Mid‐Infrared Optical Parametric Oscillator[J]. Chinese Journal of Lasers, 2024, 51(7): 0701019.