基于热效应的4F简并腔时空输出特性分析
Spatial coherence is one of the characteristics of traditional lasers. High spatial coherence enables good directivity in lasers; however, it also results in speckles. Thus, to suppress speckle noise, many methods have been developed to reduce the spatial coherence of lasers, including the mechanical vibration and photoelectric methods. Specifically, the mechanical vibration method is relatively slow and fails to achieve speckle suppression within short periods; alternatively, the photoelectric method suffers from problems pertaining to low threshold power and high cost. Notably, the laser with a degenerate cavity structure proposed by Arnaud in 1969 features low temporal and spatial coherence. Over recent years, factors influencing the spatio-temporal coherence characteristics of 4F degenerate cavity lasers have been studied; however, the influence of the thermal effect in the laser gain medium on the 4F degenerate cavity has been neglected. The thermal effect in solid-state lasers is a problem that cannot be ignored; this thermal lens effect of the laser gain medium degrades the perfect imaging characteristics of 4F degenerate cavity lasers and also alters the degenerate characteristics of the 4F resonant cavity. Accordingly, considering the thermal lens effect in the degenerate cavity gain medium, the influence of cavity length variations on the space – time output characteristics is analyzed and experimentally verified in this work. These results provide theoretical and experimental supports for research on low spacetime coherent lasers.
In this study, the transmission matrix and G-parameter equivalent cavity analysis methods are used for analyzing the spatio-temporal spectrum distribution characteristics of the beam in lasers considering the thermal lens effect in the degenerate cavity. The existence of the thermal lens, which degrade the degeneracy of the transverse mode spectrum in the degenerate cavity, is analyzed. In this experiment, a set of thermal lens measurement optical paths based on the 4F degenerate cavity laser is built. The focal length of the thermal lens, laser beat frequency signal, and maximum transverse mode order of the 4F degenerate cavity laser are measured. Additionally, considering a thermal lens in the cavity, the influence of cavity length variations on the maximum transverse mode order and the transverse mode beat frequency bandwidth in the cavity is theoretically calculated and experimentally verified.
A set of thermal lens measurement optical paths based on the 4F degenerate cavity laser is built in this experiment (Fig. 2). The influence of cavity length variations in the 4F degenerate cavity laser on the focal length of the thermal lens (Fig. 3), laser beat signal (Fig. 4), and maximum transverse mode order (Fig. 5) is studied. Experimental results reveal that the total spectral bandwidth of the degenerate laser cavity does not vary with the focal length and cavity length of the thermal lens. Further, as the cavity length of the thermal lens increases, the frequency interval of the transverse mode increases, the beat frequency bandwidth of the transverse mode fluctuates, and the maximum transverse mode order, Nmax, decreases significantly. Moreover, this study theoretically simulates the maximum transverse mode order with cavity length variations under the influence of a thermal lens, and the simulation results in the time domain are consistent with the experimental results.
Here, the influence of cavity length variations on the maximum transverse mode order and spectral structure of a degenerate laser cavity is studied considering the thermal lens effect in the laser gain medium. The existence of a thermal lens degrades the perfect imaging characteristics of the 4F degenerate cavity and the degenerate characteristics at the transverse and longitudinal mode frequencies. In this work, under the influence of a thermal lens, the beat frequency signal and maximum transverse mode order of a degenerate laser cavity are measured considering changes in the cavity length. Results indicate that the total spectral bandwidth of the degenerate laser cavity does not vary with the focal length and cavity length of the thermal lens. As the cavity length under the thermal lens increases, the transverse mode frequency interval increases and the transverse mode beat frequency bandwidth fluctuates. This suggests that changes in the cavity length affect the mode distribution in the spectrum. Further, experimental results for the maximum transverse mode order show that, owing to the existence of the thermal lens, a small distance of cavity mirror from the ideal position has a significant impact on the maximum transverse mode order. The maximum transverse mode order under the influence of a thermal lens is also theoretically simulated with changes in the cavity length. Notably, simulation results in the time domain are consistent with the experimental results, indicating that the theoretical model of the thermal lens has a practical significance for research on the spatio-temporal output characteristics of 4F degenerate cavity lases. Analyses also indicate that adjusting the laser cavity length is an effective method for controlling the spatial coherence of these lasers.
1 引言
空间相干性是传统激光器的特性之一,高空间相干性使激光具有很好的方向性,能够以极小的发散角实现长距离传输,但较高的激光相干性会产生散斑现象[1],不利于实现图像的清晰成像[2-3]、材料加工以及惯性约束中的靶丸压缩[4]。因此,为了抑制散斑噪声的出现,人们发展出多种降低激光空间相干性的方法,主要有机械振动法和光电法。机械振动法是通过引入时变的相位延迟来降低光束的空间相干性,具体方法有旋转毛玻璃法[5-6]、动态液晶光调制器[7]、高频振动扫描微镜[8]等,但这些方法都涉及机械移动,速度相对缓慢,无法实现短时间的散斑抑制。光电法则采用声光调制晶体对激光进行相位调制,激光产生声光衍射,降低了激光的空间相干性[9-10],它能够克服机械法散斑抑制缓慢的缺点,但是阈值功率低,成本高。1969年,Arnaud[11]提出了简并腔结构的激光器,该激光器能够产生具有时空低相干特点的激光。由于简并腔的完美成像特性,大量横模可同时在腔内谐振,且兼具多纵模谐振特性,因此4F简并激光器同时拥有低时间相干性和低空间相干性[12-13],这一特点对惯性约束核聚变光场的匀滑尤为重要[14]。2013年,Nixon等[15]发现,可以通过改变4F简并腔内频谱面的大小来控制简并腔的空间相干性。2015年,Chriki等[16-17]研究发现,横模数量的增加可以降低成像中的散斑对比度,105个横模数量可使散斑对比度降低到2%。2020年,Mahler等[18]在腔内插入相位散射片,在不改变简并腔光谱总带宽的情况下改变简并腔的光谱精细结构,纳秒时间内散斑对比度降低到3%,使得4F简并激光器的散斑抑制效果得到大幅提高。这些研究分析了4F简并激光器时空相干特性的影响因素,但均忽略了激光增益介质中热效应对4F简并腔的影响,而在固体激光器中热效应是一个不可忽略的问题。激光增益介质热透镜效应的存在会破坏4F简并腔的完美成像特性,进而改变4F谐振腔的简并特性。本文从简并腔增益介质存在热透镜效应的角度出发,使用传输矩阵和G参数等价腔分析法分析了腔长变化对时空输出特性的影响,并进行了实验验证。研究结果为低时空相干激光器的研究提供了理论和实验参考。
4F简并激光腔解析及热透镜影响分析
4F简并激光腔解析
4F简并激光腔的简并特性体现在两个方面,一是数万个横模可以在腔内同时起振,二是横模频率与纵模频率是简并的。这种简并特性是由4F简并腔的结构所决定的,4F简并腔原理图如
4F简并激光腔的横模频率特性可以通过传输矩阵[20]进行分析,谐振腔参数(g1和g2)与谐振腔传输矩阵元(A和D)的关系为
横模频率间隔Δνε为
完美4F简并激光腔单程ABCD矩阵为
往返一周总变换矩阵为
往返一周总变换矩阵为
A2=D2=1,可以看出,改变激光腔腔长后,激光腔恒为临界腔。根据式(3)可知,腔长变化Δz不会影响简并腔的横模频率简并性。
2.2 热透镜影响分析
存在热透镜时简并激光腔的ABCD传输矩阵为
根据G参数等价腔分析法可计算腔内基模光束的传输特性。具体分为三步:第一步,计算出简并腔内往返变换矩阵M',并推导出简并腔的G参数;第二步,根据高斯光束自再现公式
3 实验装置
4F简并腔激光器的结构如
4 实验结果分析
通过
图 3. 腔长变化对热透镜焦距的影响
Fig. 3. Influence of cavity length variation on focal length of thermal lens
为了研究存在热透镜的情况下腔长对激光器总带宽及横模拍频带宽的影响,利用光电探测器测量了激光的时间波形。对时间波形的数据进行傅里叶变换,获得了4F简并腔的拍频信号。其中激光器总带宽根据纵模拍频的数量来确定,横模拍频带宽由横模频率间隔和横模最大阶数确定。
图 4. 简并腔腔长对时间分布特性的影响。(a)简并腔拍频分布;(b)腔长变化对横模拍频带宽的影响
Fig. 4. Influence of degenerate cavity length on time distribution characteristic.(a)Beat frequency distribution of degenerate cavity;(b)influence of cavity length variation on beat bandwidth of transverse mode
为了研究存在热透镜时腔长变化对激光光场的空间相干性的影响,我们对光束的最大横模阶数进行了实验测量。采用
图 5. 腔长变化对横模最大阶数的影响
Fig. 5. Influence of cavity length variation on maximum order of transverse mode
为了分析存在热透镜时腔长变化对横模最大阶数的影响,分别采用菲涅耳数(空间域)及横模拍频带宽(时间域)两种方法对最大横模阶数Nmax进行估算。根据测量得到的热透镜焦距计算出谐振腔的传输矩阵,并使用G参数等价腔分析法[19]计算出腔内光阑处的基模半径和横模频率间隔,结果如
图 6. 热透镜下腔长变化对横模频率间隔及基模半径的影响
Fig. 6. Influence of cavity length variation on frequency interval of transverse mode and fundamental moderadius under thermal lens
5 结论
研究了在激光增益介质热效应下腔长变化对简并激光腔最大横模阶数及光谱结构的影响。热透镜的存在会破坏4F简并腔的完美成像特征及横模与纵模频率的简并特性。在泵浦功率为3倍阈值功率时,实验测得热透镜焦距约为10 m,同时获得了在热透镜影响下腔长变化时简并激光腔的拍频信号及最大横模阶数。结果表明,简并激光腔的总光谱带宽约为2.7 GHz,基本不随热透镜焦距和腔长发生变化,说明存在热透镜效应时,腔长变化对简并激光腔的时间相干性影响不大。在热透镜影响下,当腔长增加时,横模频率间隔从0.18 MHz增大到3.7 MHz,横模拍频带宽在20~33 MHz区间波动,说明腔长变化会影响光谱中的模式分布。最大横模阶数Nmax的实验结果表明,由于热透镜的存在,腔镜M2与理想位置的微小距离会对最大横模阶数产生显著影响。实验获得的最大横模阶数Nmax=205,散斑对比度C=0.5%;在Δz=10 mm时,Nmax=33,散斑对比度C=3.3%。理论模拟了热透镜影响下腔长变化时的最大横模阶数,时域的模拟结果与实验结果吻合,表明热透镜的理论模型对4F简并腔时空输出特性的研究具有实际指导意义。分析表明,调节激光腔长是控制激光空间相干性的一种有效方法。
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Article Outline
张昆鹏, 张笑琪, 冯滔, 张艳丽, 韦辉, 杨方伦, 朱健强. 基于热效应的4F简并腔时空输出特性分析[J]. 中国激光, 2022, 49(24): 2401002. Kunpeng Zhang, Xiaoqi Zhang, Tao Feng, Yanli Zhang, Hui Wei, Fanglun Yang, Jianqiang Zhu. Spatio⁃Temporal Output Characteristics of 4F Degenerate Cavity Based on Thermal Effect[J]. Chinese Journal of Lasers, 2022, 49(24): 2401002.