CN105762636A - 一种产生高空间强度对比度的飞秒涡旋光束的方法 - Google Patents
一种产生高空间强度对比度的飞秒涡旋光束的方法 Download PDFInfo
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Abstract
本发明公开一种产生高空间强度对比度的飞秒涡旋光束的方法,包括搭建具有泵浦源、增益晶体、可饱和吸收镜和输出耦合镜的激光传输腔体,基于半导体可饱和吸收镜(SESAM)锁模产生高阶飞秒厄米高斯光束,在腔外利用柱透镜模式转换器将其转换为高阶飞秒拉盖尔高斯光束,即飞秒涡旋光束。本发明产生的飞秒涡旋光束空间对比度高,解决了传统方法产生的宽带涡旋光束空间对比度低的问题;同时实现拓扑荷数可调。
Description
技术领域
本发明属于激光技术领域,具体涉及产生高阶飞秒厄米高斯光束的方法以及将飞秒厄米高斯光束转换为飞秒拉盖尔高斯光束的方法。
背景技术
飞秒涡旋光束在时域上拥有脉冲持续时间短,峰值功率高等特点;同时在空间域上具有螺旋形相位和环形光束的特点,对于带有拓扑荷数为l的涡旋光束,其光子能携带的角动量。因此,飞秒涡旋光束在强场物理、材料加工、光学操控以及光通讯等领域具有重要的应用前景。
以往飞秒涡旋光束的产生方法都是在腔外对基模的飞秒高斯光束进行空间位相调制,如利用螺旋相位板、计算全息等方法。而这些方法都无法产生纯净的飞秒涡旋光束,导致所产生的飞秒涡旋光束中心不空,即空间强度对比度低,而高的空间对比度对于飞秒涡旋光束在强场物理等方面的应用至关重要。
发明内容
本发明的目的在于克服上述现有技术的不足,提供一种产生高空间强度对比度的飞秒涡旋光束的方法。通过锁模激光器直接产生高阶飞秒厄米高斯光束,再利用柱透镜模式转换器将飞秒厄米高斯光束转换为飞秒拉盖尔高斯光束,即飞秒涡旋光束,保持了涡旋光束的单模性,大大提高了其空间对比度,解决了宽带飞秒涡旋光束空间对比度低的问题。
本发明的技术解决方案如下:
一种产生高空间强度对比度的飞秒涡旋光束的方法,该方法包括如下步骤:
步骤1,搭建具有泵浦源、增益晶体、可饱和吸收镜和输出耦合镜的激光传输腔体;首先调节基模高斯光束稳定的锁模状态,然后调节输出耦合镜改变腔轴旋转角度,使增益晶体内激光光束与泵浦光束呈一定角度,从而使厄米高斯光束的增益覆盖大于基模高斯光束的增益覆盖,并利用CCD照相机监测经输出耦合镜输出的激光光斑图样,直到CCD照相机中呈现飞秒厄米高斯光束图样,即产生高阶飞秒厄米高斯光束HG0n,其中n为整数;
步骤2,搭建柱透镜模式转换器,利用CCD照相机监测输出的激光光斑图样,直到CCD照相机中呈现LG0n模式的飞秒拉盖尔高斯光束图样,即将飞秒厄米高斯光束转换为飞秒拉盖尔高斯光束(即飞秒涡旋光束)。
所述的腔轴旋转角度θ,公式如下:
其中
式中Pth(HG0n)代表HG0n模式的厄米高斯光束对应的泵浦功率阈值,这里的n与公式中的n相同;Hn-k-2j为厄米多项式;ωl为基模高斯光束光腰位置的光斑半径,由具体激光腔结构决定,可以通过空间传输的ABCD矩阵计算得到(几何光学的标准方法);ωx与ωy为泵浦光束光腰位置的纵向与横向的光斑半径,可以直接测量得到;L为增益晶体长度;α为增益晶体对泵浦光的吸收系数,测量方法为:首先测量增益晶体对泵浦光的吸收率,再除以增益晶体长度,即为吸收系数;ηp、γ、Isat分别为泵浦效率、损耗系数以及饱和光强,具体计算时,只需考虑不同模式之间的相对关系,这三个参数为常数,不影响计算结果,所以不需要具体的数值。
所述的激光传输腔体包括泵浦源、第一凸透镜、第二凸透镜、第一凹面镜、第二凹面镜、位于该第一凹面镜和第二凹面镜中心的增益晶体,第三凹面镜、可饱和吸收镜、棱镜对和输出耦合镜;
所述泵浦源通过第一透镜和第二透镜准直聚焦入增益晶体中,通过该增益晶体后,依次经所述第一凹面镜与第三凹面镜的反射后聚焦到所述可饱和吸收镜表面,再经该可饱和吸收镜反射后原路返回再次通过增益晶体后,经所述第二凹面镜的反射后再通过所述棱镜对进行色散补偿,最后经所述输出耦合镜输出飞秒厄米高斯光束。
所述的柱透镜模式转换器包括凸透镜、第一柱透镜和第二柱透镜;第一柱透镜和第二柱透镜完全相同,且两柱透镜之间的距离为其焦距的倍,两个柱透镜轴均与水平面呈45度;
所述的凸透镜将厄米高斯光束聚焦到两个柱透镜的中心,第一柱透镜的凸面朝着激光入射的方向,第二柱透镜的凸面朝着激光出射的方向。
与现有技术相比,本发明的有益效果是:
1)利用谐振腔直接产生的厄米高斯光束保持了其单模性,即产生了纯净的飞秒厄米高斯光束;
2)可饱和吸收镜作为锁模器件在飞秒高阶厄米高斯光束的产生上显示其优势,即可以产生稳定的锁模脉冲;
3)柱透镜模式转换器可以将厄米高斯光束百分之百地转换为拉盖尔高斯光束,同样保持了其单模性,因此可大大提高飞秒涡旋光束的空间对比度;
4)通过调节输出耦合镜角度,可以实现飞秒厄米高斯光束的阶数可调,也就实现了飞秒涡旋光束的拓扑荷数可调,方法简单。
附图说明
图1是本发明实施例飞秒涡旋锁模激光器的结构示意图。
图2是高阶厄米高斯光束产生以及阶数可调的原理示意图。其中:a为增益晶体内部不同阶数的厄米高斯光束与泵浦光束空间覆盖(增益覆盖)对比示意图;b为不同阶数的厄米高斯光束振荡对应的归一化泵浦功率阈值与图2a所示的旋转角度的关系。
图3是HG01模式的锁模结果示意图。其中:a.HG01模式的光斑图;b.HG01模式的脉冲序列;c.HG01模式的自相关曲线,通过高斯拟合,显示其时间脉冲宽度为621飞秒;d.HG01模式的光谱。
图4是HG02模式的锁模结果示意图。其中:a.HG02模式的光斑图;b.HG02模式的脉冲序列;c.HG02模式的自相关曲线,通过高斯拟合,显示其时间脉冲宽度为630飞秒;d.HG02模式的光谱。
图5是利用柱透镜模式转换器转换后得到的飞秒拉盖尔高斯光束(飞秒涡旋光束)的光斑图以及与平面波的干涉图样。其中:a.LG01模式的光斑图;b.LG02模式的光斑图;c.LG01模式的飞秒拉盖尔高斯光束与平面波的干涉图样,中心呈现叉形,且有一个叉,显示其拓扑荷数为1;d.LG02模式的飞秒拉盖尔高斯光束与平面波的干涉图样,中心呈现叉形,且有两个叉,显示其拓扑荷数为2。
图6是飞秒涡旋光束的空间强度对比度测量结果示意图。其中:a.对于LG01模式,其空间对比度测量结果为34dB;b.对于LG02模式,其空间对比度测量结果为42dB。需要说明的是,相对位置是以涡旋光束中心与环上强度最大点之间的距离为参考。
图中1.激光二极管(泵浦源);2.第一凸透镜;3.第二凸透镜;4.第一凹面镜;5.增益晶体;6.第二凹面镜;7.第三凹面镜;8.可饱和吸收镜;9.棱镜对;10.输出耦合镜;11.HG02模式的飞秒厄米高斯光束光斑图样(由CCD照相机显示);12.柱透镜模式转换器;12-1.凸透镜;12-2.第一柱透镜;12-3.第二柱透镜;13.LG02模式的飞秒拉盖尔高斯光束光斑图样(由CCD照相机显示)。
具体实施方式
下面结合附图对本发明作进一步的详细说明。
通过调节输出耦合镜角度旋转腔轴,改变不同阶数的厄米高斯光束的增益覆盖(与泵浦光束的重叠区),以实现纯净的特定阶数的厄米高斯(HG0n)光束产生,其中n为整数,腔轴的方向为腔内激光传播的方向,同时,利用可饱和吸收镜进行被动锁模,以产生不同阶数的飞秒厄米高斯光束,再利用柱透镜模式转换器,将飞秒厄米高斯光束转换为飞秒拉盖尔高斯(LG0l)光束,l为整数。这里需要说明,在模式转换中,HG0n模式转换为LG0l模式,其中l=n,对于LG0l模式,每个光子携带的角动量,即为带有拓扑荷数为l的涡旋光束。
搭建如图1所示的激光器系统,其中泵浦源1为单结激光二极管,发射波长为793纳米,通过两个透镜准直聚焦到增益晶体5中心,增益晶体为Tm:CYA,发射波长为1970纳米,两个透镜的焦距均为100毫米;第一凹面镜4、第二凹面镜6、第三凹面镜7以及输出耦合镜10作为谐振腔中的反射镜组成五镜腔系统,三个凹面镜的曲率半径均为100毫米,可饱和吸收镜的参数为(BATOP,SAM-1960-8-10ps-x),增益晶体5位于第一凹面镜4和第二凹面镜6中心,两凹面镜距离为110毫米。腔内激光的传播路径为:从增益晶体出发,经第一凹面镜4与第三凹面镜7的反射聚焦到可饱和吸收镜8表面,第一凹面镜4与第三凹面镜7之间距离为600毫米,第三凹面镜7与可饱和吸收镜8距离为50毫米,激光再由可饱和吸收镜8反射原路返回再次通过增益晶体,再由凹面镜6反射通过棱镜对进行色散补偿,棱镜对间距为350毫米,最后通过输出耦合镜10实现激光输出,第二凹面镜6到输出耦合镜10之间的光程为700毫米。利用CCD照相机监测激光的光斑图样,优化激光腔结构,首先得到稳定的基模高斯光束的锁模状态,旋转输出耦合镜的角度,直到CCD照相机中呈现HG01模式的飞秒厄米高斯光束图样(图3a),其锁模脉冲序列如图3b所示。
然后搭建图1中所示的柱透镜模式转换器12,利用一个凸透镜12-1(焦距为160毫米)将厄米高斯光束聚焦到两个柱透镜的中心,两个柱透镜完全相同,焦距为50毫米,前面的柱透镜凸面朝着激光入射的方向,两柱透镜之间的距离为其焦距的倍,同时保持两个柱透镜轴与水平面呈45度,则输出的HG01模式的飞秒厄米高斯光束就会被转换为LG01模式的飞秒拉盖尔高斯光束,即带有1个拓扑荷的飞秒涡旋光束(图5a),其脉宽为621飞秒(图3c),其空间对比度达到34dB(图6a)。继续旋转输出耦合镜的角度,直到CCD照相机中呈现HG02模式的飞秒厄米高斯光束图样(图4a),其锁模脉冲序列如图4b所示。再利用柱透镜模式转换器,得到了LG02模式的飞秒拉盖尔高斯光束,即带有2个拓扑荷的飞秒涡旋光束(图5b),其脉宽为630飞秒(图4c),其空间对比度达到42dB(图6b)。该方法得到的飞秒涡旋光束的空间对比度比传统方法产生的飞秒涡旋光束的空间对比度高1-2个数量级。
当增益晶体内部激光光束与泵浦光束呈某一角度时(图2a),高阶厄米高斯光束的增益覆盖大于基模高斯光束的增益覆盖,会导致高阶横模的振荡阈值更低,从而优先振荡输出。不同阶数的厄米高斯光束对应的振荡阈值随腔轴旋转角度的变化关系为:
其中
这里Pth(HG0n)代表HG0n模式的厄米高斯光束对应的泵浦功率阈值;Hn-k-2j为厄米多项式;θ表示如图2a所示的腔轴旋转角度;ωl为基模高斯光束光腰位置的光斑半径;ωx与ωy为泵浦光束光腰位置的纵向与横向的光斑半径;L为增益晶体长度;α为增益晶体对泵浦光的吸收系数;γ为损耗系数;Isat为饱和光强;ηp为泵浦效率。根据以上公式,我们计算了不同旋转角度对应的归一化泵浦功率阈值(图2b),计算用到的参数为ωl=ωx=ωy=35微米;晶体长度L=6毫米;吸收系数α=0.8/毫米;其他参数为常数,不影响计算结果。结果显示,当腔轴与泵浦光束旋转角度为1.3度时,HG01模式的厄米高斯光束的振荡阈值小于基模(HG00)高斯光束,从而优先振荡;当旋转角度为1.5度时,HG02模式的厄米高斯光束振荡阈值变得最小,从而优先振荡。因此通过旋转腔轴可以实现输出的飞秒厄米高斯光束的阶数可调,也就实现了飞秒涡旋光束的拓扑荷数可调。
最后所应说明的是,以上实施例仅用以说明本发明的技术方案而非限制,尽管参照较佳实施例对本发明进行了详细说明,本领域的普通技术人员应当理解,可以对发明的技术方案进行修改或者等同替换,而不脱离本发明技术方案的精神和范围,其均应涵盖在本发明的权利要求范围当中。
Claims (4)
1.一种产生高空间强度对比度的飞秒涡旋光束的方法,其特征在于,该方法包括如下步骤:
步骤1,搭建具有泵浦源、增益晶体、可饱和吸收镜和输出耦合镜的激光传输腔体;首先调节基模高斯光束稳定的锁模状态,然后调节输出耦合镜改变腔轴旋转角度,使增益晶体内激光光束与泵浦光束呈一定角度,从而使厄米高斯光束的增益覆盖大于基模高斯光束的增益覆盖,并利用CCD照相机监测经输出耦合镜输出的激光光斑图样,直到CCD照相机中呈现飞秒厄米高斯光束图样,即产生高阶飞秒厄米高斯光束HG0n,其中n为整数;
步骤2,搭建柱透镜模式转换器,利用CCD照相机监测输出的激光光斑图样,直到CCD照相机中呈现LG0n模式的飞秒拉盖尔高斯光束图样,即将飞秒厄米高斯光束转换为飞秒拉盖尔高斯光束。
2.根据权利要求1所述的产生高空间强度对比度的飞秒涡旋光束的方法,其特征在于,所述的腔轴旋转角度θ,公式如下:
其中
式中Pth(HG0n)代表HG0n模式的厄米高斯光束对应的泵浦功率阈值,Hn-k-2j为厄米多项式;ωl为基模高斯光束光腰位置的光斑半径,由具体激光腔结构决定;ωx与ωy为泵浦光束光腰位置的纵向与横向的光斑半径;L为增益晶体长度;α为增益晶体对泵浦光的吸收系数;ηp、γ、Isat分别为泵浦效率、损耗系数以及饱和光强。
3.根据权利要求1所述的产生高空间强度对比度的飞秒涡旋光束的方法,其特征在于,所述的激光传输腔体包括泵浦源(1)、第一凸透镜(2)、第二凸透镜(3)、第一凹面镜(4)、第二凹面镜(6)、位于该第一凹面镜(4)和第二凹面镜(6)中心的增益晶体(5),第三凹面镜(7)、可饱和吸收镜(8)、棱镜对(9)和输出耦合镜(10);
所述泵浦源(1)通过第一透镜(2)和第二透镜(3)准直聚焦入增益晶体(5)中,通过该增益晶体(5)后,依次经所述第一凹面镜(4)与第三凹面镜(7)的反射后聚焦到所述可饱和吸收镜(8)表面,再经该可饱和吸收镜(8)反射后原路返回再次通过增益晶体(5)后,经所述第二凹面镜(6)的反射后再通过所述棱镜对(9)进行色散补偿,最后经所述输出耦合镜(10)输出飞秒厄米高斯光束。
4.根据权利要求1所述的产生高空间强度对比度的飞秒涡旋光束的方法,其特征在于,所述的柱透镜模式转换器(12)包括凸透镜(12-1)、第一柱透镜(12-2)和第二柱透镜(12-3);第一柱透镜(12-2)和第二柱透镜(12-3)完全相同,且两柱透镜之间的距离为其焦距的倍,两个柱透镜轴均与水平面呈45度;
所述的凸透镜(12-1)将厄米高斯光束聚焦到两个柱透镜的中心,第一柱透镜的凸面朝着激光入射的方向,第二柱透镜的凸面朝着激光出射的方向。
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CN107357113A (zh) * | 2017-07-20 | 2017-11-17 | 深圳大学 | 一种涡旋超短激光脉冲放大系统及方法 |
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CN109031674B (zh) * | 2018-08-07 | 2020-09-15 | 上海交通大学 | 腔内直接产生多涡旋光束的方法 |
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CN110556690B (zh) * | 2019-07-30 | 2021-10-26 | 华南理工大学 | 一种全光纤涡旋光锁模环形腔激光器 |
CN111722398A (zh) * | 2020-06-04 | 2020-09-29 | 上海理工大学 | 一种强聚焦条件下生成亚波长时空涡旋的方法 |
CN111722398B (zh) * | 2020-06-04 | 2022-03-08 | 上海理工大学 | 一种强聚焦条件下生成亚波长时空涡旋的方法 |
CN114184285A (zh) * | 2021-10-27 | 2022-03-15 | 西安石油大学 | 一种基于非线性介质中的涡旋光拓扑荷数检测装置 |
CN114184285B (zh) * | 2021-10-27 | 2023-04-18 | 西安石油大学 | 一种基于非线性介质中的涡旋光拓扑荷数检测装置 |
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