CN105428984A - 准参量啁啾脉冲放大器 - Google Patents
准参量啁啾脉冲放大器 Download PDFInfo
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Abstract
一种准参量啁啾脉冲放大器，包括信号光路、泵浦光路和放大器，信号光路依次包括钛宝石再生放大器，脉冲展宽器和脉冲压缩器；泵浦光路依次包括掺钕钒酸钇再生放大器，掺钕钇铝石榴石五级放大器，像传递系统，倍频晶体和光束收集器；泵浦和信号光路由电子位相锁定环联系起来同步工作；放大器是掺杂稀土离子的非线性晶体放大器，泵浦脉冲的能量不断流向啁啾信号脉冲并产生闲频脉冲，该闲频脉冲的能量又不断被晶体中掺杂的稀土离子吸收。本发明可以兼具传统激光放大器和光参量放大器的主要优点，为进一步提升超强激光系统的峰值功率铺平了道路。
Description
技术领域
本发明涉及激光放大器，特别是一种高效率的准参量啁啾脉冲放大器。
背景技术
啁啾脉冲放大(chirped-pulseamplification,以下简称为CPA)是产生超短超强激光脉冲最常用也是最有效的一种技术。它先将超短脉冲展宽成长脉宽的啁啾脉冲，然后利用放大器给啁啾脉冲补充能量，最后利用压缩器将其压回初始宽度，从而获得强的超短脉冲。CPA技术可有效降低放大器中的脉冲峰值功率，从而可以避免高阶非线性效应和放大介质的破坏。目前CPA主要依赖于两类放大器：能级型激光放大器和光参量放大器(opticalparametricamplifier,简称为OPA)。激光放大器的优点是皮实、效率高，缺点是增益带宽很小，无法支持周期量级超宽带脉冲的放大。在增益带宽方面，OPA要优于普通的激光放大器，它和CPA的结合是产生超高峰值功率激光最有前景的一种技术，目前已经可以实现拍瓦级峰值功率输出和数十毫焦耳的周期量级脉冲。然而所有的参量过程有一个共性问题：倒流效应。所谓倒流效应是指：在饱和放大域，能量会从信号光和闲频光重新流向泵浦光。这种倒流效应限制了光参量啁啾脉冲放大器的极限性能。首先，倒流会降低放大效率。人们常用的方法是通过各种优化设计使得在倒流发生之前尽量实现较高的能量转换，但是无法实现理论极限的转换效率(泵浦完全消耗)。使用复杂的空时脉冲整形技术可以实现目前最高的34％的能量转换效率，但是一般系统的效率普遍在20％左右，远低于理论极限效率。另一个方面，倒流使得光参量装置对位相匹配条件和泵浦强度均匀性非常敏感，从而对泵浦源的光束质量提出了非常严格的要求。由于目前的高能泵浦激光通常远离衍射极限，因此这个特性阻碍了光参量啁啾脉冲放大装置峰值功率的进一步提升。
发明内容
针对上述现有技术的不足，本发明提出一种高效率的准参量啁啾脉冲放大器(quasi-parametricamplification,简称为QPA)，本发明的核心原理是吸收放大过程新产生的闲频光，阻止信号光能量的回流；具体途径为在非线性晶体放大器中掺杂一定量的稀土离子，稀土离子用于吸收闲频光。这种QPA在小信号放大阶段仍然是一个OPA，而在饱和放大阶段呈现了非参量激光放大器的特征，可以支持宽带高效率的放大。本发明为进一步提升脉冲峰值功率铺平了道路。
本发明的原理如下：
在普通非线性晶体放大器中发生的三波相互作用有一个本质属性：可以支持正向(泵浦光→信号光+闲频光)和反向(信号光+闲频光→泵浦光)的能量转换。在OPA饱和放大阶段，逆过程的发生(即倒流)会严重降低信号光的转换效率。倒流的过程需要闲频光的参与，如果在闲频光产生的过程中不断将其吸收，则可以抑制倒流的发生。本发明正是基于这种原理。
本发明的技术解决方案如下：
一种准参量啁啾脉冲放大器，特点在于其构成包括：信号光路、泵浦光路和放大器，所述的信号光路依次是钛宝石再生放大器、脉冲展宽器和脉冲压缩器；所述的泵浦光路依次是掺钕钒酸钇再生放大器、掺钕钇铝石榴石五级放大器、像传递系统、倍频晶体和光束收集器；所述的掺钕钒酸钇再生放大器和所述的钛宝石再生放大器由电子位相锁定环连接并控制同步工作；所述的放大器是稀土离子掺杂的非线性晶体放大器，经所述的脉冲展宽器输出的啁啾信号脉冲射向所述的非线性晶体放大器，经所述的倍频晶体输出的泵浦脉冲射向所述的非线性晶体放大器，所述的泵浦脉冲的能量不断流向所述的啁啾信号脉冲，同时产生闲频脉冲，剩余的泵浦脉冲的能量由所述的光束收集器收集，所述的闲频脉冲的能量又不断被所述的非线性晶体放大器中的稀土离子吸收，经所述的非线性晶体放大器放大的啁啾信号脉冲经所述的脉冲压缩器输出。
所述的非线性晶体放大器中掺杂可以吸收闲频光的稀土离子，比如钐或钆，掺杂浓度大于10at％。
本发明的技术效果：
本发明准参量啁啾脉冲放大器在小信号放大阶段仍然是一个参量放大器，而在饱和放大阶段类似一个非参量激光放大器。
由于抑制了倒流，所以本发明有两个明显的特点：
一是可以实现高的转换效率，原理上可以实现量子极限效率。
二是对位相失配不敏感，从而降低了对泵浦光光束质量、指向性以及环境温度的要求。
本发明兼容非共线位相匹配技术，从而可以实现宽带的放大。
本发明兼具传统激光放大器的高效率和OPA宽带宽的优点，是一款理想的、有前景的啁啾脉冲放大器。它突破现有技术的局限，将啁啾脉冲放大至更高的功率水平。
附图说明
图1为本发明准参量啁啾脉冲放大器的数值模拟结果。
图1(a)为空时高斯泵浦下的信号效率ηs随晶体长度z的变化曲线；实线和虚线分别代表QPA和OPA；点线为理论最高效率。
图1(b)为空时4阶超高斯泵浦下的信号效率ηs随晶体长度z的变化曲线；实线和虚线分别代表QPA和OPA；点线为理论最高效率。
图1(c)为大位相失配情况下，输出信号能量Es随晶体长度z的变化曲线；实线和虚线分别代表QPA和OPA；插图为QPA在初始的0.2mm晶体长度内的信号演化。
图1(d)为QPA输出信号能量Es(z)随输入信号能量Es(0)的变化关系以及它的Frantz-Nodvik拟合；信号能量已对初始泵浦能量Ep(0)归一化；方块为计算值，曲线为拟合值；插图为QPA中信号效率Es随闲频吸收αLnl的变化关系。
图1(e)为归一化的信号效率ηs，闲频效率ηi和残余的泵浦能量Ep(z)随晶体长度z的演化；ω为相应光波的角频率，下标p,s,i分别表示泵浦光、信号光和闲频光。
图2为本发明准参量啁啾脉冲放大器的结构示意图。
图3为实施例所用掺钐三硼酸氧钙钇(Sm:YCOB)晶体的吸收光谱和增益光谱。
图3(a)吸收光谱，测量方向(θ＝29°，)；阴影区域代表实验中采用的闲频光波段；插图是Sm:YCOB晶体的毛坯照片。
图3(b)是泵浦强度为2.8GW/cm2时30mm长的Sm:YCOB的增益光谱。
图4为实施例的主要实验结果。
图4(a)OPA与QPA中信号效率随注入强度的变化曲线；
图4(b)OPA与QPA中归一化的信号效率随位相失配ΔkL的变化曲线，测量采用的是a图中对应的两者的最高注入强度；
图4(c)和(d)分别是OPA和QPA的信号光谱，其中虚线和实线对应的注入强度分别为6.5和325kW/cm2,阴影部分表示注入的脉冲光谱；
图4(e)和(f)分别是OPA和QPA对应的压缩脉冲宽度，其中实线代表实验数据，虚线代表理论的傅氏变换极限。
图5为本发明实施例的信号光和闲频光的效率曲线，方块和实线分别代表信号效率的实验值和理论模拟曲线，圆圈和虚线分别代表闲频光效率的实验值和理论模拟曲线。
图中：1为钛宝石再生放大器，2为脉冲展宽器，3为啁啾信号脉冲，4为掺杂稀土离子的非线性晶体放大器，5为闲频脉冲，6为光束收集器，7为脉冲压缩器，8为电子位相锁定环，9为掺钕钒酸钇再生放大器，10为掺钕钇铝石榴石五级放大器，11为像传递系统，12为倍频晶体，13为泵浦脉冲。
具体实施方式
下面结合附图和实施例进一步阐述本发明。
图1是一组数值模拟结果，用来说明准参量放大器的放大性质。计算采用包含闲频光吸收(吸收系数为α)的全维度标准非线性耦合波方程和对称分步傅里叶算法。计算中忽略所有的线性效应，比如衍射、色散和时空走离等。泵浦光、信号光和闲频光的波长分别为532nm、810nm和1550nm，对应理论最高信号效率约为66％。图1(c)采用的是空时均为4-阶超高斯的泵浦脉冲，其余图计算采用的是空时高斯的泵浦脉冲。计算中注入的高斯信号强度设定为泵浦强度的1％，信号光的光斑和脉宽均与泵浦光一致。非线性长度Lnl固定在2mm【Lnl的定义参考文献J.MosesandS.-W.Huang,J.Opt.Soc.Am.B28,812(2011)】。位相失配定义为Δk＝kp-ks-ki，k是相应光波的波矢，下标p,s,i分别表示泵浦光、信号光和闲频光。
OPA使用的是对泵浦、信号和闲频均透明的普通晶体，并且OPA与其逆过程-和频(sum-frequencygeneration,简称SFG)具有相同的位相匹配条件。这样在饱和放大域，OPA会转变成信号光和闲频光的SFG过程(图1a虚线)。这种倒流效应不利于信号光转换效率。尽管OPA中采用空时平顶型的泵浦脉冲会提高信号光效率，但是倒流过程仍然会发生(图1b虚线)。在QPA中，通过强吸收消耗闲频光，倒流效应可从根本上得到抑制，而且对任何泵浦状态均有效。这样随着晶体长度的增加，QPA的信号效率单调上升逐步接近理论极限效率(图1a实线)。同样重要的是QPA中的信号放大对位相失配不敏感(图1b实线)，从而会减小对泵浦激光的要求。在大的位相失配条件下，可以从信号光的演化行为得出QPA的一些有趣的特征(图1c)。对于没有闲频光吸收的OPA，信号光在整个晶体长度内规则地振荡，不断地在OPA和SFG间周期性地跃变。与之不同，QPA中的信号光只在初始阶段振荡，随后会一直处于上升阶段，类似一个非参量过程。
为进一步揭示QPA的性质，我们研究了它的输入-输出关系。正如图1d所示，QPA中信号光的输入输出模拟结果可以很好地与激光放大器中的Frantz-Nodvik关系拟合。Frantz-Nodvik公式为Es(z)＝Esatln{1+G0[exp(Es(0)/Esat)-1]}，其中Esat和G0分别为饱和能流和小信号增益。因此QPA的放大行为非常类似一个非参量的激光放大器。另外，QPA中泵浦与信号间的Manley-Rowe关系仍然成立，而与OPA中不同的是，闲频光由于强吸收不再遵守Manley-Rowe关系(图1e)。需要指出的是QPA在一定的闲频光吸收水平下才成立，在本模拟条件下为αLnl>0.3，如图1d插图所示，在该范围内，信号效率对闲频光吸收的变化不太敏感，这预示着在闲频光频率范围内不一定需要均匀的吸收。
图2是本发明准参量啁啾脉冲放大器的结构示意图。由图可见，本发明准参量啁啾脉冲放大器，包括：信号光路、泵浦光路和放大器，所述的信号光路依次是钛宝石再生放大器1、脉冲展宽器2和脉冲压缩器7；所述的泵浦光路依次是掺钕钒酸钇再生放大器9、掺钕钇铝石榴石五级放大器10、像传递系统11、倍频晶体12和光束收集器6；所述的掺钕钒酸钇再生放大器10和所述的钛宝石再生放大器1由电子位相锁定环8连接并控制同步工作；所述的放大器是稀土离子掺杂的非线性晶体放大器4，所述的脉冲展宽器2输出的啁啾信号脉冲3射向所述的非线性晶体放大器4，经所述的倍频晶体12输出的泵浦脉冲13射向所述的非线性晶体放大器4，所述的泵浦脉冲13的能量不断流向所述的啁啾信号脉冲3，同时产生闲频脉冲5，剩余的泵浦脉冲13的能量由所述的光束收集器6收集，所述的闲频脉冲5的能量又不断被所述的非线性晶体放大器4中的稀土离子吸收，经所述的非线性晶体放大器4放大的啁啾信号脉冲3经所述的脉冲压缩器7输出。
根据闲频脉冲5的波长不同，所述的非线性晶体放大器4可以掺杂不同种类的稀土离子，比如钐或钆，掺杂浓度大于10at％。
本实施例中所用泵浦脉冲13的波长为532nm，啁啾信号脉冲3的波长为795-825nm，产生的闲频脉冲5的中心波长为1550nm。掺杂稀土离子的非线性晶体放大器4为一块掺杂钐离子(Sm3+)的三硼酸氧钙钇(YCOB)晶体(简称Sm:YCOB)，掺杂浓度为30at％，如图3(a)所示。Sm:YCOB晶体对1550nm附近的激光存在强吸收，吸收系数为1～2cm-1，而基本不吸收532nm和800nm附近的激光。Sm:YCOB的尺寸为15.5×18.5×30mm3，沿XZ面切割，切割角度为θ＝28°,(θ为极角，是晶体表面法线与Z轴的夹角；为方位角，是晶体表面法线在XY平面内的投影与X轴的夹角)。30mm长的Sm:YCOB的增益带宽经测量为约34nm，如图3(b)所示。
本实施例采用单级QPA。泵浦源包括一个1kHz重复率的掺钕钒酸钇再生放大器9、一个10Hz重复率的掺钕钇铝石榴石五级放大器10、一块5mm厚的BBO倍频晶体12。产生的532nm泵浦脉冲13的能量为75mJ，时间宽度为420ps。像传递系统11将掺钕钇铝石榴石五级放大器10的输出端面像传递到Sm:YCOB晶体表面，减小衍射传输带来的泵浦畸变。信号源为一个1kHz的钛宝石再生放大器1，它相对泵浦源的时间抖动被一个电子位相锁定环8控制在约10ps范围。信号源产生的种子脉冲被一个单光栅双通型展宽器展宽至约380ps，产生的啁啾信号脉冲3的能量约为0.5mJ。放大后的啁啾信号脉冲由一个单光栅四通压缩器压缩至近傅氏变换极限的超短脉宽。
为了显示QPA对倒流的抑制效果，同时做了基于BBO晶体的普通OPA的对比实验，两者采用相同的泵浦光强，约2.8GW/cm2。实验中啁啾信号脉冲3的强度从弱到强跨越6个数量级，通过调节它的强度来调控放大器的饱和度。OPA采用的BBO晶体长度为12mm，QPA采用的Sm:YCOB晶体长度为30mm，两者在2.8GW/cm2的泵浦强度下具有相同的小信号增益。OPA的效率曲线(图4a虚线)在强注入阶段效率出现了明显的下降，这表明出现了倒流。倒流限制了OPA的最高效率约17％。与此不同的是，在QPA中，注入信号越强效率越高，如图4a实线所示，效率在整个注入光强范围内一直上升，说明倒流得到了抑制。除了效率，我们也测量了放大后的信号光谱【图4c和d中，虚线和实线分别代表注入信号强度为6.5和325kW/cm2时的放大后信号光谱，阴影部分表示注入光谱】。在OPA中(图4c)，放大后光谱中间(这部分光谱是与泵浦脉冲的峰值重合的)出现了凹陷，而且饱和程度越强凹陷越深，这从光谱的角度表明了倒流的发生。而在QPA中(图4d)，随着注入信号光强增强，放大后的脉冲光谱变得越来越平坦、越来越宽，没有出现倒流。QPA放大后的脉冲可以被很好地压缩至近傅氏变换极限，压缩脉宽为90fs(图4e)，与OPA的压缩脉宽非常接近(图4f)。
QPA的另一个特征是对位相失配不敏感，我们对此也进行了实验验证，如图4b所示。QPA可以允许较大的位相失配，在ΔkL≈10的范围内放大后效率基本不变。相比而言，OPA对位相失配更加敏感。QPA对位相失配不敏感性表明它对环境温度、光束发散性和指向性的变化不敏感。
为了进一步提升QPA的放大效率，我们通过减小光束口径来提升注入信号光强，此时的泵浦光强为3.0GW/cm2。在注入信号光强提升至260MW/cm2时，放大后的信号效率达到了41％(如图5中方块所示)，其实验效率均与理论值(图5中的实线)符合。如果考虑到晶体表面的反射损耗，那么晶体内部的泵浦到信号的转换效率高达47％，对应的泵浦消耗达到了70％。QPA的效率已经超过了目前最好的OPA的效率。需要注意的是，在我们实验中没有使用经过整形的泵浦脉冲和信号脉冲就达到了如此高的效率。由于晶体对闲频光存在吸收，所以闲频光的效率明显低于其应有水平(在本实施例情况下约为信号光效率的一半)，并且闲频光效率呈现先上升后下降的趋势(图5圆圈)，其变化规律符合理论预期(图5虚线)。
Claims (2)
1.一种准参量啁啾脉冲放大器，特征在于其构成包括：信号光路、泵浦光路和放大器，所述的信号光路依次是钛宝石再生放大器(1)、脉冲展宽器(2)和脉冲压缩器(7)；所述的泵浦光路依次是掺钕钒酸钇再生放大器(9)、掺钕钇铝石榴石五级放大器(10)、像传递系统(11)、倍频晶体(12)和光束收集器(6)；所述的掺钕钒酸钇再生放大器(10)和所述的钛宝石再生放大器(1)由电子位相锁定环(8)连接并控制同步工作；所述的放大器是稀土离子掺杂的非线性晶体放大器(4)，经所述的脉冲展宽器(2)输出的啁啾信号脉冲(3)射向所述的非线性晶体放大器(4)，经所述的倍频晶体(12)输出的泵浦脉冲(13)射向所述的非线性晶体放大器(4)，所述的泵浦脉冲(13)的能量不断流向所述的啁啾信号脉冲(3)，同时产生闲频脉冲(5)，剩余的泵浦脉冲(13)的能量由所述的光束收集器(6)收集，所述的闲频脉冲(5)的能量又不断被所述的非线性晶体放大器(4)中的稀土离子吸收，经所述的非线性晶体放大器(4)放大的啁啾信号脉冲(3)经所述的脉冲压缩器(7)输出。
2.根据权利要求1所述的准参量啁啾脉冲放大器，其特征在于所述的非线性晶体放大器(4)中掺杂可以吸收闲频光的稀土离子，比如钐或钆，掺杂浓度大于10at％。
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| CN108649419A (zh) * | 2018-03-02 | 2018-10-12 | 上海交通大学 | 超高平均功率光参量啁啾脉冲放大器 |
| CN109616863A (zh) * | 2019-01-15 | 2019-04-12 | 上海交通大学 | 通过闲频光倍频实现高效率非线性频率下转换的方法和装置 |
| CN113189824A (zh) * | 2021-04-21 | 2021-07-30 | 中国科学院上海光学精密机械研究所 | 一种基于双非线性光学过程的宽带光参量放大装置 |
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| CN106410577B (zh) * | 2016-10-19 | 2018-11-02 | 上海交通大学 | 温度和波长不敏感光参量啁啾脉冲放大器 |
| CN108649419A (zh) * | 2018-03-02 | 2018-10-12 | 上海交通大学 | 超高平均功率光参量啁啾脉冲放大器 |
| CN109616863A (zh) * | 2019-01-15 | 2019-04-12 | 上海交通大学 | 通过闲频光倍频实现高效率非线性频率下转换的方法和装置 |
| CN109616863B (zh) * | 2019-01-15 | 2021-02-02 | 上海交通大学 | 通过闲频光倍频实现高效率非线性频率下转换的方法和装置 |
| CN113189824A (zh) * | 2021-04-21 | 2021-07-30 | 中国科学院上海光学精密机械研究所 | 一种基于双非线性光学过程的宽带光参量放大装置 |
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Application publication date: 20160323 |