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光学参量变换是基于二阶非线性效应的三波耦合过程,按能量转移方向可以分为参量下转换过程如差频、OPO和参量上转换过程如和频、倍频。对于参量下转换过程,如果相互耦合的三个光波的角频率分别为
${\mathrm{\omega }}_{1}{\text{、}}{\mathrm{\omega }}_{2}{\text{和}}{\mathrm{\omega }}_{3}$ ,通常将频率最高的$ {\mathrm{\omega }}_{3} $ 称为泵浦光,把$ {\mathrm{\omega }}_{2} $ 称为信号光,频率最低的$ {\mathrm{\omega }}_{1} $ 称为闲频光。根据能量守恒,相互耦合的三个光波的角频率一定满足以下关系:$$ {\omega }_{1}+{\omega }_{2}={\omega }_{3} $$ (2-1) 因此,中红外波段的光学参量变换就是通过某些手段使泵浦光
$ {\omega }_{3} $ 的能量有效的流向闲频光$ {\mathrm{\omega }}_{1} $ 。这些手段主要包括差频产生(Difference Frequency Generation,DFG)和OPO。如图1(a)所示,高峰值功率的泵浦光$ {\mathrm{\omega }}_{3} $ 和信号光$ {\mathrm{\omega }}_{2} $ 同时通过非线性晶体,产生新的频率成分——闲频光$ {\mathrm{\omega }}_{1} $ 的过程就是DFG。值得一提的是,能量从泵浦光向闲频光转移的过程中,信号光的能量也会相应的得到放大,因此这个过程对于信号光而言就称为光学参量放大(Optical Parametric Amplifier, OPA)过程。当同时进入非线性晶体的两束光为泵浦光和闲频光时,则会发生对于闲频光而言的光学参量放大,文中将这中情况也归类为DFG。图 1 光学参量下转换示意图。(a) DFG;(b) OPO
Figure 1. Schematic diagram of optical parameter down conversion. (a) DFG; (b) OPO
当只有泵浦光进入非线性晶体,由于参量荧光的存在,结合谐振腔的正反馈作用,也可以同时获得可观的信号光和闲频光输出,这种方法就是OPO,如图1(b)所示。谐振腔的存在降低了OPO对入射光能量的要求,且具有高的信噪比、好的光束质量以及结构简单、光光转换效率高等优点。
对于光学参量变换,三波耦合过程可以用耦合波方程来描述。基于慢变包络近似,考虑二阶非线性极化强度、一阶和二阶色散项,描述飞秒光学参量变换过程的三波耦合方程组可以写成如下形式:
$$\begin{array}{l} \dfrac{\partial {E}_{1}\left({\textit{z}},t\right)}{\partial {\textit{z}}}+\dfrac{1}{{v}_{1}}\dfrac{\partial {E}_{1}\left({\textit{z}},t\right)}{\partial t}+\dfrac{i}{2}{k}_{1}^{''}\dfrac{{\partial }^{2}{E}_{1}\left({\textit{z}},t\right)}{\partial {t}^{2}}=\\ \dfrac{2i{\omega }_{1}^{2}}{{k}_{1}{c}^{2}}{d}_{\mathrm{e}\mathrm{f}\mathrm{f}}{E}_{3}\left({\textit{z}},t\right){E}_{2}^{*}\left({\textit{z}},t\right){{\rm{e}}}^{{i}\Delta k{\textit{z}}} \cdot\end{array} $$ $$ \begin{array}{l} \dfrac{\partial {E}_{2}\left({\textit{z}},t\right)}{\partial {\textit{z}}}+\dfrac{1}{{v}_{2}}\dfrac{\partial {E}_{2}\left({\textit{z}},t\right)}{\partial t}+\dfrac{i}{2}{k}_{2}^{''}\dfrac{{\partial }^{2}{E}_{2}\left({\textit{z}},t\right)}{\partial {t}^{2}}=\\ \dfrac{2i{\omega }_{1}^{2}}{{k}_{1}{c}^{2}}{d}_{\mathrm{e}\mathrm{f}\mathrm{f}}{E}_{3}\left({\textit{z}},t\right){E}_{1}^{*}\left({\textit{z}},t\right){{\rm{e}}}^{{i}\Delta k{\textit{z}}}\cdot\end{array} $$ $$\begin{array}{l} \dfrac{\partial {E}_{3}\left({\textit{z}},t\right)}{\partial {\textit{z}}}+\dfrac{1}{{v}_{3}}\dfrac{\partial {E}_{3}\left({\textit{z}},t\right)}{\partial t}+\dfrac{i}{2}{k}_{3}^{''}\dfrac{{\partial }^{2}{E}_{3}\left({\textit{z}},t\right)}{\partial {t}^{2}}=\\ \dfrac{2i{\omega }_{1}^{2}}{{k}_{1}{c}^{2}}{d}_{\mathrm{e}\mathrm{f}\mathrm{f}}{E}_{1}\left({\textit{z}},t\right){E}_{2}^{*}\left({\textit{z}},t\right){{\rm{e}}}^{{i}\Delta k{\textit{z}}} \end{array}$$ (2-2) 式中:
$ {E}_{3} $ 、$ {E}_{2} $ 和$ {E}_{1} $ 分别为泵浦光$ {\mathrm{\omega }}_{3} $ 、信号光$ {\mathrm{\omega }}_{2} $ 和闲频光$ {\mathrm{\omega }}_{1} $ 的电场复振幅;$ {k}_{3} $ 、$ {k}_{2} $ 和$ {k}_{1} $ 为相应的波矢;$ \Delta k= $ $ {k}_{3}-{k}_{2}-{k}_{1} $ 为相位失配量;$ {k}_{i}^{''}={{\partial }^{2}k}_{i}/\partial {\omega }_{i}^{2} $ 表示群速度色散(Group Velocity Dispersion,GVD);$ {v}_{i} $ 为相应的群速度;$ {d}_{\mathrm{e}\mathrm{f}\mathrm{f}} $ 为非线性晶体的有效非线性系数。当忽略色散项时,可以很容易得到公式(2)的一组解析解,但对于飞秒光学参量变换过程,非线性晶体中的群速度色散和群速度失配并不容忽视,这时就需要通过分步傅里叶变换算法对耦合波方程组进行数值求解[11]。为了有较为直观的了解,这里给出忽略色散项和泵浦光消耗时获得闲频光的效率:
$$ {\eta }_{1}=\frac{8{\mathrm{\pi }}^{2}{L}^{2}{\omega }_{1}^{2}}{{\epsilon }_{0}{c}^{3}}\frac{{{d}_{\mathrm{e}\mathrm{f}\mathrm{f}}}^{2}}{{n}_{1}{n}_{2}{n}_{3}}{I}_{2}{\mathrm{s}\mathrm{i}\mathrm{n}{c}}^{2}\left(\frac{\left|\Delta k\right|L}{2}\right) $$ (2-3) 式中:n1、n2和n3为闲频光、信号光和泵浦光在非线性晶体中的折射率;
$ L $ 为三波耦合相互作用距离;$ {I}_{2}=2{\epsilon }_{0}c{n}_{2}{\left|{E}_{2}\right|}^{2} $ 为信号光强度;$ {\epsilon }_{0} $ 为真空中介电常数。公式(3)给出了一个广泛适用的规律,即闲频光的产生效率与非线性晶体品质因子$ \left({F}{O}{M}=\dfrac{{{d}_{\mathrm{e}\mathrm{f}\mathrm{f}}}^{2}}{{n}_{1}{n}_{2}{n}_{3}}\right) $ 以及相位匹配情况息息相关。当满足相位匹配条件即$ \Delta k=0 $ 时,$ {\mathrm{s}\mathrm{i}\mathrm{n}{c}}^{2}\left(\dfrac{\Delta kL}{2}\right)=1 $ ,转换效率达到最大。最为常见的两种相位匹配技术分别是双折射相位匹配技术和准相位匹配技术。根据晶体的双折射特性,如果光波以合适的角度入射,使得参与相互作用的三个光波的折射率满足
$ {\omega }_{1}{n}_{1}+{\omega }_{2}{n}_{2}={\omega }_{3}{n}_{3} $ ,就能达到相位匹配的目的。因此双折射相位匹配也称作角度相位匹配。若信号光和闲频光具有相同的偏振,称之为第Ⅰ类相位匹配;若它们的偏振相互正交,则称之为第Ⅱ类相位匹配。为了充分利用非线性晶体最大的有效非线性系数分量(沿某一特定方向,但三波沿这个方向传播时不一定能满足双折射相位匹配),周期性的反转非线性晶体的非线性系数引入一个倒格矢$ {k}_{\mathrm{\Lambda }} $ 来补偿相位失配——使$ {k}_{\mathrm{\Lambda }}=\Delta k $ ,也能实现有效的频率转换,这种技术称为准相位匹配。倒格矢$ {k}_{\mathrm{\Lambda }} $ 与非线性系数反转周期$ {\varLambda } $ 的关系一般为:$ {{k}}_{\mathrm{\Lambda }}=2\mathrm{\pi }/{\varLambda } $ ,因此理论上只要选择合适的周期性结构,就能够在非线性晶体的整个透光范围内得到有效的参量变换。OPO由泵浦源、谐振腔和非线性晶体三部分构成。其中,中红外飞秒OPO的泵浦源有钛宝石飞秒激光器(中心波长~800 nm)[12]、Yb飞秒激光器(中心波长~1 µm)[13]及其近红外波段飞秒OPO[14]和Er飞秒激光器(中心波长~1.5 µm)[15]等。OPO谐振腔根据腔内振荡光的成分也可以分为两种。当腔内只有信号光或闲频光振荡时,称之为单谐振模式;当信号光和闲频光同时振荡时,称之为双谐振模式。一般而言,双谐振OPO的振荡阈值远低于单谐振OPO,但稳定性也会较差。与纳秒或连续光OPO不同,由于参量转换的即时性,飞秒OPO需要满足严格的同步泵浦条件,即只有当信号光或闲频光脉冲在腔内往返一次或几次后能够和某个泵浦光脉冲在晶体中重合才会得到增益而被放大。同步泵浦条件通过调节OPO腔长使其与泵浦源匹配来实现。一般来说有三种方式。假设OPO和泵浦源的腔长分别为LOPO和Lpump,当Lpump=NLOPO、NLpump=LOPO、LOPO=M/N∙Lpump时,都能满足同步泵浦方式。第一种模式下,OPO的重复频率为泵浦光的N倍,即信号光往返N次后被第二个泵浦脉冲放大。第二种模式则与之相反,重复频率为泵浦光的1/N,即信号光往返一次与第N+1个泵浦光相互作用。第三种模式则是信号光往返N次后被第M+1个泵浦脉冲放大。
用于产生中红外激光的常用非线性晶体包括双折射相位匹配晶体:砷酸钛氧钾(KTiOAsO4, KTA)、磷锗锌(ZnGeP2)、硒镓银(AgGaSe2)、硒化镓(GaSe)等和准相位匹配晶体:周期极化的铌酸锂(PPLN)、周期极化的钽酸锂(PPLT)、周期取向的磷化镓(OP-GaP)和砷化镓(OP-GaAs)等,表1列出了这些晶体的主要光学性质。常用的氧化物非线性晶体中,KTA具有很高的抗损伤阈值,但它对3.5 µm以上的波段具有较强的吸收,而PPLN和PPLT晶体对4 µm以上的波段具有较强的吸收,因此它们一般用于信号光单谐振的OPO。非氧化物非线性晶体具有很高的品质因子,且在长波处的通光范围更大,但由于双光子吸收等因素,无法直接利用成熟的800 nm和1 µm近红外飞秒激光泵浦。
表 1 常用中红外非线性晶体的光学性质总结
Table 1. Summary of the optical properties of widely used mid-infrared nonlinear crystals
Nonlinear crystal Transmittance range/μm FOM*/pm2·V−2 LIDT**/GW·cm−2 Ref. PPLN 0.32-5 52.4 0.039 (20 ns@1.56 μm) [16] PPLT 0.28-5.5 21 0.14 (30 ns@1.06 μm) [16] KTA 0.35-5.3 49.1 1.2 (8 ns@1.064 μm) [16] OP-GaAs 0.9-18 246.5 5 J/cm2 (5 ns, 1.064 μm) [17] OP-GaP 0.4-13 158.6 - [17] ZGP 0.74-12 196 0.03 (30 ns@1.064 μm) [16] GaSe 0.62-20 106.8 0.03 (10 ns@1.064 μm) [16] AgGaSe2 0.76-18 82.3 0.013 (30 ns@1.064 μm) [16] *FOM是按照绝对有效非线性系数的最大分量在1064 nm→1400 nm+4433 nm或1550 nm→2400 nm+4376 nm 的泵浦波长下计算的。 **LIDT:指激光诱导的晶体损伤阈值。 对于飞秒OPO,非线性晶体长度的选择至关重要。一方面,从公式(3)可知转换效率与非线性晶体长度有关,但由于三波在晶体中的群速度不同,导致它们在晶体中传播一定距离后会发生分离,从而存在一个有效相互作用的距离,又称为有效晶体长度。考虑泵浦光和振荡信号光之间的时间走离,假设泵浦光和信号光的脉冲持续均为τ,泵浦光和信号光在晶体中的群速度分别为νp和νs,则泵浦光和信号光的最大有效相互作用距离可通过下式计算:
$$ {L}_{\mathrm{max}}=2\tau {\left|\frac{1}{{v}_{\rm p}}-\frac{1}{{v}_{\rm s}}\right|}^{-1}$$ (2-4) 因此,飞秒OPO中非线性晶体的长度一般不超过其最大有效相互作用距离。另一方面,非线性晶体的相位匹配带宽与其长度成反比,如公式(5)所示:
$$\begin{array}{l} \Delta \lambda = \dfrac{{0.44{\lambda ^2}}}{{cL\left| {GVM} \right|}} \\ GVM = \dfrac{1}{{{v_{\rm i}}}} - \dfrac{1}{{{v_{\rm s}}}} \\ \end{array} $$ (2-5) 式中:νi为晶体中闲频光的群速度。因此,为了获得非常宽谱的信号光振荡,需要牺牲一定的晶体长度。相位匹配带宽还与晶体中信号光和闲频光的群速度失配(Group Velocity Mismatch,GVM)成反比。因此令飞秒OPO工作于简并状态,此时νs=νi,也可以获得非常大的相位匹配带宽。对于非共线相位匹配方式,晶体的相位匹配带宽则可用公式(6)表示:
$$ \begin{array}{l}\Delta \lambda =\dfrac{{\lambda }^{2}}{{c}L\left|GVM\right|}\\ GVM=\dfrac{1}{{v}_{\rm i}\mathrm{cos}(\alpha +\beta )}-\dfrac{1}{{v}_{\rm s}}\end{array}$$ (2-6) 式中:α和β分别为信号光、闲频光与泵浦光的夹角,由此可知,通过引入合适的非共线角,使νicos(α+β)= νs同样可以消除GVM对相位匹配带宽的限制,获得最大的相位匹配带宽,因此非共线相位匹配是获得宽带OPO振荡的另一种有效方案。
Research progress of 2-5 µm mid-IR femtosecond optical parametric oscillator (Invited)
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摘要: 自1994年首次利用克尔透镜锁模钛宝石激光器泵浦RTA光学参量振荡器实现中红外飞秒激光输出以来,在这20多年的时间内,随着高功率近红外泵浦源与各种优质非线性晶体的不断涌现,中红外飞秒光学参量振荡器在平均功率、脉冲宽度、调谐范围等方面都取得了长足的发展,为基础科学研究、生物医疗以及国防安全等领域提供了多样化的应用工具。文中将2~5 µm中红外飞秒光学参量振荡器分为波长可调谐输出型与宽光谱输出型两类,分别重点就这两类中红外飞秒光学参量振荡器的国内外研究进展进行综述,最后对进一步发展趋势进行了展望。高功率、高光束质量中红外飞秒光学参量振荡器和大能量中红外飞秒光学参量振荡器是其中两个重要的发展方向。Abstract: Since the first use of Kerr lens mode-locked Ti: sapphire laser pumped RTA optical parametric oscillator to achieve mid-infrared femtosecond laser in 1994, with the continuous emergence of high-power near-infrared pump sources and various high-quality nonlinear crystals, the mid-infrared femtosecond optical parametric oscillator had made considerable progress in terms of average power, pulse width, and tuning range in the past two decades, providing diverse application tools for basic scientific research, biomedicine, and national defense security. Mid-infrared femtosecond optical parametric oscillators were divided into two types in this paper: wavelength tunable output type and broadband-spectrum output type. The research progress of these two types of 2-5 µm mid-infrared femtosecond optical parametric oscillators at home and abroad were reviewed respectively. Finally, the further development trend was discussed. In view of the outlook, high-power, high-beam quality mid-infrared femtosecond optical parametric oscillators and high-energy mid-infrared femtosecond optical parametric oscillators are two important development directions.
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表 1 常用中红外非线性晶体的光学性质总结
Table 1. Summary of the optical properties of widely used mid-infrared nonlinear crystals
Nonlinear crystal Transmittance range/μm FOM*/pm2·V−2 LIDT**/GW·cm−2 Ref. PPLN 0.32-5 52.4 0.039 (20 ns@1.56 μm) [16] PPLT 0.28-5.5 21 0.14 (30 ns@1.06 μm) [16] KTA 0.35-5.3 49.1 1.2 (8 ns@1.064 μm) [16] OP-GaAs 0.9-18 246.5 5 J/cm2 (5 ns, 1.064 μm) [17] OP-GaP 0.4-13 158.6 - [17] ZGP 0.74-12 196 0.03 (30 ns@1.064 μm) [16] GaSe 0.62-20 106.8 0.03 (10 ns@1.064 μm) [16] AgGaSe2 0.76-18 82.3 0.013 (30 ns@1.064 μm) [16] *FOM是按照绝对有效非线性系数的最大分量在1064 nm→1400 nm+4433 nm或1550 nm→2400 nm+4376 nm 的泵浦波长下计算的。 **LIDT:指激光诱导的晶体损伤阈值。 -
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