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金刚石晶体以其优异的光学和热物性,已在非线性光学、热传导、光学窗口等领域成为目前炙手可热的晶体材料。图2为金刚石的热导率和透射光谱范围与其他常见晶体材料的对比[38-42]。从图中可以看出,金刚石的热导率远高于目前已知的晶体材料(是常用激光晶体YVO4的380多倍、YAG的140多倍);此外,相比于常见的拉曼晶体YVO4以及光学材料,金刚石的光谱透过范围更宽,在紫外、可见光、近红外及长波红外均具有极高的透过率。表1为金刚石与常见拉曼晶体的特性对比,可以看出金刚石晶体的拉曼增益和拉曼频移均高于其他拉曼晶体,其热导率也是其他常见拉曼晶体的几百倍甚至上千倍。基于以上的优良特性可知,相比于传统的晶体拉曼激光器而言,金刚石拉曼激光器可以承受更高的泵浦功率,可泵浦和发射的波长范围更广,且在相同的泵浦波长情况下只需要更少的阶次就可以达到期望波长[43-45]。除此之外,极高的热稳定性也使得金刚石能够在高温、高强度的严苛工作条件下呈现良好的性能。
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早在1963年,研究人员就在天然金刚石中观察到拉曼散射现象[49],由于天然金刚石的光学质量无法满足作为拉曼增益介质的要求,因此金刚石拉曼激光器并未引起人们的足够关注。随着化学气相沉积法(CVD)等技术手段的成熟,现在人们已获得纯度比天然金刚石高几个数量级的高质量单晶金刚石,这也推动了人们对于金刚石激光器的研究逐渐深入[41, 50-51]。针对金刚石拉曼激光器的研究,主要体现在工作波长拓展、振荡器结构特性、运转模式(如:脉冲/连续、多纵模/单纵模)、输出功率提升等。
2004年,俄罗斯科学院的Kaminskii等人[52]分别利用0.5 μm和1 μm的纳秒和皮秒脉冲泵浦金刚石晶体,并成功测得了若干阶次的Stokes和反Stokes光谱。此后,人们通过金刚石拉曼转换实现了各种不同波长的激光输出,覆盖了紫外[53]、可见光[54-55]、近红外波段(含人眼安全)[48, 56-60]和中红外波段[61],运转方式也已经覆盖了纳秒至飞秒[52, 62-63],以及连续[58, 64]和准连续[1, 38]。基于金刚石晶体的优秀的热物性,金刚石拉曼激光器在高功率输出方面展现了巨大的优势[65]。2015年,Williams等人[43]利用功率为629 W的1 060 nm光纤激光器直接泵浦外腔金刚石拉曼激光器,获得了功率高达381 W的Stokes光输出。随后,Heinzig等[45]通过高功率单通泵浦的金刚石拉曼激光器,在泵浦光功率为970 W时获得了最大输出功率329 W、转换效率40%的拉曼转换输出。Antipov等人[44]利用光束质量恶化的泵浦光对金刚石拉曼激光器进行泵浦,得到稳态功率高达1.2 kW的激光输出。图3为近十几年不同运行方式的金刚石拉曼激光器输出功率与光纤和其他晶体拉曼激光器的对比。从图中可以看出,金刚石拉曼激光器的功率提升十分迅速,其最大输出功率比其他晶体拉曼激光器的功率要高两三个数量级。虽然目前金刚石拉曼激光器所获得的功率低于光纤拉曼激光器,但是得益于金刚石优异的光学性质和热物性,以及随着人造金刚石生长工艺及泵浦功率的提升,金刚石拉曼激光器的输出功率将具有很大的提升空间。
拉曼转换的自相位匹配特性使得拉曼光的空间和相位等特性并不受泵浦光光束特征的影响,在运行功率低于拉曼晶体热畸变阈值的情况下,输出拉曼激光的光束质量可相对于泵浦光得到提升,即产生光束净化现象。过去人们在基于LiIO3[66]、Ba(NO3)2[67]等晶体的拉曼激光器中已成功验证,但受到材料本身特性的限制,传统晶体拉曼激光器的光束净化仅在较低功率密度的情况下有效,这无疑限制了拉曼光束净化在人们广泛关注的高功率激光领域的应用。直至2012年,澳大利亚麦考瑞大学Kitzler等[21]通过对金刚石拉曼激光器高功率连续波转换的研究实验,在一阶金刚石拉曼转换中发现了光束净化效果,图4为实验装置图。输出拉曼激光的光束质量因子M2从泵浦源的1.7下降到一阶Stokes光的1.16 (近场光斑分布见插图),在输出Stokes光功率达到10.1 W时没有发现光损伤情况。随后人们对金刚石拉曼激光器的光束净化以及由此带来的亮度特性展开了研究。
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在拉曼转换过程中,通常利用亮度增强因子(BEF)来表征Stokes光相对泵浦光的亮度的增强程度,由Stokes光和泵浦光亮度计算得来。其中,光束的亮度B定义为[68-69]:
$$B = \frac{P}{{\mathop \lambda \nolimits^2 \mathop M\nolimits_x^2 \mathop M\nolimits_y^2 }} = \frac{P}{{\mathop {(\lambda \mathop M\nolimits^2 )}\nolimits^2 }}$$ (1) 式中:P为激光功率;λ为光束的波长;
$\mathop M\nolimits_x^2 $ 和$\mathop M\nolimits_y^2 $ 分别为光束在x和y方向上的质量因子;M2为总的光束质量因子(${M^2} = {\left( {M_x^2 \cdot M_y^2} \right)^{{1 / 2}}}$ )。从公式(1)可以看出,光亮度与光束功率成正比、与波长和光束质量因子的平方成反比。亮度增强因子BEF等于Stokes光亮度Bs与泵浦光亮度Bp的比,即:$$BEF = \frac{{{B_s}}}{{{B_p}}} = {\eta _{eff}}{\left( {\frac{{{\lambda _p} \cdot M_p^2}}{{{\lambda _s} \cdot M_s^2}}} \right)^2}$$ (2) 式中:
${\eta _{eff}} = {{{P_s}} / {{P_p}}}$ 为Stokes功率与泵浦功率的比值,即拉曼转换效率;λp和λs分别为泵浦光和Stokes光波长;$M_p^2$ 和$M_s^2$ 分别代表泵浦光和Stokes光的光束质量因子。根据公式(2)可以得到Stokes光相对于泵浦光的亮度变化,当BEF>1时,Stokes光的亮度高于泵浦光亮度,即实现了亮度增强,该公式对于级联拉曼激光器同样适用。
Development of beam brightness enhancement based on diamond Raman conversion
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摘要: 具有不同波长的高亮度激光在国防、工业、生命科学等诸多领域发挥着重要作用。但是受限于现有工作物质固有的光谱特性和热物性,传统粒子数反转激光器的波长和输出功率难以兼顾,甚至导致激光在功率提升时光束亮度不升反降。为了克服该难题,近几年人们利用非线性光学技术对光束净化开展了大量研究,即将粒子数反转激光器输出的低光束质量的光束,通过受激拉曼或受激布里渊散射等效应转变为高光束质量激光输出。其中,金刚石晶体以其高拉曼增益系数、极高的热导率和极宽的光谱透过范围等性质,在实现高效率拉曼波长转换的同时展现出优异的光束亮度增强特性,为人们获得高功率、高亮度激光光束提供了新的技术路径。文中对基于金刚石的一阶和级联拉曼转换的光束亮度增强研究进行了综述,并围绕其潜在的应用进行了探讨。Abstract: High brightness laser sources with different wavelengths play an important role in the fields such as defense, industrial, and life sciences etc. However, due to the intrinsic spectral and thermophysical properties of current available laser gain materials, it is difficult to take into account the wavelength and output power of the traditional inversion lasers, which even leads to the decrease of beam brightness. To overcome this problem, beam cleanup by using nonlinear optical technology has been carried out in recent years, which is directly transferring the low beam quality generated from inversion lasers into the high through the effects such as stimulated Raman or Brillouin scattering. Among them, with excellent properties such as high Raman gain coefficient, high thermal conductivity and wide spectral transmission range, diamond exhibits excellent beam brightness enhancement characteristics while realizing high efficiency Raman conversion, which provides a new technical path to generate high power and high brightness laser beam. Here, the development of brightness enhancement based on first-order and cascaded Raman conversion of diamond was reviewed, and its future applications were discussed.
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Key words:
- laser technology /
- diamond /
- stimulated Raman scattering /
- brightness enhancement /
- high-power
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