自由电子激光器







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The free-electron laser FELIX at the FOM Institute for Plasma Physics Rijnhuizen


自由电子激光器（FEL），所产生激光束的光学性质与传统激光器一样，具有高度相干、高能量的特点，其不同点在于其特殊的光源产生机制。传统利用气体、液体或固体（如半导体激光器）作为激光介质的激光器，其激光产生会使原本处于束缚态的原子或分子受到激发；对于FEL，激光产生则依靠将在磁场中运动的相对论电子束的动能转换为光子能量。由于电子束可以在磁场中自由移动，故命名为“自由电子激光器”。激光产生过程中没有传统意义上的介质，不需要实现粒子数反转，因此，这种激光不依赖于受激发射。自由电子激光器的核心是电子源（通常是粒子加速器）与相互作用区（把电子动能转换为光子能量）。


由于自由电子处于连续态，从理论上说其辐射波长不受固定波长限制。自由电子激光器比任何传统激光器都具有更宽的频带，因此调谐范围更宽，当前可涵盖微波，太赫兹，远红外，可见光区，甚紫外直至X射线。


自由电子激光器发明于1976年，发明者为斯坦福大学的約翰·梅迪(John Madey)。其研究核心基于Hans Motz的关于摇摆磁场构型的工作。梅迪利用24MeV的电子束和5米长的摇摆器用于放大信号。不久之后，其他拥有加速器的实验室也加入到这种激光器的开发中来。



产生机制


为了产生自由电子激光，一束电子被加速至接近光速（相对论速度）。之后，电子束通过由周期性横向磁场（通过在光腔中设置与电子束行进方向成变化夹角的磁体产生）构成的自由电子振荡器。产生周期性磁场的磁体阵列又被称为“波荡器(undulator)”或“摇摆体(wiggler)”，这是因为它们会作用于电子束使之形成正弦形状的路径。在此路径上对电子进行加速会使之发射光子（同步辐射）。由于电子周期运动与已发射光场同相，得到的是相干叠加的光场，即自由电子激光。所发射的光波长可以通过改变电子束能量或波荡器的磁场强度进行调节。



现状


因为自由电子激光器中的电子需要具有相对论速度，产生这样速度的电子通常是极为复杂的事情。除此以外，电子的同步质量要好，这使得当前的自由电子激光器复杂而昂贵，解决方案之一便是集成到现有设备中来（如位于汉堡的DESY(德国电子加速器)）。截至2016年，全球共有21台自由电子激光器，另有15台在建或计划建造。尽管自由电子激光涵盖全部光谱范围，具体使用则是针对某一特定频率范围。例如，位于杜布纳粒子物理实验室的FEL工作于毫米波段，汉堡的FLASH 1 工作于深紫外段（6-30nm）, FLASH 2 (4-80 nm)。当前最短工作波长（0.06 nm）的 FEL 則位於日本兵庫縣的SACLA。未来的FEL（如同样建造于汉堡DESY的欧洲X射线自由电子激光器 (European XFEL)）将会覆盖软硬X射线，涵盖0.05 - 4 nm范围， 即X射线自由电子激光器 （XFEL）。



更多信息



  • Madey, John, "Stimulated emission of bremsstrahlung in a periodic magnetic field". J. Appl. Phys. 42, 1906 (1971)

  • Madey, John, Stimulated emission of radiation in periodically deflected electron beam, US Patent 38 22 410,1974

  • Boscolo, et al., "Free-Electron Lasers and Masers on Curved Paths". Appl. Phys., (Germany), vol. 19, No. 1, pp. 46-51, May 1979.

  • Deacon et al., "First Operation of a Free-Electron Laser". Phys. Rev. Lett., vol. 38, No. 16, Apr. 1977, pp. 892-894.

  • Elias, et al., "Observation of Stimulated Emission of Radiation by Relativistic Electrons in a Spatially Periodic Transverse Magnetic Field", Phys. Rev. Lett., 36 (13), 1976, p. 717.

  • Gover, "Operation Regimes of Cerenkov-Smith-Purcell Free Electron Lasers and T. W. Amplifiers". Optics Communications, vol. 26, No. 3, Sep. 1978, pp. 375-379.

  • Gover, "Collective and Single Electron Interactions of Electron Beams with Electromagnetic Waves and Free Electrons Lasers". App. Phys. 16 (1978), p. 121.

  • "The FEL Program at Jefferson Lab" [1]


  • Brau, Charles, Free-Electron Lasers, Boston: Academic Press, Inc., 1990 

  • http://www.riken.jp/en/research/environment/sacla/

  • https://web.archive.org/web/20160718050902/http://www.xfel.eu/research/beamlines/




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