Oscillators are at the heart of optical lasers, providing stable, transform-limited pulses. Until now, laser oscillators have been available only in the infrared to visible and near-ultraviolet (UV) spectral region. In this paper, we present a study of an oscillator operating in the 5- to 12-keV photon-energy range. We show that, using the Kα1 line of transition metal compounds as the gain medium, an X-ray free-electron laser as a periodic pump, and a Bragg crystal optical cavity, it is possible to build X-ray oscillators producing intense, fully coherent, transform-limited pulses. As an example, we consider the case of a copper nitrate gain medium generating ∼ 5 × 1010 photons per pulse with 37-fs pulse length and 48-meV spectral resolution at 8-keV photon energy. Our theoretical study and simulation of this system show that, because of the very large gain per pass, the oscillator saturates and reaches full coherence in four to six optical-cavity transits. We discuss the feasibility and design of the X-ray optical cavity and other parts of the oscillator needed for its realization, opening the way to extend X-ray–based research beyond current capabilities.

振荡器是光学激光器的核心，可提供稳定的，受变换限制的脉冲。到目前为止，激光振荡器仅在红外到可见光和近紫外（UV）光谱范围内可用。在本文中，我们对在5至12keV光子能量范围内工作的振荡器进行了研究。我们通过使用ķα1个过渡金属化合物作为增益介质，X射线自由电子激光作为周期泵以及布拉格晶体光学腔，可以构建产生强烈的，完全相干的，受变换限制的脉冲的X射线振荡器。例如，考虑硝酸铜增益介质产生约5×1010在8keV光子能量下，每个脉冲的光子具有37fs的脉冲长度和48meV的光谱分辨率。我们对该系统的理论研究和仿真表明，由于每通增益非常大，振荡器会在四到六个光腔传输中达到饱和并达到完全相干。我们讨论了X射线光学腔以及振荡器实现所需的其他部分的可行性和设计，从而为将基于X射线的研究扩展到当前能力以外的方式开辟了道路。