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Spectral control of a cavity-based x-ray free-electron laser via active mode locking

Nanshun Huang, Hanxiang Yang, and Haixiao Deng*

  • *Contact author: denghx@sari.ac.cn
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Phys. Rev. Research 8, 013029 – Published 14 January, 2026

DOI: https://doi.org/10.1103/2fpd-ssv5

Abstract

Precise spectral control in the hard x-ray regime remains a long-standing challenge that limits applications in atomic-scale science and ultrafast spectroscopy. We present a proposal and numerical study of an actively mode-locked cavity-based x-ray free-electron laser that achieves deterministic spectral programmability with phase-locked pulse trains and comblike spectra, by coherently modulating the electron-beam energy. Three-dimensional time-dependent simulations predict 700μJ total energy, 30GW peak power, and frequency-comb spacing of 1.55eV set by the modulation frequency. We further develop selective single-line amplification via undulator tapering and absolute frequency positioning through modulation-laser tuning with meV precision. Importantly, stable mode-locked operation persists under >80% peak-to-peak cavity-reflectivity variations, substantially relaxing requirements on x-ray optics. These results establish active mode locking as a practical route to fully coherent, spectrally agile hard x-ray sources and enable opportunities in time-resolved core-level spectroscopy, x-ray quantum optics, and precision metrology.

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Phys. Rev. Research 8, 013029– Published 14 January, 2026
  • Received 1 October 2025
  • Accepted 3 December 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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