Boosting the Coherence of X-Ray Free-Electron Lasers

Amazing stuff!

"X-ray free-electron lasers (XFELs) have transformed the study of matter by delivering femtosecond and attosecond pulses at angstrom wavelengths, enabling direct observation of ultrafast structural and electronic dynamics. Despite these successes, XFELs have long lacked a capability central to precision optical science: stable temporal phase coherence.

Most XFEL facilities operate in the self-amplified spontaneous-emission (SASE) regime, in which radiation originates from microscopic shot noise in an electron beam. This mechanism produces extremely bright pulses, but shot-to-shot fluctuations in their temporal structure limit their use in phase-sensitive experiments useful for metrology, interferometry, and ultrafast spectroscopy. ...

In conventional lasers, mode locking enforces a fixed phase relationship among an optical cavity’s longitudinal modes—discrete, equally spaced frequencies that resonate with the cavity. Without mode locking, these modes would oscillate independently, and their random phases would lead to noisy emission in time. When mode locking synchronizes their phases, the modes interfere constructively at regular intervals, producing a periodic train of ultrashort pulses. ...

XFELs, however, lack a physical resonator and therefore do not possess longitudinal modes in the usual sense of a cavity-based laser. They operate in a single pass, as electron bunches propagating through a sequence of undulators emit x-ray radiation. Nevertheless, theory has shown that an effective set of longitudinal modes can be synthesized by inserting, between undulator modules, magnetic chicanes that introduce a fixed delay between successive amplification stages. These delays define a comb of allowed frequencies—sometimes described as “synthetic” longitudinal modes—whose spacing is determined by the relative temporal slippage between the emitted x rays and the electrons. Recent experiments have demonstrated that such multichicane schemes enhance temporal coherence in SASE operation ..."

From the abstract:

"X-ray free-electron lasers (FELs) are powerful photon sources offering a wide wavelength range, subfemtosecond pulse duration, and high brightness. Most x-ray FELs are based on self-amplified spontaneous emission (SASE). SASE-FEL radiation has excellent transverse but only limited longitudinal coherence, with power and spectral profiles consisting of multiple randomly distributed spikes.

In this Letter, we present the first experimental demonstration of mode-locked SASE, which generates periodic trains of phase-locked sub femtosecond pulses, thus providing an x-ray analog of the optical frequency comb.

Our approach combines the mode-coupled SASE scheme, where magnetic chicanes between the undulator modules of the FEL increase the coherence of the output radiation, and an external optical laser that restricts the FEL amplification to periodic and short regions of the electron bunch.
The work relies on evidence in the frequency and time domains for photons and electrons, respectively, and will benefit investigations of ultrafast dynamics as well as coherent spectroscopy, and enable new types of experiments requiring phase-correlated x-ray pulses."

Physics - Boosting the Coherence of X-Ray Free-Electron Lasers "Mode locking—a laser technique that revolutionized optical physics—has been extended to x rays, producing stable trains of attosecond pulses with unprecedented phase coherence."

Demonstration of Mode-Locked Frequency Comb for an X-Ray Free-Electron Laser (open access)

Fig. 1 (a) Schematic layout of the experiment. (b) and (c) Longitudinal phase space measurements of the electron beam without and with the 790-nm seed laser, respectively.