Generating Carrier-Envelope-Phase Stabilized Few-Cycle Pulses from a Free-Electron Laser Oscillator

We propose a scheme to generate carrier-envelope-phase (CEP) stabilized few-cycle optical pulses from a free-electron laser oscillator. The CEP stabilization is realized by the continuous injection of CEP-stabilized seed pulses from an external laser to the free-electron laser oscillator whose cavity length is perfectly synchronized to the electron bunch repetition. Operated at a midinfrared wavelength, the proposed method is able to drive a photon source based on high harmonic generation (HHG) to explore the generation of isolated attosecond pulses at photon energies above 1 keV with a repetition of $>10\mathrm{MHz}$. The HHG photon source will open a door to full-scale experiments of attosecond x-ray pulses and push ultrafast laser science to the zeptosecond regime.

Figure 1

Temporal shapes of FEL pulses in a perfectly synchronized optical cavity. The profile of the electron bunch at the entrance, $z=0$, and the exit, $z=L_u$, of the undulator is also plotted. The inset is the same FEL pulses plotted with a linear scale.

Figure 2

Temporal shapes of FEL pulses in a perfectly synchronized optical cavity with an external seed laser after 1500, 2000, and 2500 round trips.

Figure 3

Contour plots of the instantaneous phase of FEL pulses in units of $\pi \mathrm{rad}$ (a) without injection seeding and (b) with injection seeding. Contour plots of the instantaneous intensity of FEL pulses normalized to the maximum intensity (c) without injection seeding and (d) with injection seeding.

Figure 4

Fluctuation of the carrier-envelope phase and pulse energy as a function of intracavity seed laser intensity. The broken line is the shot noise intensity.

Figure 5

Fluctuation of the carrier-envelope phase, $\mathrm{\Delta }\varphi$, as a function of the rms jitter in the peak current, $\mathrm{\Delta }I$. The solid curve is the best-fit curve employing a function form $\mathrm{\Delta }\varphi =\sqrt{a^2+b^2(\mathrm{\Delta }I{)}^2}$.