Coherently enhancing laser pulses in a passive cavity provides ideal conditions for high-order harmonic generation in a gas, with repetition rates around 100 MHz (refs 1,2,3). Recently, extreme-ultraviolet radiation with photon energies of up to 30 eV was obtained, which is sufficiently bright for direct frequency-comb spectroscopy at 20 eV (ref. 4). Here, we identify a route to scaling these radiation sources to higher photon energies. We demonstrate that the ionization-limited attainable intracavity peak intensity increases with decreasing pulse duration. By enhancing nonlinearly compressed pulses of an Yb-based laser and coupling out the harmonics through a pierced cavity mirror, we generate spatially coherent 108 eV (11.45 nm) radiation at 78 MHz. Exploiting the full potential of the demonstrated techniques will afford high-photon-flux ultrashort-pulsed extreme-ultraviolet sources for a number of applications in science and technology, including photoelectron spectroscopy, coincidence spectroscopy with femtosecond to attosecond resolution and characterization of components and materials for nanolithography
We demonstrate nonlinear pulse compression by multi-pass cell spectral broadening (MPCSB) from 860 fs to 115 fs with compressed pulse energy of 7.5 µJ, average power of 300 W and close to diffraction-limited beam quality. The transmission of the compression unit is >90%. The results show that this recently introduced compression scheme for peak powers above the threshold for catastrophic self-focusing can be scaled to smaller pulse energies and can achieve a larger compression factor than previously reported. Good homogeneity of the spectral broadening across the beam profile is verified, which distinguishes MPCSB among other bulk compression schemes.
We demonstrate a high-finesse femtosecond enhancement cavity with an on-axis obstacle. By inserting a wire with a width of 5% of the fun-damental mode diameter, the finesse of F = 3400 is only slightly reduced to F = 3000. The low loss is due to the degeneracy of transverse modes, which allows for exciting a circulating field distribution avoiding the obstacle. We call this condition quasi-imaging. The concept could be used for output coupling of intracavity-generated higher-order harmonics through an on-axis opening in one of the cavity mirrors
We have created a broad spectrum spanning more than an optical octave by launching femtosecond pulses from a battery operated Cr:LiSAF laser into a photonic crystal fiber. Despite the massive broadening in the fiber, the comb structure of the spectrum is preserved, and this frequency comb is perfectly suited for applications in optical frequency metrology.
We describe a novel method for providing geometrical on-axis access to an optical enhancement resonator through an on-axis aperture at one of its mirrors. A superposition of transverse modes in a stable degenerate resonator is used to form a field distribution which avoids the aperture and therefore exhibits small loss. Upon propagation in the resonator the modes acquire a different phase, and an on-axis intensity maximum is formed at a different position. We call this a quasi-imaging resonator, because it is related to imaging in the sense that a hole in the field distribution, exacted by the aperture, is reproduced after a resonator round trip.
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