Optical frequency combs are coherent light sources consisting of many spectral modes equally spaced by the repetition rate of the generating mode-locked pulse train. Their invention has revolutionized the field of optical frequency metrology, and they are now indispensable tools for high-precision laser spectroscopy to study fundamental physics and for optical frequency standards. T. Hänsch and J. Hall were awarded the 2005 Nobel Prize in Physics for the development of optical frequency combs.Optical frequency combs in the extreme ultraviolet (XUV) have become an indispensable tool for many applications, including optical clocks, high-precision spectroscopy for fundamental physics, and quantum information processing. However, current XUV comb sources require an extremely complex experimental setup based on a very high average-power infrared laser system and an external resonator to achieve sufficient pulse energy for the generation of higher-order harmonics (HHG) in the ultraviolet. This strongly limits the field’s progress, and only a few spectroscopic applications using XUV frequency combs have been demonstrated.
To make XUV comb technology more accessible, the research group of the Max Planck Institute for Quantum Optics, together with the Institute of Photonics and Nanotechnologies CNR and the Politecnico di Milano, proposed a simplified solution based on a low repetition rate optical frequency comb, allowing higher pulse energies even for modest average power, followed by single-pass HHG, which greatly simplifies the experimental setup. Furthermore, the synthesized frequency comb shows an ultra-dense structure of equally distanced modes that can find even newer applications than XUV spectroscopy. In the research published in the journal Optica (link to the article here), the authors report the first demonstration of a low-noise frequency comb with a repetition rate as low as a few tens of kHz. This comb is based on a near-infrared Yb:KYW solid-state mode-locked oscillator using an acousto-optic pulse picker and a solid-state optical amplifier. The repetition rate is adjustable over three orders of magnitude from 40 MHz to 40 kHz. One of the modes of the frequency comb is tightly phase-locked to a cavity-stabilized ultra-low-noise continuous-wave laser. In particular, the authors demonstrate that the low repetition rate comb exhibits low phase noise even after pulse picking and that the comb structure is conserved at 40 kHz repetition rate. These results represent a potential breakthrough in frequency comb spectroscopy at exotic wavelengths.
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