Coherent ultraviolet frequency combs generated by resonant dispersive waves

arXiv:2610.12620v1 Announce Type: new Abstract: Resonant dispersive wave (RDW) emission in gas-filled anti-resonant hollow-core fibers (AR-HCF) offers a wavelength-tunable route to ultraviolet (UV) and vacuum ultraviolet (VUV) frequency comb generation. RDW emission is expected to be coherent fundamentally, but various noise sources are expected to limit the coherence, including amplitude-to-phase fluctuations due to input pulse intensity noise and plasma formation. It is critical, therefore, to establish the limits of the coherence of UV RDW pulses to determine their applicability for precision spectroscopy. Here, we demonstrate the coherence of RDW emission at 260~nm generated with an amplified Yb-doped fiber system at a 2-MHz repetition rate with a self-heterodyne beat measurement between the RDW and the fourth harmonic of the source. We isolate the RDW contribution to the frequency noise by coherently subtracting the carrier-envelope offset noise contribution measured simultaneously with an f-2f interferometer. We observe a beatnote linewidth below 10~kHz over timescales longer than 100~ms, and we find a conservative upper bound of 1~rad on the integrated RMS phase deviations over a frequency range spanning 1 to 30~kHz. The dominant frequency noise contributions are consistent with environmental noise of the interferometers and AR-HCF coupling fluctuations. These results provide strong evidence that RDW emission is coherent under well-controlled experimental conditions, confirming its potential for UV and VUV frequency comb generation.