Stability and coherence of ultraviolet resonant dispersive wave emission in gas-filled anti-resonant hollow-core fibers

Ultrafast ultraviolet (UV) and vacuum-ultraviolet (VUV) light sources enable a wide range of applications in spectroscopy and metrology. These spectral regions are valuable because many electronic transitions occur at UV and VUV wavelengths, while the corresponding photon energies can also enable photoemission-based measurements. Motivated by frequency comb applications, we focus on developing UV/VUV light sources that preserve the coherence required for comb-based measurements. Such sources would combine access to short-wavelength transitions with the high precision of frequency comb measurements. However, UV/VUV frequency comb generation typically requires nonlinear frequency conversion of infrared pulses, which can transfer or amplify noise from the driving laser. The coherence of the generated UV/VUV light must therefore be characterized. We investigate nonlinear UV/VUV generation in gas-filled anti-resonant hollow-core fibers (AR-HCFs), focussing on resonant dispersive wave (RDW) emission. RDW emission can efficiently convert infrared light into the UV/VUV, but requires sub-35 fs pulses that are not directly supplied from standard laser amplifier systems. A nonlinear pulse compression stage is therefore needed to prepare the driving pulses. Since both the nonlinear compression stage and RDW emission stage can influence the stability of the generated light, we characterize the stability of both stages to identify noise sources at different points in the system. First, we examine the stability of self-phase-modulation-based pulse compression in gas-filled AR-HCFs. Through simulations and experiments, we show that the AR-HCF wall thickness strongly enhances or suppresses modulational instability, a process which amplifies stochastic noise, leading to degraded compressed-pulse stability. By choosing a wall thickness that suppresses modulational instability, stable pulse compression can be achieved. Finally, we investigate the relative coherence of UV RDW emission near 260~nm. Using a UV f-4f interferometer, the carrier-envelope-offset frequency (f₀) of the RDW is measured. After removing the driving field's own f₀ fluctuations, the resulting analysis shows that RDW emission preserves sufficient coherence to produce a measurable UV beat note, while also revealing additional noise from the nonlinear conversion process. Together, these results demonstrate the feasibility of coherent UV frequency comb generation by RDW emission.