Impact of Waveguide Radius on the Spectral Bandwidth of Frequency-Tunable Ultrashort Ultraviolet Pulses

The spectral modulation of ultrashort pulses through Cherenkov soliton radiation [1], or resonant dispersive wave (RDW) emission [2]–[3], is crucial for nonlinear frequency conversion and spectro-temporal shaping of laser pulses. Applications requiring tunable spectra with broad or narrow bandwidth underscore the need to understand and control the spectral bandwidth of RDW emission. This study investigates modifying waveguide dispersion via lateral size changes to adjust the spectral bandwidth of frequency-tunable ultrashort light sources in the deep ultraviolet spectrum. We demonstrate how coherence length―the key parameter defining the spectral bandwidth of RDW emission―can be engineered using hollow-core waveguides (HCW) filled with argon gas. RDW emission was generated in multimode HCWs with inner radii of 70 μm and 125 μm, each 1 meter long. The waveguides were pumped with ultrashort pulses at 800 nm and 1 kHz. Gas pressures were chosen so both waveguides experience similar phase mismatch, generating RDW with comparable peak central wavelength in the DUV. To achieve similar soliton fission lengths, the initial pulses were compressed to about 18 fs for the smaller HCW and approximately 10 fs for the larger one. We demonstrated that adjusting the waveguide radius to alter the coherence length can significantly broaden the spectral bandwidth, as shown in Fig. 1a. The larger HCW generated a broader spectral bandwidth, attributed to the wider coherence length bandwidth. Fig. 1b illustrates the variation of coherence length between soliton pulses and dispersive waves with wavelength, highlighting its impact on the spectral bandwidth of frequency-tunable deep ultraviolet pulses. Notably, no unwanted frequencies were generated in the visible range, as the smaller HCW exhibited a shorter coherence length in this range. This feature simplifies the subsequent frequency filtering process. It should be noted that while nonlinear and plasma effects can influence phase mismatch and coherence length, our investigation, which involved a wide range of input energy scans for both HCWs, revealed that the dispersive characteristics of the waveguide were the dominant factor in adjusting the bandwidth of these tunable DUV pulses.