Theoretical analysis of self-phase-modulation-enabled spectral selection in hollow-core fibers: effect of ionization

We propose the generation of wavelength-tunable femtosecond pulses with µJ energy based on spectral broadening in hollow-core fibers (HCFs) filled with noble gas. A proper combination of gas type, gas pressure, and the HCF core radius ensures that the broadened spectrum consists of isolated spectral lobes; the selection of the outermost spectral lobes produces nearly transform-limited pulses. We performed a detailed numerical investigation of this method based on an antiresonant HCF with a 20 µm core radius filled with 17.3 bar Xe gas. By using 1.03 µm, 200 fs pulses with up to 20 µJ energy as the excitation pulse, this method can produce 100 fs pulses tuned from 0.74 to 1.25 µm with up to 5 µJ pulse energy. Further energy scaling is limited by the onset of ionization, which reduces the wavelength tuning range and causes a significant spectral blue shift. Increasing the input pulse energy beyond 45 µJ creates a strong spectral lobe peaking in the wavelength range of 0.92–0.95 µm with >10µJ energy. These results represent a two orders of magnitude improvement in the pulse energy compared to current tunable femtosecond sources based on nonlinear wavelength conversion in solid-core optical fibers.