Spectral Bandwidth Tuning of Photoionization-Induced Blue-Shifted Solitons in gas-Filled Hollow-Core Anti-Resonant Fibers

Frequency blue shift of optical solitons, due to light-plasma interactions is an important nonlinear process of laser frequency up-conversion in gas-filled hollow-core fibers, which can be applied for generating ultrafast pulses with tunable wavelengths covering infrared and visible regimes. While the mechanism of photoionization in this frequency up-conversion process has been well studied for a few years, the spectral width tunability of output ultrafast pulses generated in such a hollow-core fiber system has not been comprehensively investigated yet. Here, we demonstrate that the theory of adiabatic soliton propagation could be used to understand the spectral bandwidth evolution of photoionization-induced blue-shifted solitons in gas-filled hollow-core anti-resonant fibers. Experimentally, through varying the gas pressure in the hollow-core fiber and the input pulse energy, high-repetition-rate (10 kHz) tunable ultrafast pulses were obtained at the fiber output with a central wavelength tuning range from ∼900 nm to ∼650 nm and a bandwidth tuning range (at 700 nm) from ∼100 nm to ∼180 nm, corresponding to chirp-compensated pulse width tuning from 7.2 fs to 5.4 fs. These experimental results agree well with the theoretical predictions, and the femtosecond visible light source with high-flexible pulse parameters, demonstrated here, may have some potentials in ultrafast spectroscopy and nonlinear-optics applications.