The Effect of Nitrogen Rotational Response on Ultra-Flat Supercontinuum Generation in Gas-Filled Hollow-Core Photonic Crystal Fiber

We experimentally and numerically investigate flat supercontinuum generation in gas-filled anti-resonant guiding hollow-core photonic crystal fiber filled with argon or nitrogen. The ability to change the dispersion profile of gas-filled hollow-core fibers by simply changing the gas pressure [1], allowed us to tune the zero-dispersion wavelength to maximize the supercontinuum bandwidth when pumping at 1030 nm. We generate an ultra-flat supercontinuum extending from 350 nm up to 2000 nm in both gases, but with significant differences in the spectral power distribution. Although argon and nitrogen exhibit similar Kerr nonlinearity and dispersion, the energy density of the continuum in the normal dispersion region, covering the visible region, is significantly lower when using nitrogen. By comparing results obtained from both gases, we determine the role of the rotational Raman response on the supercontinuum formation. Fig. 1(a) shows an example of the output spectrum for argon and nitrogen at 50 bar when pumping with $12\ \mu\mathrm{J}$ , 1 ps pulses. Clear differences in the energy density in the normal dispersion region (i.e. visible spectral region) can be seen.