Exploring nonlinear pulse propagation, Raman frequency conversion and near octave spanning supercontinuum generation in atmospheric air-filled hollow-core Kagomé fiber

Abstract

We have demonstrated Raman frequency conversion and supercontinuum light generation in a hollow core Kagomé fiber filled with air at atmospheric pressure, and developed a numerical model able to explain the results with good accuracy. A solid-state disk laser was used to launch short pulses (~6ps) at 1030nm into an in-house fabricated hollow core Kagomé fiber with negative core curvature and both ends were open to the atmosphere. The fiber had a 150 THz wide transmission window and a record low loss of ~12 dB/km at the pump wavelength. By gradually increasing the pulse energy up to 250 µJ, we observed the onset of different Kerr and Raman based optical nonlinear processes, resulting in a supercontinuum spanning from 850 to 1600 nm at maximum input power. In order to study the pulse propagation dynamics of the experiment, we used a generalized nonlinear Schrödinger equation (GNLSE). Our simulations showed that the use of a conventional damping oscillator model for the time-dependent response of the rotational Raman component of air was not accurate enough at such high intensities and large pulse widths. Therefore, we adopted a semiquantum Raman model for air, which included the full rotational and vibrational response, and their temperature-induced broadening. With this, our GNLSE results matched well the experimental data, which allowed us to clearly identify the nonlinear phenomena involved in the process. Aside from the technological interest in the high spectral density of the supercontinuum demonstrated, the validated numerical model can provide a valuable optimization tool for gas based nonlinear processes in air-filled fibers.

Strong nonlinear optical effects in micro-confined atmospheric air

“…This reveals that the Raman comb dynamics do not imply significant loss. Finally, the comb structure of the spectrum shows an insignificant to zero effect from Kerr nonlinearity, even at energy value as high as 1.03 mJ, which contrasts with those obtained with 6 ps [31].…”

Hybrid photonic-crystal fiber

“…Experimentally, simply using an air-filled broadbandguiding HC-PCF, Mousavi et al (2017) demonstrated a variety of nonlinear effects, including supercontinuum formation between 850 to 1600 nm, when pumping at 1030 nm. A more exotic supercontinuum mechanism based on the dispersion caused by the resonances in a broadband-guiding HC-PCF has also recently been reported (Sollapur et al, 2017), leading to broadening extending from 200 nm to 1.7 μm.…”