Abstract
Differentially pumped capillaries, i.e., capillaries operated in a pressure gradient environment, are widely used for nonlinear pulse compression. In this work, we show that strong pressure gradients and high gas throughputs can cause spatiotemporal instabilities of the output beam profile. The instabilities occur with a sudden onset as the flow evolves from laminar to turbulent. Based on the experimental and numerical results, we derive guidelines to predict the onset of those instabilities and discuss possible applications in the context of nonlinear flow dynamics.
Highlights
Differentially pumped capillaries, i.e., capillaries operated in a pressure gradient environment, are widely used for nonlinear pulse compression
We show that strong pressure gradients and high gas throughputs can cause spatiotemporal instabilities of the output beam profile
Because of the broadband waveguiding capabilities, the straightforward experimental handling and the possibility to realize a broad range of core diameters on the order of 0.1 mm – 1 mm, glass capillaries have been widely used in the spectral broadening step of nonlinear pulse compression
Summary
The use of gas-filled, hollow-core waveguides has revolutionized fundamental research and applications in optics and photonics. It can be shown that the gas particle density gradient along the fiber can be employed to wash out the phase-matching of unwanted parametric generation via position-dependent changes in the dispersion landscape Another successful strategy to avoid detrimental ionization effects [20] associated with too high laser intensities is to use capillaries with very large core diameters (i.e. several hundred times the operation wavelength [16]). It has been reported that certain instabilities (of the optical output) occur, if the pressure gradient for a differentially pumped capillary with 350 μm core diameter and 1 m length exceeds 3 bar (nitrogen) [22], but the authors neither investigated this effect nor explained its origin This raises the question of whether and to which extent a high gas throughput can have a detrimental impact on the output laser characteristics. This is because the higher possible gas pressures (due to the wavelength dependence of the self-focusing limit [23]) and the increased fiber dimensions (to allow for reasonable transmission and spectral broadening [20]) are not necessarily compatible with a low gas throughput
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