Compression of ultrashort laser pulses in a planar hollow waveguide

Several concepts for the compression of ultrashort laser pulses to the few-cycle regime were recently proposed. Most promising among these are filamentation in gases as well as nonlinear propagation inside gas-filled hollow-core fibres. However, both methods are limited in terms of the accessible output pulse energy (typically below ~1 mJ), either by the onset of multiple filamentation for the first case or damage of the fibre for the latter. In this paper, optical pulse generation and nonlinear propagation of laser pulses in a planar hollow waveguide are investigated. A Ti:Sapphire CPA system is utilized with 15 mJ pulse energy and 50 fs duration (FWHM) at 100 Hz. The planar hollow waveguide is composed of two fused silica slabs separated by spacers of 127 mum thickness. Optical pulse with 10-16 fs duration is generated at pulse energies higher than 9 mJ and an energy throughput of 60-70%. The field is decomposed into a set of leaky waveguide modes and nonlinear evolution of the coupled modes is modelled by means of the nonlinear envelope equation (NEE) in a (3+1)-dimensional configuration, enabling for the consideration of guided propagation along the short waveguide axis and free propagation in the perpendicular transversal direction.