UXH Unidirectional Pulse Propagation Equation UPPE - Propagates a laser pulse through a gas-filled hollow capillary. eXtreme NonLinear Optics XNLO - Calculates the single atom response for a given laser pulse. High Harmonic Generation & Propagation HHGP - Propagates the generated high harmonics through a gas-filled hollow capillary to the capillary end.