Spatial effects in the linear propagation of drastically ultrashort optical pulses in single-mode waveguides

We analyze the effect of the varying transverse field profile associated to the linear propagation of ultrashort pulses along an optical waveguide. This effect has been systematically ignored in the literature. Based on numerical calculations, we analyze the magnitude of such effect to justify the use of the standard model, at least for pulses larger than about 5–7 optical carrier cycles. Theoretical considerations are presented about the extreme difficulty of even establishing the initial mathematical condition of the propagation problem under such circumstances.