  We present a systematic study on linear propagation of ultrashort laser
pulses in media with dispersion, dispersionless media and vacuum. The applied
method of amplitude envelopes gives the opportunity to estimate the limits of
slowly warring amplitude approximation and to describe an amplitude
integro-differential equation, governing the propagation of optical pulses in
single cycle regime. The well known slowly varying amplitude equation and the
amplitude equation for vacuum are written in dimensionless form. Three
parameters are obtained defining different linear regimes of the optical pulses
evolution. In contrast to previous studies we demonstrate that in femtosecond
region the nonparaxial terms are not small and can dominate over transverse
Laplacian. The normalized amplitude nonparaxial equations are solved using the
method of Fourier transforms. Fundamental solutions with spectral kernels
different from Fresnel one are found. One unexpected new result is the relative
stability of light pulses with spherical and spheroidal spatial form, when we
compare their transverse enlargement with the paraxial diffraction of lights
beam in air. It is important to emphasize here the case of light disks, i.e.
pulses whose longitudinal size is small with respect to the transverse one,
which in some partial cases are practically diffractionless over distances of
thousand kilometers. A new formula which calculates the diffraction length of
optical pulses is suggested.
