  We theoretically investigate the generation of two entangled beams of light
in the process of single-pass type-I noncollinear frequency degenerate
parametric downconversion with an ultrashort pulsed pump. We find the
spatio-temporal squeezing eigenmodes and the corresponding squeezing
eigenvalues of the generated field both numerically and analytically. The
analytical solution is obtained by modeling the joint spectral amplitude of the
field by a Gaussian function in curvilinear coordinates. We show that this
method is highly efficient and is in a good agreement with the numerical
solution. We also reveal that when the total bandwidth of the generated beams
is sufficiently high, the modal functions cannot be factored into a spatial and
a temporal parts, but exhibit a spatio-temporal coupling, whose strength can be
increased by shortening the pump.
