  Exact Superstring solutions are constructed moving in 4-D space-time with
positive curvature and non-trivial dilaton and antisymmetric tensor fields. The
full spectrum of string excitations is derived as a function of moduli fields
$T^{i}$ and the scale $\mu^2=1/(k+2)$ which induced by the non-zero background
fields. The spectrum of string excitations has a non-zero mass gap $\mu^2$ and
in the weak curvature limit ($\mu$ small) $\mu^2$ plays the role of a well
defined infrared regulator, consistent with modular invariance, gauge
invariance, supersymmetry and chirality.
  The effects of a covariantly constant chomo-magnetic field $B$ as well as
additional curvature can be derived exactly up to one string-loop level. Thus,
the one-loop corrections to all couplings (gravitational, gauge and Yukawas)
are unambiguously computed and are finite both in the Ultra-Violet and the
Infra-Red regime. These corrections are necessary for quantitative string
superunification predictions at low energies. Similar calculations are done in
the effective field theory. The one-loop corrections to the couplings are also
found to satisfy Infrared Flow Equations.
