  We propose an efficient approach for time integration of Klein-Gordon
equations with highly oscillatory in time input terms. The new methods are
highly accurate in the entire range, from slowly varying up to highly
oscillatory regimes. Our approach is based on splitting methods tailored to the
structure of the input term which allows us to resolve the oscillations in the
system uniformly in all frequencies, while the error constant does not grow as
the oscillations increase. Numerical experiments highlight our theoretical
findings and demonstrate the efficiency of the new schemes.
