  We propose periodically-modulated entangled states of light and show that
they can be generated in two experimentally feasible schemes of nondegenerate
optical parametric oscillator (NOPO): (i) driven by continuously modulated pump
field; (ii) under action of a periodic sequence of identical laser pulses. We
show that the time-modulation of the pump field amplitude essentially improves
the degree of continuous-variable entanglement in NOPO. We develop
semiclassical and quantum theories of these devices for both below- and
above-threshold regimes. Our analytical results are in well agrement with
numerical simulation and support a concept of time-modulated entangled states.
