  Parametric down-conversion in a nonlinear crystal is a widely employed
technique for generating quadrature squeezed light with multiple modes, which
finds applications in quantum metrology, quantum information and communication.
Here we study the generation of temporally multimode pulsed squeezed light in a
synchronously pumped optical parametric oscillator (SPOPO) operating below the
oscillation threshold, while considering the presence of non-compensated
intracavity group-velocity dispersion. Based on the developed timedomain model
of the system, we show that the dispersion results in mode-dependent detuning
of the broadband supermodes of the pulsed parametric process from the cavity
resonance, as well as linear coupling between these supermodes. With the
perturbation theory up to the second order in the coupling coefficients between
modes, we obtained a solution for the supermode amplitudes given an arbitrary
number of modes and pump level. The dispersion affects the quantum state of the
supermodes by influencing their squeezing level and the rotation of the
squeezing ellipse. It also affects the entanglement among the supermodes,
leading to reduced suppression of shot noise level as measured in the balanced
homodyne detection scheme. Furthermore, our study highlights the potential of
SPOPO with group-velocity dispersion as a testbench for experimental
investigations of multimode effects in linearly evanescent coupled parametric
oscillators.
