  We present in this paper a general model for determining the quantum
properties of the light generated by a synchronously pumped optical parametric
oscillator (SPOPO) operating below threshold. This model considers time and
frequency on an equal footing, which allows us to find new quantum properties,
related for example to the carrier envelope offset (CEO) phase, and to consider
situations that are close to real experiments. We show that, in addition to
multimode squeezing in the so-called 'supermodes', the system exhibits
quadrature entanglement between frequency combs of opposite CEO phases. We have
also determined the quantum properties of the individual pulses and their
quantum correlations with the neighboring pulses. Finally, we determine the
quantum Cramer-Rao limit for an ultra-short time delay measurement using a
given number of pulses generated by the SPOPO.
