This thesis studies theoretically and experimentally the quantum properties of Synchronously Pumped Optical Parametric Oscillators (SPOPO). A symplectic formalism allows us to derive the SPOPO's input-output relations for the quantum field, both in time and frequency domain. We predict that the generated field is highly multimode, and its squeezed modes are frequency combs with different spectral/temporal profiles. This is experimentally demonstrated using a SPOPO composed of a ring cavity and a nonlinear cristal pumped by a train of femtosecond pulses at 400 nm. The obtained states are mesured by a homodyne detection with a pulse-shaped local oscillator. This technique allows the reconstruction of a covariance matrix in a basis of frequency pixels and highlights the existence of quantum correlations between different regions of the optical spectrum. By diagonalizing the covariance matrix, one gets the noise-decorrelated modes and their compression level. We have observed the generation of states described by at least 4 squeezed and decorrelated modes propagating in the same beam. Recent results show states containing up to 8 squeezed co-propagating modes, which paves the way for scalable generation of highly multimode states of light.
