Direct Observation of the Second-Order Coherence of Parametrically Generated Light

The second-harmonic spectrum of the output of an optical parametric emitter at degeneracy exhibits a narrow peak, much sharper than the spectrum of the parametric light. This peak is a manifestation of the second-order correlations in parametric light. Alternatively, it can be considered as resulting from the recombination of simultaneously generated (twin) wave packets. The twin-wave-packet correlation time is shown to be limited by the first-order coherence time of parametric light.