ABSTRACT
This chapter provides a review of both theoretical and experimental results that are obtained on the Non-Degenerate Optical Parametric Oscillator above threshold operation. In particular it is shown that most of oscillator's characteristics can be understood from the fact that the non-linear crystal emits highly correlated twin photons. The chapter studies the correlation properties between the intensities of the corresponding twin fields. It provides a simple treatment in terms of “corpuscular” photons and gives a proof of the corpuscular model in a fully quantum manner. The chapter provides a classical representation of the fields but in which vacuum fluctuations are properly accounted. This “classical” approach yields the noise spectra of the various measurable quantities in the general case. The chapter presents observation of quantum noise reduction on the difference between the intensities of the two twin beams. It deals with new possible applications of the twin photons to spectroscopy and to the generation of amplitude squeezed laser beams.
