Mirror effects and induced coherence in parametric down-conversion

Among interference effects that have been observed using parametric down-conversion light, some experiments show that second and fourth order interference effects can be observed between independent light beams. Also, it has been shown that the presence of mirrors reflecting the pump and the down converted beams can give rise to interference effects. In this paper we show that these mirror effects can be explained alternatively by means of a coherence induced between initially independent beams. Our analysis is based on a longitudinal multimode approach and leads to intensity and coincidence distributions and fringe visibilities. We also present experimental results showing that mirror effects can be observed even using a pump beam with a coherence length much smaller than the distance between crystal and pump mirror.