Spatial structures in optical parametric amplification

We analyze the spatial aspects of the down-converted field emitted by a cav ityless optical parametric amplifier, both from theoretical/numerical and e xperimental viewpoints. Our model neglects pump depletion and includes a cl assical description of the noise which initiates the process. The ringshape d angular distribution in the far field is determined basically by the phas e-matching conditions. In the near field, the configuration arises from the superposition of contributions from a broad frequency band; it is shown th at the finite duration of the pump pulse introduces a correlation among nei ghboring frequencies that allows for the appearance of a spotlike spatial m odulation. When the spatial configuration of the pump field is changed from a plane wave to a narrow Gaussian, the near-field pattern assumes the form of a roll structure. The experimental observations obtained with a lithium triborate crystal agree qualitatively well with the theoretical picture. [ S1050-2947(99)05608-5].