Spatial structures in optical parametric amplification

We analyze the spatial aspects of the down-converted field emitted by a cavityless optical parametric amplifier, both from theoretical/numerical and experimental viewpoints. Our model neglects pump depletion and includes a classical description of the noise which initiates the process. The ringshaped angular distribution in the far field is determined basically by the phase-matching conditions. In the near field, the configuration arises from the superposition of contributions from a broad frequency band; it is shown that the finite duration of the pump pulse introduces a correlation among neighboring frequencies that allows for the appearance of a spotlike spatial modulation. 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].