Abstract

Recent experiments [1] give evidence of transverse pattern formation in Optical Parametric Oscillators (OPO’s). The variety of possible patterns is highly enhanced when considering the extra degree of freedom associated with the polarization of light. For example, in a type-II OPO the down converted beams (signal and idler) are orthogonally polarized and patterns in the transverse plane for each linear polarization can be formed. Here we address the question of vectorial properties of transverse pattern formation and Bloch domains walls in type-II OPO when a direct polarization coupling between the down converted fields (DCF) is taken into account. First, we show that the presence of a birefringent and/or dichroic mirror into a type-II OPO give rise to Bloch walls between domains of different phase. In these walls the field amplitude does not vanish while the phase rotates when going from one domain to the neighboring one. These domain appear here for positive or null detunings, where the zero wavenumber is selected [2]. We find field defects at points along the wall, where sections of the wall of different chirality join (Fig. 1). Two dynamical regimes are found: in a first one the vector field approaches a final homogeneous state, while in a second one walls are continuously generated and annihilated. In the first case flat Bloch walls do not move and in the second one, walls of different chirality move spontaneously with opposite velocities. Secondly we consider a type-II OPO with an intracavity quarter wave-plate which also provides a polarization coupling. For negative DCF’s detunings transverse intensity patterns are predicted and numerically observed. The structures arise spontaneously and exhibit a competition between two concentrics rings of unstable modes in the far field (Fig. 2). For long times stripe intensity patterns with an arbitrary orientation can be selected in the signal and idler near field (Fig. 3).

© 2001 EPS