Ward H, Ouarzazi M N, Taki M, Glorieux P
Laboratoire de Physique des Lasers, Atomes et Molécules, CNRS UMR No. 8523, Centre d'Etudes et de Recherches Lasers et Applications, Université des Sciences et Technologies de Lille, UFR de Physique, Bâtiment P5, F-59655 Villeneuve d'Ascq Cedex, France.
Phys Rev E Stat Nonlin Soft Matter Phys. 2001 Jan;63(1 Pt 2):016604. doi: 10.1103/PhysRevE.63.016604. Epub 2000 Dec 19.
Convective and absolute nature of instabilities in nondegenerate optical parametric oscillators with large transverse section, for negative detunings and in the presence of walkoff, is examined. The asymptotic response of the signal and idler fields to a transverse localized two-dimensional perturbation is evaluated. The presence of walkoff breaks the rotational symmetry in the transverse plane, and the system, at the absolute instability threshold, selects traveling waves propagating in the walkoff direction among an infinity of unstable spatiotemporal modes. We show that in optical parametric oscillators (OPO's) with negative detunings, contrary to the case of positive detunings, the walkoff shrinks the region of convective instabilities, and even may suppress the convective/absolute transition. Hence, in a certain range of parameters, signal field envelopes in the form of wave packets of zero group velocity are found where the instability is absolute at the onset, although the walkoff is present. We also show that nonlinear pattern selection is ruled by the cross-coupling terms appearing in the asymmetric coupled Ginzburg-Landau equations derived near-threshold of the signal and idler generation. The numerical solutions of the original OPO equations confirm the analytical predictions for the values of the instability thresholds and the corresponding selected patterns.
研究了具有大横向截面的非简并光学参量振荡器中，在负失谐且存在走离效应时不稳定性的对流和绝对性质。评估了信号场和闲频场对横向局域二维微扰的渐近响应。走离效应的存在打破了横向平面内的旋转对称性，并且在绝对不稳定性阈值处，系统在无穷多个不稳定时空模式中选择沿走离方向传播的行波。我们表明，在负失谐的光学参量振荡器（OPO）中，与正失谐情况相反，走离效应会缩小对流不稳定性区域，甚至可能抑制对流/绝对转变。因此，在一定参数范围内，发现了零群速度波包形式的信号场包络，尽管存在走离效应，但在起始时不稳定性是绝对的。我们还表明，非线性模式选择由在信号和闲频产生阈值附近导出的非对称耦合金兹堡 - 朗道方程中出现的交叉耦合项决定。原始OPO方程的数值解证实了关于不稳定性阈值值和相应选定模式的分析预测。
