Abstract

Multi-conical emission was predicted¹ to occur in the passive ring cavity with thin cell and free-space diffraction, in both side of the resonance. This work deals with biconical emission numerically observed in a tuned cavity, in the defocusing case, when the ratio of the two first critical wavenumbers is very close to $\sqrt{2}$. The numerical simulations and the analytical study of the equations display very different transverse structures whether mono- or bi-conical emission occurs. Indeed the hexagonal order which spontaneously emerges 23 in the monoconical case, is destroyed in the bi-conical case, leading to a (4-4) bi-square structure with 4 peaks on two concentric rings in the far-field, (Fig. 1), near the instability threshold.The destruction of the hexagonal order is a signature of the strong intermodal coupling, whereas quasi-pattern is expected with weak coupling⁴. As the intensity of the plane-wave input beam increases, a drift bifurcation occurs leading to a new (4-4) pattern with smaller wavelength. Then a (8-8) structure is formed, (Fig.2), which consists in two twisted bisquares, with a 53° rotation angle ensuring the wavevector locking, or the spatial periodicity of the structure.

© 1996 IEEE