Self-generating high-order diffraction under Bragg mismatching in LiNbO₃

The self-generated high-order diffraction associated with a conical ring in heavily-doped LiNbO₃:Fe was observed while two coherent beams intersect inside the crystal. It is proposed that multiple scattering may occur under the Bragg mismatching condition if the corresponding coupling coefficient is large enough. The possible mechanisms for generating the high-order diffraction and the conical ring are presented, including large refractive index change in the crystal, Bragg-mismatching diffraction, Bragg degeneracy, and multiple scattering.