PURPOSE: To improve the doubling of interaction in the conversion efficiency of II type phase matching even in a single passage geometric arrangement in addition to a resonance ring geometric arrangement, to improve other non- linear interaction such as the generation of increasing/decreasing frequency and to improve the characteristics of a beam passing a double refractive ctystal requiring phase matching independently of non-linear interaction.

CONSTITUTION: The system consists of a double refractive crystal material 58 capable of forming an angular difference between pointing vectors related to two polarized states of a beam in the material and provided with an incident face 60 and a device for selecting an incident angle so as to compensate the angular difference between the pointing vectors by an angular difference between wave vectors, making a beam incident upon the material 58 at an incident angle inclined from the incident face 60 so as to form the angular difference between the wave vectors related to two polarized states by a refractive effect generated on the incident face 60 and orienting the crystal axis of the material 58 so that requirement for phase matching on the incident face 60 is substantially satisfied.