A cavity has a longitudinal hollow having opposite open ends. An excitation source, such as a YAG rod, is accommodated in the hollow of the cavity. A hollow, cylindrical rod holder is inserted into each of the opposite open ends of the hollow and is fitted over a corresponding end of the YAG rod. A seal is fitted into a hollow of the rod holder adjacent to a portion of the rod holder fitted over the end of the YAG rod. The seal includes an annular seal body with a U-shaped section, e.g. made from Teflon®, and fitted over an outer periphery of the YAG rod to grip the YAG rod, as well as a spring fitted in a groove of the seal body provided by the U-shaped section so as to circumferentially surround a gripped portion of the YAG rod.