Experimental determination of the fundamental-mode diameter in solid-state lasers

A method to determine directly the radius of the fundamental mode in a laser crystal has been developed. The radius is measured by comparison of the distribution of the spontaneous emission in the pumped region during laser operation with the fluorescence distribution without laser emission. Measuring the mode radius with various pump powers enables one to optimize the overlap between the pump and the cavity beam and to determine the dioptric power of the thermally induced lens.