Existence of two coupling constants in microchip lasers

We implemented two different experimental setups to probe directly the nonl inear coupling between modes in microchip lasers. We show that, remarkably, the results can be interpreted by use of Lamb's coupling constant. In an E r, Yb:glass microchip laser, we measured C-12 = 0.80 between longitudinal m odes and C-xy = 0.95 between orthogonally linearly polarized eigenstates. T he high values obtained give some physical insight into the single-frequenc y operation of such lasers. (C) 2000 Optical Society of America. OCIS codes : 140.3500, 140.3480.