Analysis of a Room-Temperature cw Ruby Laser of 10-mm Resonator Length: The Ruby Laser as a Thermal Lens

A nominally plane-parallel ruby laser of 10-mm length was investigated in continuous operation in an ellipsoidal pumping system under water cooling. The laser emits pure transverse modes of low order. The zero-order mode can be described by Gaussian distributions; it corresponds approximately to the mode of a confocal resonator of 9-cm radius of curvature. The resonator curvature is induced by a thermal curvature of the ruby end faces and by a bulk thermal lens effect. The axial-mode frequencies and the emission-center frequency shift to smaller values with increasing pump power as a consequence of the increasing ruby temperature in cw operation. The relaxation pulses of single axial modes do not overlap in this short laser, and the interaction of different axial modes can be observed in the emission of one mode. Consequences of the observed thermal curvature for the transverse mode selection of cw and pulsed crystal lasers are discussed.