Optical bistability in a bulk heavily doped Er:Yb:phosphate glass laser

We consider the output of a longitudinally pumped heavily doped Er:Yb:phosphate glass laser. We experimentally observe that the characteristic curve output-versus-input is not linear and exhibits bistability. A theoretical approach, based on a thermal lensing effect that is different depending on whether the laser is ON or OFF and on the overlapping mismatch between the pump and the laser spot sizes, provides an excellent description of the experimental results.