Avalanches of resonant phonons generated by stimulated emission within
the optically excited (E) over bar(E-2) doubler are observed in a 400
-mu m thick fiber of single-crystalline rubp. In comparison with bull;
ruby, the avalanches develop fully in a shorter rime because the phon
ons are confined to the active medium by reflection from the fiber bou
ndaries. Position-independent rate equations adequately describe tile
dynamics.