We study the Lorenz-Haken instability in a laser with arbitrary mirrors ref
lectivity R. In the limit of slow population difference we demonstrate that
in order to observe the instability the reflectivity must exceed the criti
cal value R-c similar or equal to 0.5379, which coincides with that found r
ecently for the multimode (Risken-Nummedal-Graham-Haken) instability. Sever
al other differences with respect to the uniform field limit R --> 1 are pr
esented. (C) 2000 Elsevier Science B.V. All rights reserved.