Multimode instability in ring fiber lasers

We analyze the conditions under which the Risken-Nummedal-Graham-Haken mult imode instability can be observed in ring-cavity class-B three- and four-le vel lasers. Relevant examples of these are rare-earth doped fiber lasers. W e show that the standard two-level laser model can be applied to three- and four-level laser systems under quite general conditions. We take advantage of this isomorphism to derive the conditions for the observation of the in stability. The analysis is extended to the case in which the uniform field limit does not apply in order to cover more realistic situations. [S1050-29 47(99)06509-9].