We present experimental evidence of coexisting periodic attractors in
a semiconductor laser subject to external optical injection. The coexi
sting attractors appear after the semiconductor laser has undergone a
Hopf bifurcation from the locked steady state. We consider the single-
mode rate equations and derive a third-order differential equation for
the phase of the laser field. We then analyse the bifurcation diagram
of the time-periodic states in terms of the frequency detuning and th
e injection rate and show the existence of multiple periodic attractor
s.