Abstract
The locking states of a delayed negative optoelectronic feedback system are studied experimentally. Harmonic locking is observed in this system. Instead of locking to the frequency of the delay loop, the pulsing frequency of the system locks to a harmonic of the loop frequency. Moreover, a period-adding route of the locking states is found and locking states of Farey fractions up to order 17 are reported. The plot of the rotation number of the locking states shows a Devil's staircase structure. © 2002 American Institute of Physics.