  The discrete circle map is the archetypical example of a driven periodic
system, showing a complex resonance structure under a change of the forcing
frequency known as the devil's staircase. Adler's equation can be seen as the
direct continuous equivalent of the circle map, describing locking effects in
periodic systems with continuous forcing. This type of locking produces a
single fundamental resonance tongue without higher order resonances, and a
devil's staircase is not observed. We show that, with harmonically modulated
forcing, nonlinear oscillations close to a Hopf bifurcation generically
reproduce the devil's staircase even in the continuous case. Experimental
results on a semiconductor laser driven by a modulated optical signal show
excellent agreement with our theoretical predictions. The locking appears as a
modulation of the oscillation amplitude as well as the angular oscillation
frequency. Our results show that by proper implementation of an external drive,
additional regions of stable frequency locking can be introduced in systems
which originally show only a single Adler-type resonance tongue.
