  We present a novel locking scheme for active length-stabilization and
frequency detuning of a cavity optomechanical device based on the optical
spring effect. The scheme can be used as an alternative to the
Pound-Drever-Hall locking technique but in contrast doesn't require signal
processing on time-scales of the cavity decay rate. It is therefore
particularly suited for stabilizing micro cavities, where this time-scale can
be extremely fast. The error signal is generated through the optical spring
effect, i.e. the detuning-dependent frequency-shift of a nanomechanical
oscillator that is dispersively coupled to the intra-cavity light field. We
explain the functional principle of the lock and characterize its performance
in terms of bandwidth and gain profile. The optical spring locking scheme can
be implemented without larger efforts in a wide variety of optomechanical
systems in the unresolved sideband regime.
