  The 'optical spring' results from dynamical back-action and can be used to
improve the sensitivity of cavity-enhanced gravitational-wave detectors. The
effect occurs if an oscillation of the cavity length results in an oscillation
of the intra-cavity light power having such a phase relation that the light's
radiation pressure force amplifies the oscillation of the cavity length. Here,
we analyse a Michelson interferometer whose optical-spring cavity includes an
additional optical-parametric amplifier with adjustable phase. We find that the
phase of the parametric pump field is a versatile parameter for shaping the
interferometer's spectral density.
