  The radiation pressure induced coupling between an optical cavity field and a
mechanical oscillator can create entanglement between them. In previous works
this entanglement was treated as that of the quantum fluctuations of the cavity
and mechanical modes around their classical mean values. Here we provide a
fully quantum approach to optomechanical entanglement, which goes beyond the
approximation of classical mean motion plus quantum fluctuation, and applies to
arbitrary cavity drive. We illustrate the real-time evolution of optomechanical
entanglement under drive of arbitrary detuning to show the existence of high,
robust and stable entanglement in blue detuned regime, and highlight the
quantum noise effects that can cause entanglement sudden death and revival.
