  A significant challenge in the development of chip-scale
cavity-optomechanical devices as testbeds for quantum experiments and classical
metrology lies in the coupling of light from nanoscale optical mode volumes to
conventional optical components such as lenses and fibers. In this work we
demonstrate a high-efficiency, single-sided fiber-optic coupling platform for
optomechanical cavities. By utilizing an adiabatic waveguide taper to transform
a single optical mode between a photonic crystal zipper cavity and a
permanently mounted fiber, we achieve a collection efficiency for intracavity
photons of 52% at the cavity resonance wavelength of 1538nm. An optical
balanced homodyne measurement of the displacement fluctuations of the
fundamental in-plane mechanical resonance at 3.3 MHz reveals that the
imprecision noise floor lies a factor of 2.8 above the standard quantum limit
(SQL) for continuous position measurement, with a predicted total added noise
of 1.4 phonons at the optimal probe power. The combination of extremely low
measurement noise and robust fiber alignment presents significant progress
towards single-phonon sensitivity for these sorts of integrated
micro-optomechanical cavities.
