  Integrated micro and nanophotonic optomechanical experiments enable the
manipulation of mechanical resonators on the single phonon level. Interfacing
these structures requires elaborate techniques limited in tunability,
flexibility, and scaling towards multi-mode systems. Here, we demonstrate a
cavity optomechanical experiment using 3D-laser-written polymer membranes
inside fiber Fabry-Perot cavities. Vacuum coupling strengths of ~ 30 kHz to the
fundamental megahertz mechanical mode are reached. We observe optomechanical
spring tuning of the mechanical resonator by tens of kHz exceeding its
linewidth at cryogenic temperatures. The extreme flexibility of the laser
writing process allows for a direct integration of the membrane into the
microscopic cavity. The direct fiber coupling, its scaling capabilities to
coupled resonator systems, and the potential implementation of dissipation
dilution structures and integration of electrodes make it a promising platform
for fiber-tip integrated accelerometers, optomechanically tunable multi-mode
mechanical systems, or directly fiber-coupled systems for microwave to optics
conversion.
