Cooling and sensing using whispering gallery mode resonators
This thesis reports on a detailed exploration of the optomechanical interaction between a tapered optical fibre and a silica microsphere mounted on a cantilever. The amount of light evanescently coupled from the fibre into the optical whispering gallery mode of the sphere is exquisitely sensitive to their separation allowing fast measurement of picometre displacements of both the microsphere cantilever and the fibre. By exploiting this enhanced transduction, strong active feedback damping/cooling of the thermal motion of both the fibre and microsphere cantilever have been demonstrated to the noise limit of the system. The cavity enhanced optical dipole force between the fibre and the sphere was used to damp multiple mechanical modes of the tapered fibre, while a piezo stack at the clamped end of the microsphere cantilever allowed for cooling of its centre of mass motion and the second mechanical eigenmode. The effect of noise within the feedback loop was shown to invert the measured mechanical mode spectrum at high feedback gain as the noise itself is fed into the resonator. A rich variety of feedback induced spring stiffening and softening of the mode is measured when time delays are introduced. Cooling of the mechanical modes of the taper, which are ubiquitous to many whispering gallery mode experiments and are considered as unwanted noise, has not been achieved previously. Simultaneous operation of both feedback schemes was demonstrated for the first time, providing stabilization of the system. By using the microsphere cantilever as an inertial test mass, measurement of its displacement induced by acceleration can resolve micro g accelerations at high bandwidth.