Observation of thermal feedback on the optical coupling noise of a microsphere attached to a low-spring-constant cantilever

A silica microsphere on a low-spring-constant cantilever (pendulum) is fabricated and evanescently coupled to a tapered optical fiber. The motion of the pendulum is detected as variations in the transmitted laser power through the tapered fiber. The optical coupling noise created by the pendulum motion is recorded by taking a fast Fourier transform of the transmitted laser power and the fundamental mechanical mode of the pendulum at 1.16 kHz is observed. The thermal damping and amplification of the coupling noise is investigated and the effect of the thermal feedback on the noise spectrum is examined. The response of the thermo-optical feedback to small transient and driven variations in the taper-pendulum separation for different values of laser detuning is demonstrated. Preliminary results on the optical force between the pendulum and the tapered fiber are also presented. Microspherical pendulums, with low mechanical spring constant, could be used for studying nanoscopic optical and mechanical forces, or optical cooling.