As thermo-optic locking is widely used to establish a stable frequency detuning between an external laser and a high Q microcavity, it is important to understand how this method affects microcavity temperature and frequency fluctuations. A theoretical analysis of the laser-microcavity frequency fluctuations is presented and used to find the spectral dependence of the suppression of laser-microcavity, relative frequency noise caused by thermo-optic locking. The response function is that of a high-pass filter with a bandwidth and low-frequency suppression that increase with input power. The results are verified using an external-cavity diode laser and a silica disk resonator. The locking of relative frequency fluctuations causes temperature fluctuations within the microcavity that transfer pump frequency noise onto the microcavity modes over the thermal locking bandwidth. This transfer is verified experimentally. These results are important to investigations of noise properties in many nonlinear microcavity experiments in which low-frequency, optical-pump frequency noise must be considered.
