  Silicon carbide silicon carbide (SiC SiC) composites are often used in
oxidizing environments at high temperatures. Measurements of the thermal
conductance of the oxide layer provide a way to better understand the oxidation
process with high spatial resolution. We use time domain thermoreflectance
(TDTR) to map the thermal conductance of the oxide layer and the thermal
conductivity of the SiC SiC composite with a spatial resolution of 3 {\mu}m.
Heterodyne detection using a 50 kHz modulated probe beam and a 10 MHz modulated
pump suppresses the coherent pick-up and enables faster data acquisition than
what has previously been possible using sequential demodulation. By analyzing
the noise of the measured signals, we find that in the limit of small
integration time constants or low laser powers, the dominant source of noise is
the input noise of the preamplifier. The thermal conductance of the oxide that
forms on the fiber region is lower than the oxide on the matrix due to small
differences in thickness and thermal conductivity.
