We exploit the strong polarization dependence of the thermooptic coefficients in a lithium niobate whispering-gallery-mode resonator to create a self-referenced thermometer. An unprecedented temperature sensitivity of 3.0 GHz/K in the frequency difference between modes of orthogonal polarizations is demonstrated. In order to lock the lasers to the mode resonances, we use a simple intracavity phase modulation approach that provides for superbly low frequency instability. We demonstrate a record room-temperature thermometer detectivity of 40 nK with 1 s of averaging time. Simulations based on the fluctuation-dissipation theorem are performed to calculate the fundamental thermorefractive noise, showing that the detectivity could be improved with reduced laser-locking instabilities. (c) 2018 Optical Society of America
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