Sub-microkelvin resolution temperature sensing using orthogonal polarization modes in a fiber ring resonator

This paper proposes a sub-microkelvin resolution temperature sensing system operating in the millihertz to hertz band, based on a polarization-maintaining fiber ring resonator. In order to suppress the fiber fundamental thermal noise inherent in the fiber ring resonator, we leverage the birefringence effect of the polarization-maintaining fiber through common-mode rejection and utilize the resonant frequency difference between the orthogonal polarization modes as the temperature sensing signal. Using a 2-meter-long panda-type polarization-maintaining fiber resonator with a Q factor > 10⁸, we achieve a temperature sensitivity of -101.23 MHz/K at room temperature over a measurement range of 7.32 °C. Further, by implementing a photo-electric negative feedback loop to suppress laser-power-fluctuation-induced self-heating noise and a vacuum chamber to isolate environmental interference, the system finally attains 0.8 μK/√Hz resolution at 0.1 mHz, more than an order of magnitude lower than the theoretical single-mode thermal noise limit of the fiber resonator. This approach provides an innovative solution for high-precision temperature measurement applications in the field of space-borne and ground-based precision measurements.