Sensitive thermometry with a fiber-integrated optical microcavity

High-sensitivity thermometry is critical for advanced cutting-edge technologies such as gravimeters, quantum clocks, and lithography systems. Although recent transformative advances in resonator-based optical microthermometers achieve high sensitivity, the input and output setups are complex and require precise control, severely limiting their practical applications and achievable footprint. Here, we experimentally demonstrate a fiber-integrated thermometer made of a high-Q optical Fabry-Pérot fiber microcavity, filled with transparent but thermal-sensitive ultraviolet glue. The input and transmitted probe laser fields are coupled to the microther-mometer via optical fibers, significantly facilitating operation and minimizing the device size. Combining the large temperature dependence of the ultraviolet glue and the micrometer-scale cavity, sensitive thermometry is achieved by independently measuring the resonance shift or the transmission change. Based on evaluation of equivalent temperature noise of the power spectral density, the resonance-shift method achieves microkelvin-level sensitivity. The fixed-wavelength transmission-change method yields a nanokelvin-level readout-referred equivalent temperature-noise floor at high frequencies using quasi-static response calibration. This low-cost, simple, and fiber-integrated microthermometer holds significant potentials for advanced applications in quantum device characterization, biocalorimetry, new optoelectronic technologies, and next-generation clocks.