Photonic thermometer with a sub-millikelvin resolution and broad temperature range by waveguide-microring Fano resonance.

Fano resonance theoretically is an effective approach for sensitivity enhancement in photonic sensing applications, but the reported methods suffer from complicated structure and fabrication, narrow dynamic range, etc. In this article, we propose a photonic thermometer with sub-millikelvin resolution and broad temperature measurement range implemented by a simple waveguide-microring Fano structure. An air hole is introduced at the center of the coupling region of the waveguide of an all-pass microring resonator. The effective refractive index theory is used to design its equivalent phase shift and therefore the lineshape of the Fano resonance. Experimental results showed that the quality factor and the Fano parameter of the structure were invariant in a broad temperature range. The wavelength-temperature sensitivity was 75.3 pm/℃, the intensity-temperature sensitivity at the Fano asymmetric edge was 7.49 dB/℃, and the temperature resolution was 0.25 mK within 10℃ to 90℃.