Whispering-gallery mode silica microsensors for cryogenic to room temperature measurement

Optical resonance shifts are measured against a wide range of temperatures from cryogenic to room temperature for silica microspheres operating at whispering-gallery modes. The sensor head microsphere is coupled to a fiber taper and placed in an insulated cell where the air temperature first cools down to below 110 K and then rises steadily and slowly. The transmission resonance spectrum of a distributed feedback laser at 1531 nm exciting the microsphere–taper system is monitored and recorded for every 1 K temperature increment. The resonance wavelength shifts against the temperature changes are analyzed. Several microspheres with size from 85 to 435 µm are tested. No significant dependence of the sensor sensitivity is seen with the sphere size. A cubic dependence of the wavelength shift versus the temperature is least-squares fitted. The measured sensitivity increases from 4.5 pm K−1 to 11 pm K−1 with increasing temperature in the test temperature range, and this behavior is consistent with the temperature dependence of the sum of thermal expansion and thermo-optic coefficients of silica material. The resolution of the sensors with the current instrument could reach 3 mK.