Optical resonance shifts are measured against temperature changes for different silica beads ranging from 80 to 450 µm in diameter. A micro-bead is fabricated by hydrogen flame fusing the tip of a single mode silica fibre/taper, and coupled to a fibre taper of submicrometre diameter. The coupling system in whispering-gallery modes is placed in an insulated cell. The air in the cell is slowly heated up from room temperature to about 10 K higher, and red shifts of a resonance wavelength during the heating process are recorded. Linear dependence of the wavelength shift versus the temperature rise is observed for all the tested micro-resonators. The measured sensitivity for beads greater than 200 µm in size closely matches the analytical value based on bulk material properties of silica thermal expansion and the thermo-optic effect. For smaller micro-beads, however, the measured sensitivity increases with shrinking bead size. The ultra-high resolution of such a kind of temperature micro-sensor and its potential applications are addressed.

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.

The spectral shift response of optical whispering-gallery modes due to the adsorption and desorption of water molecules into a SiO 2 nano-coating is investigated. This coating is applied to a silica microsphere and optical-shift measurements are made at a very low humidity level (<10%). The micro-optical coupling system is incorporated into a sealed vacuum chamber for controlled humidity testing. The experimental observation of the whispering-gallery-mode spectral shift with refractive index is consistent with a theoretical analysis using Mie theory. A significant hysteresis effect is observed in the spectral shift of individual whispering-gallery modes during variable humidity tests. To understand the hysteresis mechanism, tests with step changes in humidity are performed, which reveal complex and dynamic water molecule transport phenomena between the nano-coating and surrounding environment.

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