Fig. 1. Schematic of the experimental setup combining an MBR with the DOFC technique. TL, tunable laser; PC, polarization controller; AWG, arbitrary waveform generator; PD, photodetector; OSC, oscilloscope; SI, signal input; LO, local oscillator. (a) Microscopic image of the MBR with a diameter of 209 μm. (b) SEM image of the cross section of the MBR. (c) Schematic of the ultrasound detection using two MBRs.

Fig. 2. Schematic of the experiments based on (a) TL frequency-tuning and (b) DOFC methods. (c) The transmission spectrum using TL scanning. (d) Resonant dip in the transmission spectrum using TL scanning, and the Q factor is estimated as 3.02×107. (e) Resonant dip in the transmission spectrum using DOFC.

Fig. 3. (a) Intensity and (b) phase responses in the MBR-based DOFC without ultrasonic stimulation. (c) Intensity and (d) phase responses in the MBR-based DOFC with ultrasonic stimulation. The insets on the right part of (a)–(d) are enlarged contour parts.

Fig. 4. (a) Frequency response of a resonance peak in a single ultrasound response period. (b) The retrieved frequency spectrum of point D obtained by FFT.

Fig. 5. Measured transmission spectra of the temperature-induced frequency shifts of MBR-A and MBR-B.

Fig. 6. (a) Schematic of an ultrasound detector based on two MBRs. (b) Frequency shift of the resonant peaks of MBR-A and MBR-B varies with time under the ultrasonic response. Δφ is the phase difference between the two resonators. (c) Relationship between the Δφ and the distance between the ultrasound and the MBR-A. (d) Schematic of a 3D structure testing model with an array of MBRs.