Fig. 1. Concept of the microcomb-assisted ultra-high-resolution and broad-range WGM photonic thermometer. The sensing element is an on-chip microring WGMR, which generates WGM red shift with increasing temperature (positive thermo-optic coefficient). The PDH locking ensures linewidth reduction and tracking of the WGM. The soliton microcomb provides broadband frequency references, thereby ensuring a broad range. By combining PDH-locking microring with a soliton microcomb, the temperature sensing achieves ultra-high resolution and broad range.

Fig. 2. Beatnote ambiguity during microcomb-assisted temperature sensing. The blue dashed line, red solid line, and green dashed line are the beatnote signals, real temperature curve, and non-ambiguous temperature range (NATR) boundary, respectively.

Fig. 3. (a) SEM image of fabricated Si3N4 microring resonator; (b) image of fabricated MgF2 microdisk resonator; (c) measured transmission spectrum of Si3N4 microring resonator; (d) experimental setup of microcomb-assisted temperature sensing system: AWG, arbitrary waveform generator; ECDL, external-cavity diode laser; EOM, electro-optic modulator; EDFA, erbium-doped fiber amplifier; PD, photodetector; FBG, fiber Bragg grating; PA, preamplifier; RSA, real-time spectrum analyzer; NTC, negative temperature coefficient thermistor; TEC, thermoelectric cooler; Atten., attenuator.

Fig. 4. (a) Optical spectrum of single-soliton microcomb; (b) electrical spectrum of MC-WGM beatnote signal.

Fig. 5. (a) Measured waterfall spectrum shows signal ambiguity near the NATR boundary (white dash line); (b) measured voltage gradient of the feedback signal for ambiguity resolving process; (c) reconstructed temperature curve (blue solid line) and reference temperature curve (red solid line).

Fig. 6. (a) Measured temperature curve shows ultra-high resolution in a broad temperature range, and the temperature deviation curve reflects the heat transfer rate, proving our proposed sensor has a fast response (temp. deviation = meas. temp.−TEC temp.); (b)–(d) mK-level temperature measurement in various temperature regions proves the ultra-high resolution of 136 μK.

Fig. 7. (a) Transmission spectra of two different soliton locking regimes; (b) repetition frequency stability and corresponding temperature stability of two different soliton locking regimes; (c) pump frequency stability and corresponding temperature stability of two different soliton locking regimes.