Fig. 1. (a) Schematic of the Brillouin laser operation and the spiral-ring resonator design. (b) Conceptual illustration of the Brillouin lasing conditions. Brillouin lasing occurs when the free spectral range (FSR) of the cavity precisely matches the Brillouin frequency shift. (c) The simulated group index of the spiral-ring resonators as a function of the wavelength. The inset shows the fundamental TE mode profile of the SRR. (d) The dependence of the coupling coefficient on the SRR gap with a coupling length l_cl of 80 µm. The inset depicts the diagram of the SRR coupling region with a coupling coefficient of K.

Fig. 2. (a) Microscope image of the GeSbS SRR device with a total length of ∼1.7 cm. The bending radius is 100 µm. (b) SEM image of the cross section of the resonator with 3.5-µm-thick SiO₂ top cladding. The size of the waveguide core is 2.2 µm × 850 nm.

Fig. 3. (a) Optical transmission of the GeSbS SRR with a resonance linewidth of 190 MHz; (b) the total and intrinsic resonance linewidths of the SRR in the wavelength span from 1530 to 1600 nm. (c) The transmission of the GeSbS SRR redshifts slightly with the increasing temperature. (d) The resonance frequency experiences a 32-GHz blueshift with the temperature fluctuation ranging from 25°C to 41°C with a step of 2°C, illustrating a slope efficiency Δυ/ΔT of −2.023.

Fig. 4. Experimental setups to characterize (a) threshold and slope efficiency and (b) linewidth and phase noise of the Brillouin laser in the GeSbS SRR. ECDL, external cavity diode laser; EDFA, erbium-doped fiber amplifier; ISO, optical isolator; PC, polarization controller; CIR, circulator; DUT, device under test; PM, power meter; AOM, acousto-optical modulator; APD, avalanche photodetector; ESA, electrical spectrum analyzer.

Fig. 5. (a) First-order Stokes signal power versus on-chip pump power; (b) high-resolution optical spectrum of the back-reflected pump and the first-order Stokes signal; inset shows the recorded beat-note on the ESA between them. (c) Electrical power spectrum of the self-heterodyne signal for laser linewidth characterization; (d) single-sideband phase noise of the first-order Stokes emission at different output powers.