The stimulated Brillouin scattering (SBS) laser features narrow linewidth and has garnered considerable attention in the past few years. Brillouin lasers are capable of producing highly coherent, spectrally pure emissions and can be designed to operate from the visible to the infrared regions^[1–3]. They have been used to narrow the pump laser linewidth^[4] and, if cascaded, can be utilized to obtain low-phase-noise microwave frequency synthesizers^[5]. Compact chip-scale Brillouin lasers are highly desired to improve efficiency and reduce system complexity. Efficient Brillouin lasers with low threshold and high slope efficiency are highly desirable in optical communications, lidar, precision spectroscopy, etc. Microring resonators have emerged as intriguing photonic integrated devices to produce low-threshold Brillouin emission due to their high-quality factors (Q-factors) and small mode volumes. Brillouin lasing has been demonstrated in recent years on a variety of material platforms, including fluoride^[6], fused silica^[5,7–9], chalcogenide glasses (ChGs)^[10–12], silicon^[13], and silicon nitride^[2,14,15]. However, some of them, such as SiO2, GaF2, and diamond^[1,16], are not compatible with planar-scale integrated photonics, which restricts their practical applications. Silicon, as the ideal medium to enable integrated photonic circuits, has been employed to explore on-chip forward stimulated Brillouin scattering (FSBS) interactions^[17] and Brillouin lasers^[13]. However, the silicon waveguide for FSBS needs to be suspended to obtain good acoustic mode confinement, which is fragile and complicates the fabrication process. Silicon nitride, one of the most well-developed photonic platforms to date, has been widely used in nonlinear integrated photonics owing to its ultralow loss that enables high Q-factor microresonators^[18,19] and large Kerr effective nonlinearity^[20]. Nevertheless, the Brillouin gain coefficient of silicon nitride is three orders lower than its counterparts in ChGs^[21,22]. In addition, the previously reported Brillouin lasing in the integrated Si3N4 waveguide was attained within the SiO2 cladding^[2,14,15].

The ChGs photonic integrated platform, showing a broad transparent window covering from the visible to the mid-infrared regions and a high Brillouin gain coefficient, is very promising for producing chip-scale Brillouin lasers over a wide wavelength range^[23–25]. Brillouin lasing on a ChG chip was first demonstrated using an As2S3 waveguide and a fiber ring cavity, which showed a lasing threshold of 360 mW^[10]. Morrison et al. reported an initial demonstration of Brillouin lasing in a planar integrated circuit with a threshold of 50 mW, which was realized in an As2S3 ring resonator with a Q-factor of 4×105^[11]. The lasing threshold has been further reduced to 0.53 mW in a high-QAs2S3 resonator fabricated by an approach where the light-guiding structure was deposited on a pre-wet-etched SiO2 microring with an extremely smooth surface, thus avoiding directly dry etching the As2S3 core^[12]. However, this design requires a flip-chip coupling scheme to achieve high coupling efficiency for low lasing threshold, which poses extra challenges with respect to stability and sensitivity to environmental conditions. Among the various approaches for compact on-chip Brillouin lasers, planar integrated microresonators are beneficial to practical applications that require narrow emission linewidth and high stability because the all-waveguide resonator design is key to improving environmental robustness. Although there has been tremendous progress in the development of planar integrated ChG devices, to date, the performance of the reported Brillouin laser based on all-waveguide ChG resonator still remains a relatively high lasing threshold and a low slope efficiency due to the insufficient Q-factor^[11]. It is essential to improve the on-chip Brillouin lasing in ChG-based planar integrated circuit.

Here we present the demonstration of Brillouin lasing with a low threshold and high slope efficiency from a high-Q spiral-ring resonator (SRR) in an alternative Ge25Sb10S65 (GeSbS) platform, which has a free spectrum range (FSR) closely matching the Brillouin frequency shift (BFS) and enables simultaneous confinement of optical and acoustic modes. Compared with the typical materials containing the arsenic (As) component for ChG devices, GeSbS is environmentally friendly, more compatible with the CMOS fabrication procedure, and exhibits a high damage threshold. What is more, it has a superior intrinsic Brillouin gain coefficient of 0.524×10−9m/W^[26], which is 10 times higher than the fused silica and 1000 times higher than the silicon nitride. It provides a good opportunity to enable a highly coherent on-chip light source, which will be beneficial to accelerate the development of monolithic integration on a single ChG chip.

SBS is an acousto-optic interaction that originates from coherent nonlinear coupling between optical waves and sound waves. The schematic of the Brillouin laser based on the GeSbS SRR and the spiral-ring design is shown in Fig. 1(a). When an external pump wave is coupled into an optical resonator and generates Brillouin gain that equals the round-trip losses throughout the cavity, Brillouin laser oscillation occurs and counterpropagates with the pump. With a pump power surpassing the lasing threshold, incident pump photons are efficiently converted to an emitted optical Stokes wave that builds up from spontaneous noise. Matching of the resonator FSR to the BFS (ΩSBS), as shown in Fig. 1(b), plays a critical role in achieving low-threshold Brillouin lasing in the ring resonators. The FSR of the SRR is given by FSR=cngL,(1)where ng is the group index, and L is the round-trip length of the resonator. The BFS is related to effective refractive index neff of the optical mode and the acoustic velocity of the acoustic mode vac by ΩSBS=2neffvac/λp, where λp is the pump wavelength. Thus, to achieve a low-threshold Brillouin laser, it is necessary to appropriately determine the round-trip length of the resonator by considering both the group index and the effective index that are related to the geometry of the resonator waveguide. The side of the resonator waveguide is 2.2µm×850nm, and the radius of the ring resonator is chosen to be 100 µm to reduce the bending loss as much as possible. To determine the round-trip length, we perform waveguide mode simulation using the finite-element method (FEM) for the calculation of neff and ng. Figure 1(c) shows the normalized fundamental transverse-electric (TE) mode distribution of the resonator waveguide; its group index is 2.353 at 1550 nm. The required round-trip length of the resonator to enable Brillouin lasing is ∼1.7cm, estimated from Eq. (1). To verify the size in the coupling region, we also simulate the coupling coefficients varying with different gaps at 1551.45 nm using the finite-difference time-domain (FDTD) method, as shown in Fig. 1(d). It indicates that the coupling strength weakens with the increasing gap. When the gap is 150 nm, the corresponding coupling coefficient reaches 0.323. Given the actual fabrication accuracy and resolution, the gap is set to be 150 nm with a fixed coupling length of 80 µm in the following experimental demonstration.

Figure 2(a) shows the microscope image of the ring resonator with a round trip of ∼1.7cm in a spiral-ring configuration. The detailed fabrication flow of the device is provided in the Supplementary Material. The coupling gap distance is chosen to be 150 nm, exhibiting the critical coupling state closely. The SRR is coupled to a bus waveguide with a width of 2 µm, which is tapered down to 170 nm over a length of 600 µm near the end of the chip facet to increase the coupling efficiency and reduce the strong reflection between the chip and the lensed fiber. The scanning electron microscope (SEM) image of the resonator cross section is shown in Fig. 2(b) with vertical and smooth core sidewalls, which is beneficial to the high Q-factor and low laser threshold in the subsequent demonstration. The measured fiber-to-chip coupling loss is ∼3.5dB per facet.

Optical transmission of the fabricated device in the wavelength range from 1530 to 1600 nm is measured.

The observed FSR is 7.4 GHz around 1549.21 nm. As shown in Fig. 3(a), the extinction ratio (ER) is 6 dB, and the total resonance linewidth is 190 MHz, corresponding to a loaded Q-factor of 1.01×106. The calculated intrinsic Q-factor and resonance linewidth are 1.39×106 and 139.4 MHz, respectively. The Q-factor of the GeSbS SRR is 2.5 times higher than that of the previously reported As2S3 ring resonator used to produce Brillouin lasers^[11]. Figure 3(b) presents the total (green circle) and intrinsic (red circle) linewidths for the GeSbS SRR from 1530 to 1600 nm. The results indicate that the fabricated device has narrow resonance linewidths and thus high Q-factors over a large wavelength range.

The thermo-optic stability of the material plays a crucial role in achieving highly efficient nonlinear interactions. To investigate the thermal stability of GeSbS, we also characterize the thermo-optic coefficient (TOC) of GeSbS by observing the resonance frequency shifts with the temperature fluctuations. As shown in Fig. 3(c), the transmission curve of the SRR encounters a slight redshift with increasing temperature. This can be attributed to the increased effective refractive index in the resonant condition in SRRs. Figure 3(d) records the resonant frequency experiencing a 32-GHz shift in the temperature range of 25°C to 41°C, with a negative slope efficiency Δυ/ΔT of −2.023. Consequently, the estimated TOC of GeSbS is 2.405×105K−1 by the equation dn/dT=−(ΔυΔT)×(nυ), which is the same order of magnitude as that of the well-developed silicon nitride (2.45×10−5K−1)^[27] and thin-film lithium niobite (ne∼3.34×10−5K−1)^[28] platforms.

The experimental setup to demonstrate Brillouin lasing is shown in Fig. 4(a). The pump is generated from an external cavity diode laser (ECDL, linewidth <10kHz), which is followed by an erbium-doped fiber amplifier (EDFA). The amplified pump is coupled to the chip by a lensed fiber before passing through a circulator via port 1 to port 2. A high-resolution optical spectrum analyzer (OSA, AP2088A) is used to record the spectra of the back-reflected pump and Brillouin Stokes wave, which pass through the circulator via port 2 to port 3. The pump wavelength is finely tuned at the step of 1 pm from short to long wavelengths. The detuning between the pump wave and the cavity resonance would change, then reaching the thermal locking state on the targeted thermally shifted resonance^[29]. The back-reflected optical waves are monitored by the OSA for a range of pump power levels.

The redshifted Stokes peak is observed when the on-chip pump power reaches the lasing threshold of 24.8 mW, which outperforms the previous record of Brillouin lasing in a planar integrated all-waveguide resonator made of As2S3^[11]. Theoretically, the lasing threshold Pth of a Brillouin laser in a resonator with optimum matching of the SBS shift to the cavity FSR can be estimated from the following expression^[30]: Pth=π2neff2λp2LGSBSQL2(1+K)3K,(2)where λp is the pump wavelength, GSBS denotes the Brillouin gain coefficient in the unit of m−1W−1, QL is the loaded Q-factor of the resonator, and K is the coupling coefficient, which is related to the transmission coefficient such that T=[(1−K)/(1+K)]2. Equation (2) shows that the threshold Pth scales inverse-quadratically with the loaded Q-factor of the resonator. Therefore, a low lasing threshold can be expected for the integrated GeSbS SRR with the enhanced Q-factor. At a fixed on-chip pump power of 36 mW, the optical spectrum of the Brillouin lasing signal and the back-reflected pump is obtained by the OSA, as plotted in Fig. 5(b). The beat-note frequency between the Stokes wave and the reflected pump measured by an electrical spectrum analyzer (ESA) is at 7.49 GHz, as shown in the inset of Fig. 5(b). Moreover, other nonlinear phenomena such as four-wave mixing (FWM) and Raman scattering are not observed at the relatively large pump powers. This can be attributed to the non-optimal dispersion property of our SRR for good phase matching of FWM, the relatively low Raman gain coefficient of GeSbS (7.37×10−12m/W)^[25], which is about 2 orders of magnitude lower than the Brillouin gain coefficient (5.24×10−10m/W), and the larger effective mode volume with a long round-trip span of ∼1.7cm.

The slope efficiency η is characterized by measuring the output Stokes power versus the on-chip pump power as plotted in Fig. 5(a). In our experiment, the on-chip pump power is evaluated by subtracting the fiber-to-chip coupling loss from the pump power at the output of the lensed fiber. Similarly, the on-chip Stokes power is calibrated by the Stokes power recorded by the OSA plus the optical path loss and the chip-to-fiber coupling loss. We thus estimate the slope efficiency to be 8.3%. It can be theoretically derived that the slope efficiency of Brillouin lasing from a microcavity depends mainly on the coupling parameter (K) such that η≈2(1+1/K)−2^[30]. The coupling parameter of our GeSbS SRR estimated from the measured ER is 0.33, which is in good agreement with the simulation result shown in Fig. 1(b). Thus, we can achieve a slope efficiency more than 20 times higher than that of the As2S3 resonator, which has a lower coupling efficiency of 0.04 and a resultant moderate slope efficiency of 0.3%^[11]. Moreover, further increasing the pump power can increase the Stokes power, while a second Stokes wave will also occur, leading to power depletion of the initial Stokes wave and laser linewidth broadening^[31].

The linewidth and the single-sided phase noise of the Brillouin laser are measured using the delayed self-heterodyne method^[32]. The experimental setup is shown in Fig. 4(b). After being routed from port 2 to port 3 of the circulator, the optical waves are sent to an optical bandpass filter with a narrow bandwidth to remove any residual back-reflected pump. Then the Stokes output is split into two paths by a 50:50 coupler. In the upper path, the frequency of the Stokes signal is shifted by 40 MHz by an acousto-optic modulator (AOM). The signal in the lower path is delayed by a 2-km-long fiber. The two signals are recombined and detected by an avalanche photodetector (Thorlabs 430C). The self-heterodyne signal is measured using an electrical spectrum analyzer (ESA) to extract the linewidth and the phase noise of the Brillouin laser. The electrical power spectrum directly detected by the ESA is shown by the blue curve in Fig. 5(c). The fringes in the spectrum can be attributed to the fact that the obtained signals in the two paths are partially coherent because the fiber delay length is shorter than the coherence length of the Brillouin laser. The laser linewidth is estimated by applying a fitting scheme to the measured spectrum^[32], as shown by the red curve in Fig. 5(c). The estimated linewidth is 8 kHz for a Stokes output power of 1.56 mW, which is approximate to the linewidth of the pump. The measured phase noise spectra of the Brillouin laser at different Stokes output powers are plotted in Fig. 5(d). The red and black traces correspond to output powers of 0.79 and 1.56 mW, which are the points (1) and (2) in Fig. 5(a), respectively. The far-from-carrier white-frequency-noise levels by the emission fundamental linewidth are also indicated by the dotted lines in Fig. 5(d). By averaging the measured phase noise from 2.4 to 2.9 MHz, the estimated phase noise level corresponding to the lower output power (0.79 mW) is 118.27dBc/Hz, which decreases to 122.31dBc/Hz by increasing the pump power to achieve a higher Stokes output power of 1.56 mW. The outcome is higher than the counterparts in previous results^[33,34], which can be attributed to the deviation from perfect phase-mismatching condition in Brillouin lasing. The results indicate the unique feature of Brillouin lasers that the phase noise level is lowered with the increase of the Stokes output power, which has also been demonstrated in a Brillouin laser on an integrated Si3N4 waveguide platform^[14]. In comparison to Brillouin lasing previously demonstrated in other ChG-based devices, the Brillouin lasing based on our fabricated high-Q-factor GeSbS SRR has improved performance in terms of lasing threshold, slope efficiency, and stability. The threshold is lower than its counterparts in the planar As2S3 ring resonator^[11] and the 7-cm long As2S3 rib waveguide within a ring fiber cavity^[10]. The corresponding slope efficiency is lower than that in the unetched As2S3 configuration^[12], yet it is still higher than the on-chip planar case^[11] and can be further increased by optimizing the SRR coupler design (see Table 1 in the Supplementary Material).

The linewidths of the lasing Stokes line of the presented device are the same order of magnitude as that of the pump light. This can be attributed to the fact that the resonance linewidth of the optical resonator is larger than the Brillouin gain linewidth (i.e., ∼48MHz^[24]). To achieve a significant linewidth narrowing, the linewidth of the optical cavity should be narrower than the Brillouin gain linewidth. Several strategies might be employed to improve the linewidth and phase-noise performance. For example, advanced fabrication methods such as multipass exposure^[35] and resist reflow techniques can be exploited to further reduce the sidewall scattering roughness and thus increase the Q-factor of the SRR as well as narrow the optical resonance linewidth. In addition, perfect matching of the resonator FSR to the BFS is important in minimizing the Brillouin laser linewidth. It has been demonstrated that the linewidth is obviously enhanced when operating the Brillouin laser slightly away from perfect matching^[31]. Thus, the round-trip length of the SRR needs to be designed precisely and the Pound–Drever–Hall (PDH) approach could further be helpful to lock the pump wave into the cavity mode. Moreover, inhibiting the Brillouin lasing cascade by incorporating Bragg gratings can help to increase the output power of the first-order Stokes wave and reduce the lasing linewidth^[36].

In conclusion, we have demonstrated an efficient Brillouin laser from a planar GeSbS SRR with a loaded Q-factor exceeding 1×106. A low threshold of 24.8 mW has been obtained for Brillouin lasing from ChG planar integrated circuits. The slope efficiency of the Brillouin laser is 8.3%. We have also characterized the linewidth and phase noise of the laser by the delayed self-heterodyne method. The estimated emission linewidth is 8 kHz at an output power level of 1.56 mW. This work shows the great potential of the GeSbS photonic integrated platform to realize Brillouin lasers in compact all-waveguide ring resonator configurations. Moreover, due to the wide transparency window of GeSbS, which ranges from the visible to the mid-infrared regions, the Brillouin laser based on the GeSbS spiral-ring resonator is a promising candidate for a narrow-linewidth optical source operating over a broad range of wavebands.