Brillouin-Kerr Soliton Frequency Combs in an Optical Microresonator

By generating a Brillouin laser in an optical microresonator, we realize a soliton Kerr microcomb through exciting the Kerr frequency comb using the generated Brillouin laser in the same cavity. The intracavity Brillouin laser pumping scheme enables us to access the soliton states with a blue-detuned input pump. Because of the ultranarrow linewidth and the low-noise properties of the generated Brillouin laser, the observed soliton microcomb exhibits narrow-linewidth comb lines and stable repetition rate. Also, we demonstrate a low-noise microwave signal with phase noise of $-49\mathrm{dBc}/\mathrm{Hz}$ at 10 Hz, $-130\mathrm{dBc}/\mathrm{Hz}$ at 10 kHz, and $-149\mathrm{dBc}/\mathrm{Hz}$ at 1 MHz offsets for a 10.43 GHz carrier with only a free-running input pump. The easy operation of the Brillouin-Kerr soliton microcomb with excellent performance makes our scheme promising for practical applications.

Figure 1

(a) Schematic diagram of Brillouin-Kerr soliton frequency comb. (b) Photograph of the 6.3-mm-diameter disk resonator. (c) Illustration of red-detuned Brillouin laser with Kerr self-phase modulation. Dash orange curve, cavity response of Brillouin mode with Kerr self-phase modulation. See more details in Fig. S1 of the Supplemental Material [38].

Figure 2

(a) Typical transmission spectrum of the microresonator. (b) The transmission spectrum of the Brillouin mode. (c) The transmission power spectra of the pump laser, comb, and the backward laser (including reflected pump laser, Brillouin laser, and comb) at a pump power of 167 mW. (d) Close-up of transmission power spectra of the comb and backward laser, respectively. (e) The optical spectrum of backward laser before the comb generation. (f) The radiofrequency (rf) spectrum of the beat note between the reflected pump laser and Brillouin laser with a 10 kHz resolution bandwidth (RBW).

Figure 3

(a) The optical spectra of different soliton states. I, II, and III show the measured optical spectra and fitted envelopes of the three-soliton state, two-soliton state, and single-soliton state, respectively. Insets show the relative positions of solitons circulating in the microdisk. (b) The rf spectra of the corresponding soliton repetition frequency in (a). (c) Measured dispersion $D_{\mathrm{int}}$ for the mode family of Brillouin mode. (d) Autocorrelation trace (AC trace) and autocorrelation function of ${\mathrm{sech}}^2$ fitting for the single-soliton state in (a).

Figure 4

(a) Close-up spectrum for the single-soliton state in Fig. 3 with 2 pm resolution nearby the Brillouin laser. (b) Forward optical spectrum for the single-soliton state in Fig. 3. (c) Single-sided frequency noise spectra of the pump laser, the Brillouin laser below parametric oscillation threshold, as well as the Brillouin laser and a comb line (1551.69 nm) with a single-soliton state. The dash-dotted lines represent the white-frequency-noise levels. The corresponding fundamental linewidths are also indicated.

Figure 5

(a) Single sideband (SSB) phase noise of the soliton repetition rate. (b) Allan deviation of the soliton repetition rate measured with 0.01 s gate time.