Fig. 1. Schematic of Brillouin frequency comb based on free-space optical cavity (inset: working principle)

Fig. 2. Output power versus pump power(inset: near-field profiles of pump and Stokes beams)

Fig. 3. Output spectra of Stokes beams. (a) Single Raman frequency; (b) first-order SBS frequency; (c) Brillouin frequency comb