  The coherent interaction between optical and acoustic waves via stimulated
Brillouin scattering (SBS) is a fundamental tool for manipulating light at GHz
frequencies. Its narrowband and noise-suppressing characteristics have recently
enabled microwave-photonic functionality in integrated devices based on
chalcogenide glasses, silica and silicon. Diamond possesses much higher
acoustic and bandgap frequencies and superior thermal properties, promising
increased frequency, bandwidth and power; however, fabrication of low-loss
optical and acoustic guidance structures with the resonances matched to the
Brillouin shift is currently challenging. Here we use intense cavity-enhanced
Raman generation to drive a diamond Brillouin laser without acoustic guidance.
Our versatile configuration - the first demonstration of a free-space Brillouin
laser - provides tens-of-watts of continuous Brillouin laser output on a 71 GHz
Stokes shift with user switching between single Stokes and Brillouin frequency
comb output. These results open the door to high-power, high-coherence lasers
and Brillouin frequency combs, and are a major step towards on-chip diamond SBS
devices.
