Stable microwave-frequency source based on cascaded brillouin lasers

A method for generating a microwave-frequency output electrical signal at an output frequency fM, the method comprising:

• (a) using a pump laser source, pumping at a pump frequency vpump an optical resonator so as to generate stimulated Brillouin laser outputs at respective first and third Stokes Brillouin-shifted frequencies v1=vpump−vB and v3=vpump−3vB, where vB is a Brillouin shift frequency of the optical resonator;
• and
• (b) directing the stimulated Brillouin laser outputs at the first and third Stokes Brillouin-shifted frequencies onto a photodetector so as to generate an output electrical signal at a beat frequency fM=v1−v3=2vB,
• wherein:
• (c) a free spectral range vFSR of the optical resonator is substantially equal to an integer submultiple of the Brillouin shift frequency vB of the optical resonator so that vB=MvFSR where M is an integer;
• (d) the pump laser source is frequency-locked to a corresponding resonant optical mode of the optical resonator;
• (e) the pump laser source and the optical resonator are arranged so that pumping the optical resonator with output of the pump laser source at the pump frequency vpump results in stimulated Brillouin laser oscillation in the optical resonator at the respective first, second, and third Stokes Brillouin-shifted frequencies v1=vpump−vB, v2=vpump−2vB, and v3=vpump−3vB;
• and
• (d) the photodetector is arranged to receive the first and third Stokes stimulated Brillouin laser outputs and to generate therefrom the output electrical signal at the beat frequency fM=v1−v3=2vB.