A dual-frequency optical source comprises: (a) first and second pump laser sources arranged to generate optical pump power at respective first and second pump laser frequencies vpump1 and vpump2; and (b) a fiber optical resonator characterized by a Brillouin shift frequency vB and a free spectral range that is substantially equal to an integer submultiple of the Brillouin shift frequency. Each one of the first and second pump laser sources is frequency-locked to a corresponding resonant optical mode of the fiber optical resonator. First and second optical output signals of the dual-frequency optical reference source at respective first and second output frequencies v1=vpump1−vB and v2=vpump2−vB comprise stimulated Brillouin laser output generated by simultaneous optical pumping of the fiber optical resonator by the first and second pump laser sources, respectively. An output difference frequency v2−v1 is greater than about 300 GHz.