Abstract: An optical communications system with terminal devices connected to an optical pathway for communications with one another. A system wide reference signal is introduced into the optical pathway by a reference signal generator for propagation throughout the system. Each terminal device includes a resonant cavity to support one of a plurality of available sets of resonant modes, each set having an associated free spectral range between resonant modes, and controls the resonant cavity structure to lock one of the resonant modes of the resonant cavity to the system wide reference signal. The resonant characteristics of the resonant cavity at each terminal device can be changed to vary the number of integral wavelengths supported in the resonant cavity. Plural terminal devices can communicate with one another to support a set of resonant modes common to one another with one mode of the common set locked to the system wide frequency reference.