Tunable laser operation with locally commensurate condition

A tunable laser, comprising:

• a gain medium pumped to emit a plurality of photons in response to an electrical input;
• a first optical cavity, optically coupled to the gain medium, in which said plurality of photons resonate in accordance with a plurality of lasing modes, said first optical cavity having a first free spectral range;
• a second optical cavity optically coupled to the first optical cavity and having a second free spectral range that is substantially equal to an integer multiple of the first free spectral range;
• and
• a channel selector optically coupled to the first and second optical cavities, adjustable to select a lasing mode from among said plurality of lasing modes having a frequency corresponding to a selected channel,
• wherein the relationship between the free spectral ranges of the first and second optical cavities produces a globally commensurate condition under which changes between communication channels having respective wavelengths are effectuated substantially without requiring adjustment to an optical length of the first optical cavity.