A long-long coupled-cavity glass laser includes a pair of active waveguide cavities that are coherently coupled using a passive waveguide cavity. The active and passive waveguide cavities are of sufficient length so that multiple supermodes and multiple peaks in the coupling coefficient are created over the width of the gain spectra. The supermodes are gain flattened so that the one supermode that coincides with a peak in the coupling coefficient will oscillate. Tunability is achieved by changing the optical path length of either the passive or active waveguide cavities to match a different supermode to a different coupling coefficient peak.