Single frequency laser arrays

A device comprising:

• a passive waveguide structure attached to a substrate, the passive waveguide structure comprising a dielectric layer forming a passive waveguide core;
• and
• an active waveguide structure attached to a top surface of the passive waveguide structure, the active waveguide structure comprising a quantum well layer;
• wherein the active waveguide structure has a first mesa of width Wm1 defining a first laser supporting a first optical mode, and a second mesa of width Wm2 defining a second laser supporting a second optical mode;
• wherein a grating structure, defined by pitch P, present in the passive waveguide structure, overlaps with at least an evanescent part of each of the first and second optical modes;
• and
• wherein the lasing wavelengths of the first and second lasers are in part defined by the first and second mesa widths Wm1 and Wm2 respectively, and by the grating structure pitch P.