A laser source includes an optical cavity having a length exceeding a first predefined distance (such as 6 mm), where a wavelength spacing between optical modes associated with the optical cavity is less than a second predefined distance (such as 100 pm). Moreover, a gain medium in the laser source amplifies the optical signal. Furthermore, tunable-grating waveguides in the laser source, which are optically coupled to ends of the optical cavity, reflect a portion of the optical signal back into the optical cavity, and at least one of the tunable-grating waveguides transmits a remainder of the optical signal out of the optical cavity.