The present invention provides one or more injection-lockable whistle-geometry semiconductor ring lasers, which may be cascaded, that are integrated on a common silicon-on-insulator (SOI) substrate with a single-frequency semiconductor master laser, wherein the light output from the semiconductor master laser is used to injection-lock the first of the semiconductor ring lasers. The ring lasers can be operated in strongly injection-locked mode, while at least one of them is subjected to direct injection current modulation.