Phase noise measurements of external- cavity semiconductor lasers

With the current interest in coherent sources for optical communication systems, much effort is being directed at obtaining semiconductor lasers with a high degree of temporal coherence. This corresponds to producing a laser with a linewidth of a few kilohertz. This is significantly narrower than the several megahertz typical of solitary single-mode diode lasers. By placing a diode laser in an external cavity, the linewidth can be narrowed significantly.1,2