Modal Output Power For Short External Cavity Single Mode Lasers

There are a number of hurdles to overcome when realizing a practical coherent fiber optic communication system; one of them is a narrow linewidth semiconductor laser source with frequency stability. Three mechanisms exist to suppress the side modes of a multimode laser diode: a) frequency selective feedback by incorporating a grating near the active region during growth as in distributed feedback (DFB) or distributed Brag reflection (DBR) laser, b) external feedback where the external cavity length determines the tuned frequency, and c) injection locking the laser diode by using a master laser diode. At present the best sources for coherent communications are obtained by combining mechanisms a) and b); i.e., by adding a long external cavity to an antireflection coated DFB laser. In this report we describe progress made in achieving a single mode laser source by adding a short external cavity to a multimode laser diode. Photon output power per mode as a function of injection current is calculated using rate equations for cases of with and without short external cavity feedback. Experimentally, side mode suppression ratios ranging from 15dB to 26dB are observed in the laboratory using a flat mirror as a feedback element.