OPTICAL FEEDBACK EFFECTS IN EXTERNAL CAVITY SEMICONDUCTOR LASERS

Abstract

Optical feedback effects in external cavity semiconductor lasers have been investigated in many laser systems, both in conventional bulk lasers as well as Quantum Well (QW) lasers. The work begins with a brief review of some conventional results of optical feedback effects in these laser systems. A theoretical model based on a three-mirror -compound-cavity laser is presented to study the spontaneous emission effects on external cavity semiconductor lasers. It is found that inclusion of the spontaneous emission term in the secular equation resulting from matching the boundary conditions on the coherent optical fields at the interfaces in the compound cavity of the laser significantly affects the gain and phase conditions of the external cavity semiconductor laser. It is also found that there is a strong asymmetry in the contributions of the spontaneous emission to the right or left travelling field in relation to the external mirror on the longitudinal linewidth. Subsequently, this model is extended to study the secondary effects of the refractive index change induced by free carriers. This is done by modifying the secular equations to include a phase shift ?g in the gain term (g* = gi?g). The model presented has been found to explain the experimentally observed output intensity undulations and spectral changes of semiconductor lasers when the pumping current is varied above the threshold current. Finally, double cavity semiconductor laser is investigated and found to have significant effects on the phase and gain conditions of the secular equation.