EXTERNAL FEEDBACK OPERATION OF SEMICONDUCTOR LASERS

Abstract

The aim of this thesis is to contribute to the understanding of the dynamics of semiconductor lasers on a 10 to 100 ps time scale when their output is altered by external feedback. The optimization of parameters involved in optoelectronic feedback results in a reduction in the measured minimum pulse FWHM relative to previous work, and gives the conditions necessary to produce 25 ps FWHM optical pulses using this technique. To aid in the understanding of the modelocking of diode lasers, the CW spectral and threshold characteristics of a diode laser coupled to a mirror external cavity are examined. It is found that multiple reflections in the compound cavity cause an increase:). in mode linewidth with increasing bias current up to a maximum for a particular level of feedback for an external mirror coupled laser, but not for a grating coupled laser. As a result of the CW optical feedback analysis, grating feedback is utilized as a probe of the laser gain in a refinement of a previously reported technique for direct gain measurement. Also, the total loss reduction induced by external optical feedback is found to be made up of the sum of mirror loss and mode propagation loss reductions. This has significance in simulations of active modelocking and injection locking tuning characteristics carried out in this work. Simulations for active modelocking of a diode laser with low residual facet reflectivity in. an external mirror cavity account for the previously unexplained extreme sensitivity of the modelocking process to oscillator stability. Grating external cavity injection locking experiments and simulations present a new scheme for obtaining nearly transform limited optical pulses using semiconductor lasers.