COHERENCE AND FREQUENCY STABILITY OF SEMICONDUCTOR LASERS UNDER CW OPERA TI ON AND MODULATION

Abstract

This work is an investigation into the coherence and frequency stability of semiconductor lasers. The work begins with a brief review of the many varied techniques which have been proposed to achieve highly coherent. and stable semiconductor lasers. A computer simulation model of a semiconductor laser is then presented to investigate the effects of inhomogeneous broadening on the (longitudinal) mode stability of the laser. In particular, the model developed here enables the electronic intraband relaxation scattering to be developed beyond the well-known relaxation time approximation which is normally used in similar analyses. The results of spectral hole burning arising from long scattering times with corresponding detrimental effects on modal stability are presented. With further incorporation of a nonlinear gain suppression term arising from population beating (due to coupling of the modes), single moded stable operation is shown to be possible, but multimoded operation will again result if the suppression term is too large. Experimental investigations begin with the use of optical feedback from an external cavity to improve the coherence properties of a semiconductor laser. The corresponding effects on modal stability are also observed. Linewidths as low as several hundred kHz are achieved, and the behaviour of the power-independent linewidth under feedback is also investigated. Intensity noise properties of the laser in the external cavity are reported, which are closely related to the modal behaviour and stability of the laser. The ability of feedback to reduce mode-hopping in the laser is demonstrated. Finally, the behaviour of the laser under feedback and direct current modulation is observed, with a reduced frequency modulation response due to feedback. The ability of direct current modulation in stabilising the single mode operation of the feedback laser is also investigated.