Abstract
This paper investigates numerically influence of the external-cavity length on the type of the route-to-chaos of semiconductor lasers under external optical feedback. The study is based on numerical solution of a time-delay model of rate equations, and the solutions are employed to construct bifurcation diagrams and to examine the Fourier frequency spectrum of the laser output. The ratio of the relaxation frequency to the external-cavity resonance frequency is employed to measure the influence of the length of the external cavity. The route-to-chaos is period doubling when this frequency ratio is less than unity. The route is sub-harmonic when the frequency ratio increases up to 2.25. When the frequency ratio increases further, the transition to chaos becomes quasi-periodic characterized by the compound-cavity frequency and the relaxation frequency as well as their difference. Copyright (C) 2009 John Wiley & Sons, Ltd.