This paper investigates dynamics of semiconductor lasers under optical feedback during transitions to chaos. Influence of the external-cavity length on the route-to-chaos is elucidated. The study is based on numerical solution of a time-delay model of rate equations. The route to chaos is period doubling when the ratio of the relaxation frequency to the external-cavity resonance frequency is less than unity. The route is sub-harmonic when the frequency ratio is slightly higher than unity, and is quasi-periodic characterized by the compound-cavity frequency and the relaxation frequency as well as their difference when the ratio is far above unity