Locking conditions and stability properties for a semiconductor laser with external light injection

We present a theoretical and experimental investigation of injection locking of semiconductor lasers. The theoretical analysis takes into account the dependence of refractive index on the carrier density expressed by the linewidth enhancement factor α. Locking conditions and dynamic stability are analyzed. The nonzero value of α results in an increased locking bandwidth, where only part of the range corresponds to a dynamically stable state. Asymmetric characteristics are obtained for the locked power and phase as a function of frequency detuning between the master and slave laser. Outside the stable range, light injection gives rise to beat phenomena and intensity pulsations. The theoretical results were confirmed by experiments on 830 nm CSP lasers and 1.3 μm BH lasers. The experiments include the first measurements of locking bandwidth characteristics reported for 1.3 μm lasers. Power spectra are recorded under locked and near-locked conditions and compared with theory. The 1.3 μm lasers are found to have a better dynamic stability than 830 nm lasers. Even so, the stability problems may exclude the particular application of injection locking where phase modulation is generated for coherent transmission.