Distant lasers with mutual optical injection are subject to a delayed coupling. We consider the barely investigated case of delays shorter than the relaxation oscillation period. In order to illuminate the role of these short delays, the ultimate zero-delay limit is considered as a reference. We use a traveling wave equation model, which fully resolves the spatio-temporal distributions of optical fields and carriers in the lasers and treats the wave propagation between the lasers by delayed boundary conditions. Wavelength detuning between the otherwise identical single-mode DFB lasers is used as primary bifurcation parameter. The zero-delay reference exhibits a synchronisation scenario typical for coupled oscillators. The nonsynchronised regimes represents a self-pulsation of the nonlinear carrier-photon system. Additional effects appear when including a short delay. Resonances of the cavity formed by the laser pair cause a staircase dependence on detuning of the pulsation
frequency. Irregular dynamics is observed at the borders of the
locked regions as well as at the edges of the stairs.
We have applied a comprehensive simulation tool for multisection DFB lasers to systematically investigate bifurcations caused by combining a single mode DFB laser with a feedback section. Strength and phase of the optical feedback are considered as main bifurcation parameters. The recording of output power, optical and power spectra and carrier density yield evidence of fold bifurcations of stationary states as well as subcritical and supercritical Hopf bifurcations
towards self-pulsations. Furthermore, a homoclinic bifurcation is detected and indications for a fold of limit cycles are observed in qualitative agreement with a bifurcation diagram very recently calculated with path following techniques 7 under simplifying assumptions. The present simulations show that these bifurcations are stable with respect to these approximations. They offer a method how the bifurcations can be determined experimentally.