Lasers with nonlocal feedback, diffraction, and diffusion

An off-axis feedback loop originates a two-point nonlocality that changes the instabilities of lasers of class A and B: after crossing a first threshold these devices amplify, while they lase above a second one. We obtain the instability diagram in presence of off-axis feedback and show that transverse phase and group velocities can be tuned to have the same or opposite sign, depending on control parameters. Counter-propagating noise sustained patterns and localized perturbations are found for an out-of-phase feedback, when the laser operates as a signal splitter. We predict these effects in a broad class of devices, with either positive or negative refractive index, diffusion and in presence of noise.