Lasers with nonlocal feedback, diffraction, and diffusion

An off-axis feedback loop originates a two-point nonlocality that changes the instabilities of lasers of classes 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. Counterpropagating 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.