Drift Instability for a Laser Beam Transmitted through a Rubidium Cell with Feedback Mirror

We present the characteristics of the spatio-temporal instability that occurs when rubidium atoms interact with an incident laser beam and with the beam reflected from a tilted feedback mirror. We show that there is a drift of the roll pattern that appears above threshold. Measurements of the drift frequency are in excellent agreement with recent predictions. An experiment done with two incident waves shows a more complex spatio-temporal behaviour.