We measure the temporal evolution of the intensity of an edge emitting semiconductor laser with delayed optical feedback for time spans ranging from 4.5 to 65 ns with a time resolution from 16 to 230 ps, respectively. Spectrally resolved streak camera measurements show that the fast pulsing of the total intensity is a consequence of the time delay and multimode operation of the laser. We experimentally observe that the instabilities at low frequency are generated by the interaction among different modes of the laser. [S0031-9007(98)08077-6] PACS numbers: 05.45. + b, 05.40. + j, 42.60.Mi Nonlinear systems with delayed feedback are of interest because they can be widely found in economy, biology, chemistry, and physics [1]. These systems are in principle infinite dimensional, and from this point of view, it is difficult to classify them a priori as deterministic dynami-cal systems because the existence and uniqueness of a solution have to be demonstrated for each particular model [2]. It is also difficult to separate the role of noise from de-terminism, because complex solutions display a Gaussian-Markovian behavior as if they were solutions of a Langevin equation [3,4], thus nonconventional measurement techniques are required [5]. During the past years, there has been particular interest in understanding the dynamical behavior of one such system: a semiconductor laser with optical feedback. It was experimentally shown that weak optical feedback may induce bistability [6], oscillations, and chaos [7]. At moderate feedback levels, a special dynamical regime occurs which is characterized by the appearance of aperiodic, fast reductions of the total intensity of the laser [8,9], followed by a slower recovery stage. The average time between such intensity drops is much longer than all characteristic times of the system itself, so they are called low-frequency fluctuations (LFF). From numerical simulations it was recently proposed that LFF originate from a deterministic chaotic attrac-tor which encompasses a large number of unstable fixed points, either saddle-node points or foci [9-11]. This chaotic attractor coexists with one or more stable fixed points. The trajectory, if in the chaotic attractor, wanders in phase space around the foci until it approaches a saddle. The collision with a saddle gives rise to a large, fast excursion in phase space until the evolution recovers towards the foci. Since this process lasts for several delay times, it generates a long-time scale dynamics which contributes to the lower part of the spectrum, and it was called "chaotic itinerancy with a drift" because it involves a drift in the laser frequency. Furthermore, the model-which assumes single-mode operation of the laser and very weak feedback-predicted that the temporal behavior of the laser intensity is characterized by pulses departing from zero intensity level and whose amplitude grows steadily until the collision with a saddle takes place leading to a sudden reduction of the intensity. However, recent measurements of the time-res...