We present experimental and numerical investigations of the dynamics of two device-identical, optically coupled semiconductor lasers exhibiting a delay in the coupling. Our results give evidence for subnanosecond coupling-induced synchronized chaotic dynamics in conjunction with a spontaneous symmetry-breaking: we find a well-defined time lag between the dynamics of the two lasers, and an asymmetric physical role of the subsystems. We demonstrate that the leading laser synchronizes its lagging counterpart, whereas the synchronized lagging laser drives the coupling-induced instabilities.

We give experimental and numerical evidence for a new dynamical regime in the operation of semiconductor lasers subject to delayed optical feedback occurring for short delay times. This short cavity regime is dominated by a striking dynamical phenomenon: regular pulse packages forming a robust low-frequency state with underlying fast, regular intensity pulsations. We demonstrate that these regular pulse packages correspond to trajectories moving on global orbits comprising several destabilized fixed points within the complicated phase space structure of this delay system.

We present a comprehensive study of the emission dynamics of semiconductor lasers induced by delayed optical feedback from a short external cavity. Our analysis includes experiments, numerical modeling, and bifurcation analysis by means of computing unstable manifolds. This provides a unique overview and a detailed insight into the dynamics of this technologically important system and into the mechanisms leading to delayed feedback instabilities. By varying the external cavity phase, we find a cyclic scenario leading from stable intensity emission via periodic behavior to regular and irregular pulse packages, and finally back to stable emission. We reveal the underlying interplay of localized dynamics and global bifurcations.

We report the first experimental observation of irregular picosecond light pulses within the coherence collapse of a semiconductor laser subject to delayed moderate optical feedback. This pulsing behavior agrees with the recent explanation of low frequency fluctuations as chaotic itinerancy with a drift. Theory and experiments show very good agreement. PACS numbers: 42.65.Sf, 05.45.+b, 42.55.Px Delayed feedback-induced instabilities have been studied since the late 1970s in various dynamical systems. Besides these very early interests they are nowadays of particular interest because of their intrinsic high dimensionality and, related to that, their rich variety of dynamical phenomena [1]. Optical systems have played an important role for these investigations, and have boosted the interest in high-dimensional nonlinear dynamics [2][3][4].A very popular delay system is the semiconductor laser subject to external optical feedback, because of its high sensitivity to external signals [5]. Semiconductor lasers show a sudden increase in their spectral linewidth from about 100 MHz to typically several tens of GHz for delay times not much smaller than the relaxation oscillation period and for moderate feedback levels. This phenomenon has been called coherence collapse [6], and has attracted a lot of research (e.g., [7-11]).One dynamical phenomenon within the coherence collapse regime frequently attributed to is the so-called low frequency fluctuations phenomenon (LFF). It refers to fluctuations in the emitted light intensity with distinctly lower frequencies in comparison to the underlying relaxation oscillation frequencies and mode beating frequencies [12 -14]. Recently, LFF has been explained as chaotic itinerancy with a drift [15]. This theory predicts erratic picosecond pulsing of the output power. In this paper we give experimental evidence confirming these predictions. We do this by comparing measured intensity pulses with pulse trains obtained by numerical modeling. The very good agreement supports the recent identification of the essential physical mechanism leading to the LFF behavior.The dynamics of a single-mode semiconductor laser subject to moderate amounts of optical feedback from an external reflection is modeled by the following delaydifferential rate equations [5]:Here we have written the complex optical field as E ͑t͒ E͑t͒ exp͑iv 0 t͒ p P͑t͒ exp͑iv 0 t͒, where v 0 is the optical angular frequency of the stand-alone (solitary) laser, P͑t͒ is the photon number, and n͑t͒ is the excess number of electron-hole pairs with respect to the solitary value N 0 . The parameters in (1a) and (1b) have their usual meaning: j is the bulk differential gain, e accounts for gain saturation, a is the linewidth enhancement factor, G 0 is the inverse photon lifetime, pJ th is the electrical pump current (J th is its value at threshold), and T 1 is the electron-hole pair lifetime. The feedback is accounted for via the delay time t and the feedback rate g. The dimensionless effective feedback strength is defined as C ϵ gt p...

Two delay-coupled semiconductor lasers are studied in the regime where the coupling delay is comparable to the time scales of the internal laser oscillations. Detuning the optical frequency between the two lasers, novel delay-induced scenarios leading from optical frequency locking to successive states of periodic intensity pulsations are observed. We demonstrate and analyze these dynamical phenomena experimentally using two distinct laser configurations. A theoretical treatment reveals the universal character of our findings for delay-coupled systems.