We experimentally demonstrate the existence of non dispersive solitary waves associated with a 2π phase rotation in a strongly multimode ring semiconductor laser with coherent forcing. Similarly to Bloch domain walls, such structures host a chiral charge. The numerical simulations based on a set of effective Maxwell-Bloch equations support the experimental evidence that only one sign of chiral charge is stable, which strongly affects the motion of the phase solitons. Furthermore, the reduction of the model to a modified Ginzburg Landau equation with forcing demonstrates the generality of these phenomena and exposes the impact of the lack of parity symmetry in propagative optical systems. PACS numbers: Valid PACS appear hereDissipative solitary waves have been observed as selflocalized optical wave packets along the direction of propagation in many out of equilibrium and nonlinear optical systems. In spite of their huge variety, many of the observations reported so far can be cast in two main categories depending on the presence or absence of coherent energy input, ie the (lack of) phase symmetry of the system [1]. In systems with phase symmetry, mode-locked laser pulses have been analyzed as dissipative solitons of the cubic-quintic Ginzburg Landau equation [2]. Their optical phase can wander in the course of time due to the neutral mode created upon the formation of a coherent wave. On the contrary, dissipative solitons in forced systems [3,4] have been analyzed in the framework of the Lugiato-Lefever equation [5], which includes a coherent forcing term acting as a phase reference to which solitons will lock. In both cases, the use of paradigmatic equations in addition to system-specific models has allowed to formally connect these optical solitary waves to localized states as they appear in fluid dynamics, plant ecology, granular media or reaction-diffusion systems [6][7][8][9]. In fact, optical dissipative solitons can often be explained as perturbed solitons of the nonlinear Schrödinger equation (in the weak dissipation limit [10-12]) or as locked fronts (in strongly dissipative systems [13,14]).In this Letter, we report on dissipative solitons which fundamentally consist of self-confined 2π phase rotations embedded in a homogeneously phase locked background. These "phase solitons" are generic features of spatially extended oscillatory media under nearly resonant forcing [15,16] and result from the mismatch between the natural periodicity and the forcing. Here, the mismatch between the free running laser and the external forcing frequencies leads to the formation of phase kinks * Electronic address: stephane.barland@inln.cnrs.fr as result of a commensurate-incommensurate transition [17,18]. This connects our observations with the kink solutions observed in many physical systems described by the Frenkel-Kontorova model [19], such as fluxons in Josephson arrays [20], local deformations in DNA chain [21], or excitable waves in chemical and biological systems [22,23]. In nonvariational systems, chirality acquire...

Quantum dot lasers can lase from the ground state only, simultaneously from both the ground and first excited states and from the excited state only. We examine the influence of optical injection at frequencies close to the ground state when the free-running operation of the device is excited state lasing only. We demonstrate the existence of an injection-induced bistability between ground state dominated emission and excited state dominated emission and the consequent hysteresis loop in the lasing output. Experimental and numerical investigations are in excellent agreement. Inhomogeneous broadening is found to be the underlying physical mechanism driving the phenomenon.

Multiple time scales appear in many nonlinear dynamical systems. Semiconductor lasers, in particular, provide a fertile testing ground for multiple time scale dynamics. For solitary semiconductor lasers, the two fundamental time scales are the cavity repetition rate and the relaxation oscillation frequency which is a characteristic of the field-matter interaction in the cavity. Typically, these two time scales are of very different orders, and mutual resonances do not occur. Optical feedback endows the system with a third time scale: the external cavity repetition rate. This is typically much longer than the device cavity repetition rate and suggests the possibility of resonances with the relaxation oscillations. We show that for lasers with highly damped relaxation oscillations, such resonances can be obtained and lead to spontaneous mode-locking. Two different laser types--a quantum dot based device and a quantum well based device-are analysed experimentally yielding qualitatively identical dynamics. A rate equation model is also employed showing an excellent agreement with the experimental results.

Abstract:With conventional semiconductor lasers undergoing external optical feedback, a chaotic output is typically observed even for moderate levels of the feedback strength. In this paper we examine single mode quantum dot lasers under strong optical feedback conditions and show that an entirely new dynamical regime is found consisting of spontaneous mode-locking via a resonance between the relaxation oscillation frequency and the external cavity repetition rate. Experimental observations are supported by detailed numerical simulations of rate equations appropriate for this laser type. The phenomenon constitutes an entirely new mode-locking mechanism in semiconductor lasers.