We study the hierarchical structuring of islands of stable periodic oscillations inside chaotic regions in phase diagrams of single-mode semiconductor lasers with optical injection. Phase diagrams display remarkable accumulation horizons: boundaries formed by the accumulation of infinite cascades of self-similar islands of periodic solutions of ever-increasing period. Each cascade follows a specific period-adding route. The riddling of chaotic laser phases by such networks of periodic solutions may compromise applications operating with chaotic signals such as e.g. secure communications. [12] reported diagrams obtained by numerical integration of the rate equations for an optically injected semiconductor laser showing some islands of periodic laser signals embedded in a sea of chaos. These important findings raise an interesting question concerning the precise structuring of laser chaotic phases. In fact, this question is the tip of a much wider problem that we consider here.Phase diagrams for discrete-time models described by mappings are common nowadays [13,14]. But the much more difficult problem of building detailed phase diagrams for models ruled by sets of nonlinear differential equations has been much less investigated. Of course, diagrams recording complex bifurcations and providing valuable insight for a few of the lowest periods have been obtained in a number of in-depth bifurcation studies using powerful continuation methods [7,15,16,17]. However, complete diagrams, discriminating simultaneously regions of arbitrarily high periods and regions with chaotic phases, remain essentially unexplored for continuous-time autonomous models. This is the problem we attack here.Our numerical simulations revealed surprising regularities existing inside the chaotic phases of the laser. As illustrated in Figs. 1 and 2, the parameter space has wide regions characterized by chaotic solutions. These chaotic phases contain both single accumulations as well as accumulations of accumulations. More specifically, chaotic laser phases are riddled with infinite sequences of periodadding cascades, each one converging toward curves that look simple (structureless), denoted "accumulation horizons", for simplicity. One example is indicated by the arrow marked A in Fig. 2a. From a theoretical point of view, a key novelty here is that the differential equations ruling the laser are autonomous equations, i.e., they do not involve time explicitly. Thus, the remarkable organization of the parameter space reported here must originate from an intrinsic interplay between variables and parameters of the laser. We found accumulation horizons to exist abundantly also in electronic circuits, atmospheric and chemical oscillators, and several other systems [18]. To fix ideas, here we focus just on the laser case. Incidentally, we mention that accumulation cascades in semiconductor lasers have been investigated by Krauskopf and Wieczorek [17] quite recently. However, their accumulations are of a very different nature than ours [19].The laser w...