Dynamics of semiconductor lasers subject to delayed optical feedback: the short cavity regime.

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. Delayed feedback dominated systems are encountered extensively in the physical world and are of fundamental importance. They are found in models of diffusion and thermochemical reactions. In biology they occur in blood cell production, neural control, and drug delivery, and have applications in respiratory physiology [1]. Physical understanding of the dynamics of delay systems has been boosted during recent years by investigations using semiconductor lasers (SL) subject to delayed optical feedback. Basic nonlinear dynamical phenomena as period doubling [2], a quasiperiodic route to chaos [3], the Ikeda scenario [4], and bifurcation cascades [5] have already been observed in this system. In parallel, SLs are of essential importance for modern telecommunication, data transmission, and data storage technologies. There, the performance of SL systems is often degraded due to instabilities caused by even small amounts of unavoidable optical feedback from distant reflectors as, e.g., the facet of an optical fiber or a compact disc. This then requires careful isolation of the laser, thus increasing the complexity and cost of such systems. Therefore, the understanding of delayed feedback induced instabilities and its fundamental dynamical phenomena is indispensable for a wide range of practical applications. Interestingly, all the above mentioned investigations of the dynamics of SL subject to delayed optical feedback have been performed in the so-called long cavity regime (LCR) in which the external cavity length L is chosen such that the round trip frequency of the light nEC c2L is some hundreds of MHz, and thus substantially lower than the GHz range relaxation oscillations frequency nRO. However, in many practical applications, e.g., in fiber couplers or in compact discs, the external cavity is only a few cm long. Such a reduction of L has important physical consequences. First, the ratio between the two basic system frequencies nRO and nEC is reversed. Second, the number of possible degrees of freedom is reduced. Thus, qualitatively new dynamical phenomena are to be expected for such short delay times. However, experimental investigations focusing on the nonlinear dynamical behavior of SL subject to delayed optical feedback from short external cavities are still lacking; solely stability and noise properties have been studied, e.g., see [6], and references therein. In this paper, we present the first temporally resolved investigations of the dynamics of SLs operating in the short cavity regime (SCR). We demonstrate that this new dynamical regime is characterized by striking regular pulse packages (RPP) in the intensity dynamics of the system. Our numerical analysis demonstrates that RPP correspond to well-defined global orbits in phase space always along the same series of destabilized attractors and unstable saddle points. Figure 1 depicts a schematic of the experimental setup. A temperature-stabilized laser diode (Sharp LT015MDO, Hitachi HLP1400) is subject to delayed optical feedback from a semitransparent dielectric mirror. The laser beam is collimated using an aspheric lens, and feedback strength is controlled with a polarizer (Pol.). The optical isolator (Iso.) shields this external cavity configuration from eventual perturbations from the detection branch. For detection, 10% (LT015MDO) and 70% (HLP1400), respectively, of the intensity in the external cavity is coupled out via the semitransparent mirror. Yet, we achieve a