Two edge-emitting lasers mutually coupled through orthogonal optical injection exhibit square-wave oscillations in their polarization modes. The TE and TM modes within each individual laser are always in antiphase, but the TE mode of one laser leads the TM of the other by the one-way time of flight between lasers. The duty cycle of the square waves is tunable with pump current and coupling strength, while the total period remains close to the roundtrip time. Numerical simulations give similar results and reveal the role of noise in stabilizing the oscillations. Delay-differential systems are of great importance in most fields of basic and applied sciences. They appear readily in control systems in which feedback is reinjected after a time delay, with examples that abound in biology, chemistry, mechanical systems, traffic flow, and others ͓1͔. Nonlinear optical systems in particular, with feedback that is nonlocal in space and time, are subjects of continuous interest in several applications, including phase distortion suppression and stabilization, optical beam shaping, pattern formation, and alloptical image processing.A fundamental property of many nonlinear dynamical systems is bistability as typified by double-well systems and those displaying hysteresis. Two-state solutions form the basis of all binary logic applications, and examples are plentiful in electronics, beginning with the most common multivibrator circuits used for logic, clocks, and gates. Optical digital logic is an area of rapidly increasing importance, with optical data storage and telecommunications as primary applications, as well as resurgent interest in optical computing. Therefore, two-state laser systems are of critical importance. There are a variety of useful applications stemming from the all-optical production of high-frequency optical pulses ͓2͔.Recent work has emphasized semiconductor laser systems ͓edge-emittings lasers spectroscopy ͑EELs͒ and verticalcavity surface-emitting lasers ͑VCSELs͔͒ with polarizationrotated feedback ͓3-7͔ as a source of optical square pulses generated through polarization self-modulation ͓8-12͔. These solutions are of fundamental interest in part because their dynamic properties can be examined in more detail than for conventional ͑nonrotated͒ optical feedback and because they relate to optoelectronic systems ͓13,14͔. Pulse trains generated in all such single-laser systems are symmetric square waves with duty cycles of 0.5. This paper describes optical generation of pulse trains in a system of two mutually coupled semiconductor lasers. Experimentally, we observe solutions of square pulse trains with duty cycles that are tunable as functions of the coupling strength and pump current. Self-consistent timing relationships between polarization modes are observed, and similar solutions are obtained through numerical simulations.For an overview, the dynamical system consists of two EELs, mutually coupled through optical injection, where the linear polarization state of each injected beam is rotated orthogonal ...
