The confirmation of chaotic oscillation in a single-mode He-Ne(6328Å) standing-wave class A laser, by means of delayed feedback with an external mirror is reported. Varying the reflectivity of the external mirror, the time series of the laser output power are observed. By constructing the phase portraits and estimating Lyapunov exponents, the transition from a point attractor to a strange attractor is confirmed by the positive Lyapunov exponents in the chaotic region.

We experimentally demonstrate synchronization of chaos in one-way coupled He-Ne lasers with optical feedback. We observe different types of synchronization such as identical synchronization, inverse synchronization, and random amplification. These dynamics are maintained only for a short duration of several hundred milliseconds. We also observe generalized synchronization of chaos by using one master and two slave lasers. The generalized synchronization is achieved for a long duration of tens of seconds under injection locking. The generalized synchronization is always maintained while the injection locking is achieved.

Experiments on the iron-pnictide superconductors appear to show some materials where the ground state is fully gapped, and others where low-energy excitations dominate, possibly indicative of gap nodes. Within the framework of a five-orbital spin fluctuation theory for these systems, we discuss how changes in the doping, the electronic structure or interaction parameters can tune the system from a fully gapped to a nodal sign-changing gap with s-wave (A 1g ) symmetry (s ± ). In particular, we focus on the role of the hole pocket at the (π, π ) point of the unfolded Brillouin zone, identified as crucial to the pairing by Kuroki et al (2009 Phys. Rev. B 79 224511), and show that its presence leads to additional nesting of hole and electron pockets, which stabilizes the isotropic s ± state. The pocket's contribution to the pairing can be tuned 7 2 by doping, surface effects and by changes in interaction parameters, which we examine. Analytic expressions for orbital pairing vertices calculated within the random phase approximation (RPA) fluctuation exchange approximation allow us to draw connections between aspects of the electronic structure, interaction parameters and the form of the superconducting gap. Contents

Chaotic oscillation in a single-mode He-Ne laser (6328 Å), which belongs to a class A laser, is investigated through experiments I which were reported in our previous paper [F. Kuwashima, I. Kitazima and H. Iwasawa: Jpn. J. Appl. Phys. 37 (1998) L325.], and the improved experiments II. The results of experiments II, which were conducted using a neutral density (ND) filter instead of the tilted mirror used in experiments I, demonstrate the effect of the tilted mirror and a pumping parameter on the chaotic behavior of this laser. We also confirm and discuss the threshold value for chaos vs control parameters from the estimated Lyapunov exponents. The route to chaos in this laser is also discussed. Lastly, the laser equation of the class A laser with delayed feedback is introduced and numerical results which reveal chaotic oscillations are reported.

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