Controlling chaos in a single-mode laser

Summary form only given.We demonstrate experimentally, and analyze theoretically, three different methods of controlling chaos in the optically pumped, single-mode /sup 15/NH/sub 3/ ring laser operating at 153 /spl mu/m. We have concentrated on methods involving modulation of the pump power using an acousto-optic modulator. The laser was modeled using the Lorenz equations with an appropriately varying pump parameter. Suppression of chaos was demonstrated with open-loop sinusoidal modulation of the pump power near resonant with a natural pulsation (unstable periodic orbit) frequency of the laser, and also with very slow large amplitude modulation. In the near-resonant case suppression of chaos in favor of period 1, 2 and 4 pulsations was demonstrated in the experiment in appropriate ranges of modulation frequency and amplitude, consistent with theory. Closed-loop control using continuous time-delay feedback has also been achieved, with the pump power varied in response to a continuous error signal generated by comparing the laser output with its value at an earlier time, using a cable delay line. Not only is this method very simple to implement, but we found that stabilization to cw operation is possible, as well as to the expected periodic orbits. By varying the error signal generation time delay relative to the free-running laser pulsation period, various periodic states could be stabilized. In fact, in our experiments it was more convenient to fix the delay and vary the laser pulsation frequency. Theory shows that it is possible to skip several pulse periods, both in the error signal generation, and in the control signal feedback, and still gain control of the laser and ensure supercritical high-power cw operation.