Two edge-emitting lasers coupled through polarization-rotated optical injection exhibit square-wave oscillations
provided the roundtrip time from laser to laser and back is sufficiently large. If the mutual coupling between
the lasers is relatively weak, the two plateaus of the square-waves exhibit different durations even though the
total period remains close to the roundtrip time. This asymmetry progressively disappears as the feedback
strength is increased. The experimental observations are confirmed by numerical simulations. The simulations
also reveal that the square-wave regimes appear through a series of complex bifurcations and that a sufficiently
large roundtrip time is needed.
The square-wave response of edge-emitting diode lasers subject to a delayed polarization-rotated optical feedback is studied experimentally and theoretically. Square-wave self-modulated polarization intensities of a period close to twice the delay τ of the feedback gradually appear through a sequence of bifurcations starting with a Hopf bifurcation (Gavrielides et al, Proc. SPIE 6115, to appear, 2006). In Gavrielides et al (submitted, 2006), squarewave solutions were determined analytically from the laser equations in the limit of large τ. A condition on the laser parameters was derived explaining why square-wave oscillations are preferentially observed for suffciently large feedback strength. In this paper, we concentrate on the relaxation oscillations that always appear at each intensity jump between the plateaus of the square-wave. We show analytically that if the feedback strength is
progressively decreased, a bifurcation to sustained relaxation oscillations is possible for one of the two plateaus.
The response of a diode laser resulting from an incoherent delayed optical feedback is considered from numerical
and experimental perspectives. We concentrate on a class of solutions that appear as regular square waveforms.
A two-field model is used and the bifurcation diagram of these square-wave regimes is studied. Conditions under
which they typically appear are determined. The roles of various parameters are examined, particularly with
regard to the gains and losses of the two polarization modes. Numerical results are in close agreement with
experiments.
The chaotic dynamics of a semiconductor laser subject to a delayed polarization-rotated optical feedback is investigated theoretically and experimentally. An extension of the usual one-polarization model is derived to account for two orthogonal polarizations of the optical field. The two-polarization model is motivated by observations of lag synchronization in our experiments using polarization-rotated optical feedback and uni-directional injection. Experimental data confirm the predictions of the two-field model. We also show that the two-polarization model can be reduced to the one-polarization model.
We demonstrate analytically and numerically that multiple mixed external cavity mode solutions are possible for a laser subject to optical feedback from two external cavities. Such solutions exhibit a series of bifurcations and can be easily identified from optical spectra and their frequency content. Similar states have been proposed and analyzed within the framework of the usual Lang-Kobayashi equations describing a semiconductor laser subject to a single optical feedback in short cavities and with moderate pumping. We will present experimental results demonstrating the existence of mixed-mode states in a two-cavity system. We also find that the bifurcation sequence can terminate in low frequency fluctuation states before restabilization on a new maximum power external cavity mode.
We demonstrate numerically and experimentally that low- frequency fluctuations (LFF) in a laser diode subject to delayed optical feedback can be suppressed or stabilized by a second optical feedback with a short delay. The second feedback suppresses LFF by shifting antimodes far away from the external cavity modes in phase space, or by making them disappear, with the consequence that the crises that induce the power dropouts are no longer possible. Moreover, as the second feedback strength increases, the laser undergoes a bifurcation cascade with successive regions where it exhibits chaos or LFF and regions where it locks to a newly-born stable maximum gain mode. This all-optical stabilization technique is easier to implement from an experimental point of view than many existing methods since it does not require modification of any laser parameters or of the first optical feedback.
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