Double optical feedback and PT-symmetry breaking induced nonlinear dynamics in semiconductor lasers
A central aim of this research is to probe the nonlinear dynamics that arise in a semiconductor laser due to optical feedback. We investigate two schemes of optical feedback. The first scheme subjects the laser to optical feedback from two external cavities (or two loops), wherein each cavity contains a spectral filter. Using two filtered optical feedbacks, we experimentally demonstrate the ability to elicit and control unique dynamics in the optical emission frequency (wavelength) of the laser. These results are compared to a deterministic model describing the evolution of the complex electric field and carrier density of the laser. As the feedback rate from one cavity is increased, we observe a period doubling route in the frequency dynamics. To determine the influence of quantum noise on the period doubling route, we examine an augmented model of the rate equations which includes the effects of spontaneous emission and shot noise. One of the more surprising results is that in the presence of noise a larger feedback strength is required to induce chaotic dynamics. We find that noise drives the system toward stable attractors and the effects of the time-delay on the periodic dynamics are more pronounced. The second scheme we use is a system consisting of two time-delayed, optically coupled semiconductor lasers. We show that coupled lasers are an excellent test-bed to study parity (P) and time-reversal (T) symmetry breaking. Not only do optically coupled SCLs capture many of the characteristic signatures of PT symmetry breaking, but the time-delay between the lasers introduces novel and surprising features. We develop a simple PT model (analogous to a 2x2 Hamiltonian) that includes the effects of the time-delay. By examining the eigenvalues of the PT model, we can predict the intensity fluctuations by scanning the PT parameter, i.e. the frequency difference between the lasers. We experimentally observe the intensity fluctuations and find excellent agreement with the rate equation model which includes the dynamics of the carrier inversion and optical field.
Vemuri, Purdue University.
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