Tunable switching between stable and periodic states in a semiconductor laser with feedback.

Feedback-induced switching between two nonlinear dynamical states is observed in a semiconductor laser. The single-mode laser is subject to optical feedback in the long-cavity regime. In every round-trip time τ, the feedback is found to switch the laser from a stable state to a periodic state. The stable state corresponds to a continuous-wave emission at a single optical frequency. The periodic state corresponds to emission at another optical frequency with sidebands generated from a sustained relaxation oscillation. Such regular switching between the stable and periodic states is first unveiled numerically. Experimentally, the resultant intensity time series is confirmed as comprising of a square-wave envelope repeating in τ, which is modulated on a microwave carrier near the relaxation resonance frequency. Additionally, the duty cycle for the periodic state is found as continuously tunable by adjusting the feedback strength. The tunable state switching is applicable to square-wave modulated photonic microwave generation.