High frequency dynamics in delayed semiconductor lasers with short external cavity

We analyze the dynamics of a laser diode subject to optical feedback from a short external cavity (EC), i.e. with an EC round-trip time much smaller than the period of the laser relaxation oscillations (RO). Our numerical simulations are based on the Lang-Kobayashi (LK) equations for single mode edge-emitting lasers subject to weak/moderate optical feedback. A new, detailed, Hopf bifurcation analysis shows that LK equations admit both supercritical and subcritical Hopf bifurcation points. Subcritical Hopf points lead to time-periodic pulsating intensity solutions with a frequency close to half the RO frequency. In contrast, from supercritical Hopf bifurcations emerge harmonic intensity oscillations with a frequency either close to the RO frequency or to the EC frequency. Microwave oscillations are obtained, as a result of a beating between two EC modes. In general, these high frequency dynamics are stable only for a small range of feedback parameters. However, we find that decreasing the a factor largely improves the stability of the microwave oscillations and makes it possible to observe pulsating intensity solutions for a larger range of EC length. The high frequency intensity solutions of laser diodes with short EC are thought to be of great interest for new applications in all optical signal handling. Our results motivate new theoretical studies of LK equations with short EC.