Feedback phenomena in a semiconductor laser induced by distant reflectors

Multiple pass resonances in the noise spectrum and low-frequency self-oscillations in asymmetric geometrical configurations are described. The double peaked structure of the external cavity resonance f_c (at c/2L), the `subharmonic cascading´ from f_c, and the generation of subharmonics of a modulated injection current are detailed. The exact field intensity distribution in the passive external resonator is shown to command these phenomena which reveal intermingled spatial and dynamical effects. The different behaviors are related to the characteristic curves of the laser. The authors have found that when the external cavity laser operates in its unstable regime, a locking of the otherwise randomly distributed intensity drops can occur giving a narrow resonance in the noise spectrum. In addition, based on the authors´ observations, adapted rate equations are built starting from the Lang and Kobayashi model. Results show an adequate match between theory and experiments for the detailed microwave spectra, multiple pass resonances of the external cavity, and low-frequency resonances