Behavior of the relaxation oscillation frequency in vertical cavity surface-emitting laser with external feedback

The influence of weak optical feedback on the dynamics of a single longitudinal mode vertical-cavity surface-emitting laser (VCSEL) is studied experimentally under conditions when the feedback power is sufficiently low so as to avoid the onset of chaos. The VCSEL dynamics divides into two regimes depending on the ratio of the relaxation oscillation frequency to the external cavity mode spacing. In the long cavity regime, the modulation frequency response of the VCSEL exhibits resonances corresponding to the frequencies of the external cavity modes, while for the short cavity regime the modulation response of the VCSEL at the cavity resonances remains very low while the relaxation oscillation frequency is shifted and the modulation response at the shifted frequency exhibits a sharpened and enhanced resonance.