Secondary pulsations driven by spatial hole burning in modulated vertical-cavity surface-emitting laser diodes

It is shown that significant optical power is generated in secondary pulsations after switch-off of vertical-cavity surface-emitting laser diodes (VCSEL’s) when the laser is modulated from an above-threshold state to a current that is at or below the threshold value. It is found from simulations that the optical power in the secondary pulsations can be as much as 25% of the power in the lasing state. The initial reduction of optical output power subsequent to the switch-off permits a spatial redistribution of charge carriers that in turn permits a transient recovery of the modal gain above the threshold value, thus enabling the secondary pulsations to occur. The phenomenon occurs as a result of the interplay between the transverse-mode structure and the gain medium and, as such, cannot be predicted by spatially independent rate equation models. Implications of the phenomenon for practical applications of VCSEL’s with pseudorandom nonreturn-to-zero modulation formats are pointed out.