Thermal-gradient enhanced current spreading in d.h. lasers

The observed differences in light output against current (P/I) curves between c.w. and short-pulse operation of (GaAl)As lasers have generally been considered as arising from the increase in active-region temperature under c.w. operation. This temperature increase results in a decrease in optical output at a given current level because of the reduced carrier confinement at the heterojunctions. However, by carefully compensating the temperature rise in the active region in the c.w. mode, we have experimentally established that, for exactly the same active-region temperature and total pumping current, the laser emits significantly less light in the c.w. mode than in the pulse mode of operation. This difference becomes larger as the input power increases. The thermoelectric (Seebeck) effect can explain this observation. The temperature gradient induced by c.w. operation causes current to flow away from the stripe region, leading to excess wasted current and reduced light output.