Thermal-impedance ageing characteristics of c.w. stripe lasers

Laser threshold currents increase with temperature, and appreciable heat has to be dissipated from the device while it is lasing. Low thermal impedance is therefore essential for c.w. operation at high temperatures and will also extend c.w. operating life at any temperature. Life tests at elevated temperatures on our lasers showed that, in the initial stages of ageing, increases in thermal impedance were dominant. Recent improvements in chip processing have increased the stability of the chip lasing parameters to the point where thermal impedance ageing is significant even at room temperature.1 In this paper, we describe failure analyses of early lasers, discuss possible solutions to the thermal-impedance ageing problem, and finally report controlled life tests of several improved metallisations which eliminate the problem.