Connecting Theory and Practice in Optoelectronics
A fresh look at high-power laser analysis
2016-10-26Posted by on
One of the key rules of semiconductor laser physics relates to the carrier density inside the active layer. As long as I can remember, this rule states that the carrier density remains constant when the injection current rises above the lasing threshold. The reason lies in the stimulated emission of photons which consumes all additional carriers injected above threshold. The threshold carrier density delivers the threshold optical gain that compensates for the optical loss, which is usually not dependent on the injection current. Thus, the threshold carrier density should also remain constant. However, my recent analysis of high-power lasers yields different results (see picture).
In the pictured simulation of a high-power GaN-based laser  the carrier density increases strongly with rising current injection – but only if the self-heating of the laser is included in the calculation. Self-heating is an undesired but common side-effect of laser operation and it leads to the sub-linearity of the measured power vs. current characteristic. Increasing laser temperature is known to reduce the optical gain provided by the active layer at any given carrier density. Consequently, the threshold carrier density needs to rise in order to maintain the threshold gain required for lasing.
This may not be big news to some of you, but it may change the common performance characterization of high-power lasers. The power sub-linearity is often interpreted as a declining slope efficiency – assuming that the threshold current remains constant . Such decline usually reflects increasing optical loss and/or carrier leakage. However, internal absorption and carrier leakage are very small in the pictured simulation. Instead, Auger recombination is identified as main reason for the measured power roll-off, due to the rising carrier density. Thus, the common assumption of constant carrier density and constant threshold current may lead to misinterpretations of high-power laser characteristics.
Nevertheless, advanced laser simulation is not always needed for a more correct analysis of measurements. With constant optical loss, the rising threshold current can simply be extracted by shifting the pictured dashed blue line in direction of the current axis. This way, a new threshold current value is assigned to every power value. But be aware that the analysis gets more complicated with strongly rising optical loss and/or carrier leakage .