The influence of carrier density non-pinning on the output power of 1.55 μm lasers at high temperature

Maximising the output power is of key importance in the development of lasers in many applications. When operating at several times laser threshold, the threshold current, Ith, and its temperature sensitivity, T_o (=I_th(dI_th/dT)^-1) are often ignored since optimisation schemes typically aim to minimise optical losses which degrade the differential quantum efficiency, η_d. In an ideal laser, the carrier density pins at threshold and hence, in principal, all of the injected current above threshold is available to produce stimulated emission. In this paper we show that this not the case since, under the high injection conditions required to achieve high powers, internal heating causes non-pinning of the carrier density due to increased thermal broadening of the Fermi distribution of carriers. This means that above laser threshold one has to inject a higher carrier density (n) to maintain the threshold gain condition. This non-pinning of the carrier density above I_th consequently leads to an increase in the recombination currents which constitute the threshold current. This is particularly problematic if Auger recombination dominates, due to its n³ dependence and correspondingly strong temperature dependence, and results in significantly less current available to produce stimulated emission. We show that the degree of non-pinning can in fact be significant, and have a large impact on the maximum obtainable power through its effect on non-radiative recombination processes.