Unstable characteristics of complex coupled laser diode with InGaAs absorptive grating

Recently, there have been many efforts on developing loss coupled (LC) laser diodes, one type of complex coupled (CC) DFB-LD, using a periodic light intensity change by an absorption medium adjacent to an active layer. In practice, the fabrication of LC DFB-LD is more stable and more reproducible compared to gain coupled (GC) DFB-LDs which use active layer etching and regrowth on it. However, the effects of the selection of an absorption layer on the characteristics of LC DFB-LD have not been analyzed enough yet. One of them is the dynamic property of LC DFB-LD depending on the doping type of the InGaAs absorbing layer. There was a first report on the abnormal lasing properties of LC DFB-LD in which the p-InGaAs absorptive grating was surrounded by p-type cladding and placed above an active layer, like most of LC DFB-LD structures. It was estimated that the saturation of the light absorption in the p-InGaAs layer caused the unstable lasing characteristics, because such phenomena were not found in n-InGaAs absorption gratings embedded in p-cladding layers, so called the conduction inverted type of LC DFB-LD. On the other hand, there was a theoretical report that the unstable lasing property resulted from the long carrier lifetime and the high differential gain in the absorption layer. Here we show first experimental evidence that the abnormal lasing property comes from the long carrier transport effect of p-doped absorption material rather than the combination between an absorptive layer and the adjacent layers like the conduction inverted type.