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Efficient and cost-effective Si-compatible lasers are a long standing wish ofthe optoelectronic industry. In principle, there are two options. For manyapplications, lasers based on III-V compounds provide compelling solutions,even if the integration is complex and therefore costly. However, where lowcosts and also high integration density are crucial, group-IV-based lasers -made of Ge and GeSn, for example - could be an alternative, provided theirperformance can be improved. Such progresses will come with better materialsbut also with the development of a profounder understanding of their opticalproperties. In this work, we demonstrate, using Ge microbridges with strain upto 6.6%, a powerful method for determining the population inversion gain andthe material and optical losses of group IV lasers. This is made by derivingthe values for the injection carrier densities and the cavity losses from themeasurement of the change of the refractive index and the mode linewidth,respectively. We observe a laser threshold consistent with optical gain andmaterial loss values obtained from a tight binding calculation. Lasing in Ge -at steady-state - is found to be limited to low temperatures in a narrow regimeof tensile strain at the crossover to the direct band gap bandstructure. Weexplain this observation by parasitic intervalence band absorption thatincreases rapidly with higher injection densities and temperature. N-dopingseems to reduce the material loss at low excitation but does not extend thelasing regime. We also discuss the impact of the optically inactive carriers inthe L-valley on the linewidth of group IV lasers.

Investigation of Lasing in Highly Strained Germanium at the Crossover to Direct Band Gap