Recent advances in electrically pumped Ge lasers

Electrically pumped Ge lasers, integrated on a CMOS platform, are promising candidates as integrated light sources for on-chip photonic systems. The wide gain spectrum from 1520 nm to 1700 nm makes the Ge lasers ideal light sources for WDM applications. There are two main challenges for efficient electrically pumped Ge lasers, high n-type doping concentration and low-loss coupling from Ge waveguides to Si waveguides. The high n-type doping is necessary to overcome the indirect bandgap of Ge and reach gains comparable to other compound semiconductor lasers. We will show that an in-situ delta doping process will yield active Phosphorous concentrations of about 5 × 10¹⁹ cm^-3. The first demonstrated lasers were made from Ge waveguides and did not couple light to Si waveguides, commonly used for on-chip photonic systems. Due to the large refractive index difference between Ge (4.0) and Si (3.5) low loss coupling from a Ge waveguide to a Si waveguide is challenging but a requirement for efficient Ge lasers. We will discuss the different device designs to provide low loss coupling for low threshold lasing.