Wavelength splitting is a problem in QD lasers; coupling two lasers together fixes it
Eliminating wavelength splitting in quantum-dot lasers boosts spectral purity and brightness.
A team from the University of Utah (Salt Lake City, UT), Georgia Institute of Technology (Atlanta, GA), the Air Force Research Laboratory (Wright-Patterson Air Force Base, OH), and Hongik University (Sejong, South Korea) has discovered how to fix a major problem that occurs in lasers that use quantum dots (QDs) as the lasing medium.1 QD-based lasers are a candidate for light sources on photonic integrated circuits (PICs).
QD-based lasers because they can tune output wavelengths and other properties simply by growing the crystals in different sizes by using different semiconducting materials and choosing different laser geometries. The downside is that QD lasers often contain minuscule defects creating parasitic intracavity modes that split the light into multiple wavelengths, reducing the laser's spectral brightness.
The new study sought to correct this defect. First, collaborators from the Georgia Institute of Technology made 50 microscopic disk-shaped QD lasers out of cadmium selenide. The Utah researchers then showed that that almost all of the individual lasers had defects that caused wavelength splitting.
The researchers then coupled two lasers together to correct the wavelength splitting. They ran one laser at full gain; when the second laser had no gain (no pumping), the difference between the two lasers prevented any interaction and splitting still occurred. However, when the team (optically) pumped the second laser and the gain in the two lasers became similar, the interaction between the two lasers corrected the splitting and focused the energy into a single wavelength. This is the first time anyone has observed this phenomenon. The findings have implications for optics and photonics research.
"It's not impossible that someone could make a defect-free laser with quantum dots, but it would be expensive and time-consuming," says Evan Lafalce, lead author of the study. "In comparison, coupling is a quicker, more flexible, cost-effective way to correct the problem. This is a trick so that we don't have to make perfect quantum dot lasers."
1. Evan Lafalce et al., Nature Communications (2019); https://doi.org/10.1038/s41467-019-08432-6