Gain Compression and Linewidth Enhancement Factor in Mid-IR Quantum Cascade Lasers

We have observed and quantified the adiabatic and transient chirp in a directly modulated quantum cascade laser (QCL). Those wavelength tuning effects are well-characterized in diode lasers, and the rate equation model that successfully describe the diode laser behavior also provides an excellent fit for the QCL data. In this study, we have extracted the linewidth enhancement factor (α_H) from the transient chirp and the gain compression factor from the adiabatic chirp. We postulate that the extraction of the α_H from the transient chirp is valid for the QCL case, despite additional tuning effects in QCLs (e.g., voltage tuning) that are negligible in diode lasers. Also in the QCL the adiabatic chirp coefficient strongly increases with laser output power but still stays an order of magnitude below typical values known from diode lasers. We hypotesize possibility of the adiabatic chirp to be connected to the χ⁽³⁾ nonlinearity (responsible for four-wave mixing that was recently attributed to the FM self-locking in QCLs), but the exact origin remains to be experimentally confirmed in future work.