Linewidth Enhancement Factor of Mid-IR Quantum Cascade Lasers

Schawlow and Townes theorized that the ratio between the spontaneous emission and stimulated emission in a semiconductor laser defines its linewidth lower limit [1] . However, experiments showed that this intrinsic linewidth is larger, which was later demonstrated to be due to the coupling between the optical gain and the phase noise [2] . The linewidth enhancement factor (LEF) was introduced to quantify this process and as a correction to the linewidth formula in [1] .