First Direct Observation of Small Linewidth Enhancement Factor of Fabry–Perot Quantum Cascade Laser

We investigated the linewidth enhancement factor α of a Fabry–Perot quantum cascade laser by self-mixing interferometry. For the first time, the theoretically predicted characteristic waveform resulting from a small α of about -0.3 could be obtained when the optical feedback parameter was greater than one. The shape of this waveform was totally different from that of a conventional distributed-feedback laser diode with a large α. We demonstrated that subband nonparabolicity is one of the reasons why quantum cascade lasers have a small, but non-zero, α value.