Free-running 9.1-microm distributed-feedback quantum cascade laser linewidth measurement by heterodyning with a C18O2 laser.

We report spectral linewidth measurements of a 9.1-microm distributed-feedback quantum cascade laser (QCL). The free-running QCL beam was mixed with a waveguide isotopic C18O2 laser onto a high-speed HgCdTe photomixer, and beat notes were recorded from a radio-frequency spectral analyzer.