Free-running 9.1-<i>μ</i>m distributed-feedback quantum cascade laser linewidth measurement by heterodyning with a C¹⁸O₂ laser

We report spectral linewidth measurements of a 9.1-<i>μ</i>m 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. Beating was performed at two operating conditions, first near the QCL laser threshold (beating with the C¹⁸O₂<i>R</i>10 line) and then at a high injection current (beating with the C¹⁸O₂<i>R</i>8 line). Overall, beat note widths of 1.3–6.5 MHz were observed, which proves that a free-running QCL can have a short-term spectral width near 1 MHz.