Frequency locking of single-mode 3.5-THz quantum cascade lasers using a gas cell

We report frequency locking of two 3.5-THz third-order distributed feedback (DFB)quantum cascade lasers(QCLs) by using methanol molecular absorption lines, a proportional-integral-derivative controller, and a NbN bolometer. We show that the free-running linewidths of the QCLs are dependent on the electrical and temperature tuning coefficients. For both lasers, the frequency locking induces a similar linewidth reduction factor, whereby the narrowest locked linewidth is below 18 kHz with a Gaussian-like shape. The linewidth reduction factor and the ultimate linewidth correspond to the measured frequency noise power spectral density.