Frequency and phase-lock control of a 3?THz quantum cascade laser

We have locked the frequency of a $3\mathrm{THz}$ quantum cascade laser (QCL) to that of a far-infrared gas laser with a tunable microwave offset frequency. The locked QCL line shape is essentially Gaussian, with linewidths of 65 and $141\mathrm{kHz}$ at the $-3$ and $-10\mathrm{dB}$ levels, respectively. The lock condition can be maintained indefinitely, without requiring temperature or bias current regulation of the QCL other than that provided by the lock error signal. The result demonstrates that a terahertz QCL can be frequency controlled with 1-part-in-${10}^8$ accuracy, which is a factor of 100 better than that needed for a local oscillator in a heterodyne receiver for atmospheric and astronomic spectroscopy.