High-resolution frequency and phase control of a terahertz laser

We report on the high-resolution frequency and phase control of a terahertz (THz) quantum cascade laser (QCL). The 2.0 THz QCL is locked to a stable microwave reference frequency via an all-fibre infrared frequency comb. The frequency of the QCL is controlled by optical injection and the phase is independently controlled by introducing a phase-lock loop that provides electronic feedback to the QCL, forming an optical injection phase locked loop (OIPLL). By implementing this, (1) the instrument limited linewidth of the locked QCL was $ {<; }1$ Hz (2) for a fixed frequency, the phase of the THz QCL relative to the microwave reference frequency can be controlled within a range 0.3 π.