Phase control of a terahertz quantum cascade laser using an optical injection phase-lock loop (Conference Presentation)

Absolute frequency and phase control of a coherent terahertz (THz) source is desirable for high-resolution spectroscopy of atoms and molecules, coherent communications and advanced imaging techniques. Here we report on the phase control of a 2.0 THz quantum cascade laser (QCL). The QCL is optical injection locked (OIL) to an infrared frequency comb, which is generated by the successive modulation of a ‘C-band’ laser in a recirculating fibre loop. A stable microwave source defines the spacing between the comb lines resulting in the QCL being locked to an integer harmonic of the microwave frequency; the reference frequency. Within the locking range the frequency of the injection locked QCL is locked to the reference frequency, whereas the phase of the QCL undergoes a ‘π’ phase shift across the locking range. In this work, we control the phase of the QCL by introducing a phase lock loop to the OIL system to provide feedback to the QCL current forming an optical injection phase locked loop. This has several advantages over the bare OIL system: (1) the underlying frequency of the QCL is stabilised so that the QCL remains within the locking range for long periods of time. (2) The QCL frequency and phase track the microwave frequency so that the QCL may be tuned with extremely high precision. (3) By changing the locking point for the PLL the phase of QCL relative to the reference frequency could be controlled within a range 0.4π, limited by the PLL, with a constant amplitude.