Phase-locking of a 3.1THz quantum cascade laser to terahertz reference generated by a frequency comb

We are developing a low noise heterodyne receiver at 3THz band for applications of atmospheric and astronomical research and for application of wireless communications. We have developed a phase-locking system of a terahertz quantum cascade laser (THz-QCL) by using a hot electron bolometer mixer. The THz-QCL was locked to terahertz reference generated by an optical frequency comb and a photo mixer. The beat signal with IF frequency of 230MHz was compared with a microwave reference, and the error signal was applied to a loop filter with feedback to the bias current of the THz-QCL. The line width of the phaselocked beat signal of narrower than 1Hz which is limited by the resolution bandwidth of a spectrum analyzer was achieved.