Phase locking a 4.7 THz quantum cascade laser using a super-lattice diode as harmonic mixer

We have phase locked a 4.7 THz Quantum Cascade Laser (QCL) to a stable microwave reference signal. The QCL is a 3
^rd
order distributed feedback (DFB) laser with a low power consumption of 0.5 W and radiation power of about 0.25 mW. One of the key elements in our experimental setup is a superlattice diode operating as a harmonic mixer and providing the beat signal between the QCL and the 24
^th
harmonic of a 198 GHz reference signal. A phase locked beat signal is about 20 dB above the noise level for a 30 KHz resolution bandwidth.