Phase-locking of 2.4 and 2.7 Terahertz Quantum Cascade Lasers to a femtosecond mode-locked Er-fiber laser

We have used the Pockels effect in ZnTe to demonstrate that the frequency and the phase of Quantum Cascade Lasers emitting at 2.4 and 2.7 Terahertz can be actively stabilized to the n-th harmonic of the 90 MHz repetition rate of a commercial, mode-locked Er-doped fiber-laser. The beating between the stabilized QCL frequency and the harmonic of the repetition rate yielded a signal-to-noise ratio >; 80dB in 1 Hz bandwidth (limited by the resolution of our spectrum analyzer). The technique demonstrated is inherently broadband, in the sense that it allows to directly stabilize any QCL source provided that its frequency is within the spectral bandwidth of the fs-laser (~5 THz).