Precise Evaluation of a Phase-Locked THz Quantum Cascade Laser

We have demonstrated the phase-locking of a THz quantum cascade laser (THz-QCL) toward the realization of an accurate and stable local oscillator for a high-resolution receiver. The beat note between the THz-QCL and a THz reference was obtained by heterodyne mixing in a superconducting hot electron bolometer mixer (HEBM) and used for stabilizing the phase of 3.1 THz radiation from the THz-QCL. The phase-locked 3.1 THz radiation was fully evaluated with a superlattice harmonic mixer operating in the THz band, and this revealed that the THz-QCL synchronized with the microwave reference with a fractional frequency instability of
${\hbox{3}}\times {\hbox{10}}^{-15}$
at an averaging time of 100 s, corresponding to a center frequency deviation within 1 mHz, and the imposed phase noises during the heterodyne mixing were negligibly small.