  Frequency combs are powerful tools for many applications and high
performances are achieved by stabilizing these lasers. For operation in the
mid-infrared, quantum cascade lasers (QCL) are ideal candidates as they present
numerous advantages. However, stabilized QCL-combs lack of a detailed
characterization of their noise properties due to the sensitivity limits of
current analyzing techniques. To overcome these challenges, we developed what
we believe to be the first tightly locked dual QCL-comb system. Its use is
twofold. First, phase noise analysis of the dual-comb signal shows residual
phase noise below 600 mrad for all comb lines, and we characterize the comb
coherence as well as the performances of the repetition frequency locking
mechanism. Second, we demonstrate coherent averaging with a $7 \times 10^5$
Hz$^{1/2}$ figure-of-merit system, which is compatible with high precision
spectroscopy.
