  Employing two frequency combs with a slight difference in repetition
frequencies, the dual-comb source shows unique advantages in high precision
spectroscopy, imaging, ranging, communications, etc. In the terahertz (THz)
frequency range, the electrically pumped quantum cascade laser (QCL) offers the
possibility of realizing the compact dual-comb source due to its
semiconductor-based chip-scale configuration. Although the active stabilization
of a THz QCL dual-comb source was demonstrated by phase locking one of the
dual-comb lines, the full stabilization of all dual-comb lines is still
challenging. Here, we propose a self-reference method to obtain a fully
stabilized dual-comb signal on a pure THz QCL platform. Without using any
external locking components, we filter out one dual-comb line and beat it with
the whole dual-comb signal, which eliminates the common carrier offset
frequency noise and reduces the dual-comb repetition frequency noise. It is
experimentally demonstrated that the self-reference technique can significantly
improve the long-term stability of the dual-comb signal. A record of the
``maxhold" linewidth of 14.8 kHz (60 s time duration) is obtained by
implementing the self-reference technique, while without the self-reference the
dual-comb lines show a ``maxhold" linewidth of 2 MHz (15 s time duration). The
method provides the simplest way to improve the long-term stability of THz QCL
dual-comb sources, which can be further adopted for high precision
measurements.
