  Broadband dual-comb spectroscopy has attracted increasing interests due to
its unique advantages in high spectral resolution, fast detection, and so on.
Although the dual-comb technique is relatively mature in the infrared
wavelengths, it is, currently, not commercially capable of practical
applications in the terahertz regime due to the lack of high performance
broadband terahertz dual-comb sources. In the terahertz frequency range, the
electrically pumped quantum cascade laser (QCL) is a suitable candidate for the
dual-comb operation. However, free running terahertz QCL dual-comb sources
normally show limited optical bandwidths ($\sim$100-200 GHz). Although the
resonant microwave injection locking has been widely used to broaden the
emission spectra of terahertz QCLs by modulating the laser drive current at the
cavity round-trip frequency, it is hard to be employed to broaden the dual-comb
bandwidths due to the large phase noise induced by the resonant injection and
non-ideal microwave circuits. Therefore, it is challenging to obtain broadband
terahertz dual-comb sources that can fully exploits the laser gain bandwidth.
Here, we employ an off-resonant microwave injection to significantly broaden
the dual-comb bandwidth of a terahertz QCL dual-comb source emitting around 4.2
THz. The measured optical dual-comb bandwidth is broadened from 147 GHz in free
running to $>$450 GHz under the off-resonant injection. The broadened dual-comb
bandwidth is experimentally proved by the transmission measurements of a filter
and a GaAs etalon. By performing a simple numerical analysis based on a rate
equation model, we explain that the broadband dual-comb operation under the
off-resonant microwave injection could be resulted from a wider lasing
bandwidth and a higher degree of phase matching.
