  By using two mutually phase-locked optical frequency combs with slightly
different repetition rates, we demonstrate asynchronous optical-sampling
terahertz time-domain spectroscopy (ASOPS THz-TDS) without using any trigger
signals or optical delay lines. Due to a tight stabilization of the repetition
frequencies, it was possible to accumulate the data over 48 minutes in a
triggerless manner without signal degradation. The fractional frequency
stability of the measured terahertz signal is evaluated to be
$\sim$8.0$\times$10$^{- 17}$ after 730 s. The frequency accuracy of the
obtained terahertz spectrum is ensured by phase-locking the two frequency combs
to a frequency standard. To clarify the performance of our system, we
characterized the absorption line of water vapor around 0.557 THz. The good
agreement of the measured center frequency and linewidth with the values
predicted from the HITRAN database verifies the suitability of our ASOPS
THz-TDS system for precise measurements.
