Comparison of Interferometric and Electronic Phase Detection for TDS Systems

Terahertz time-domain spectroscopy requires a precise knowledge of the phase difference between two pulse trains in order to obtain high-quality terahertz spectra. In terahertz time-domain spectroscopy with systems based on repetition-rate modulation, the phase difference is commonly measured electronically utilizing a detection of the pulse trains’ higher-harmonics’ difference signal via fast photodiodes in combination with a frequency mixer. However, the electronic phase detectors used to conduct these measurements are prone to exhibit both a non-negligible timing jitter and a systematic error. Here, we present the results of our recent attempt to reduce these errors in our own electronic phase detection systems. A more than six-fold timing-jitter reduction from 59.0 fs to 8.6 fs led to a significant increase in both exploitable terahertz bandwidth and signal-to-noise ratio. Additionally, utilizing our interferometrically monitored delay line as a calibration standard, the systematic error could be removed almost entirely and thus, excellent resolution of spectral absorption lines be accomplished.