Continuous wave terahertz spectroscopy based on photomixing offers the attractive feature of detecting both amplitude and phase of the terahertz radiation. Experimentally, it is challenging to achieve sufficient accuracy at a high data acquisition rate. We use two fiber stretchers as fast phase modulators in a symmetric setup. Compared to a mechanical delay stage, the fiber stretchers are rather fast ($\approx 1\mathrm{kHz}$), which enables us to record a spectrum up to 1.8 THz with a step size of 1 GHz in only 10 min. We achieve a stability of the optical path difference of around 10 μm and use low-doped Si as an example to demonstrate the performance of our spectrometer.

A theoretical model for the responsivity and noise-equivalent power (NEP) of photoconductive antennas (PCAs) as coherent, homodyne THz detectors is presented. The model is validated by comparison to experimental values obtained for two ErAs:InGaAs PCAs. The responsivity and NEP were obtained from the measured rectified current, the current noise floor in the PCAs, and the incoming THz power for the same conditions. Since the THz power measurements are performed with a pyroelectric detector calibrated by the National Metrology Institute of Germany (PTB), the experimentally obtained values are directly traceable to the International System of Units (SI) for the described conditions. The agreement between the presented model and the experimental results is excellent using only one fitting parameter. A very low NEP of 1.8 fW/Hz at 188.8 GHz is obtained at room temperature.

In a continuous-wave terahertz system based on photomixing, the measured amplitude of the terahertz signal shows a variability due to drifts of the responsivities of the photomixers and of the optical power illuminating the photomixers. We report a simple method to substantially reduce this variability. By normalizing the amplitude to the DC photocurrents in both the transmitter and receiver photomixers, we achieve a significant increase in stability. If, e.g., the optical power of one laser is reduced by 10%, the normalized signal is expected to change by only 0.3%, i.e., less than the typical uncertainty due to short-term fluctuations. This stabilization can be particularly valuable for terahertz applications in nonideal environmental conditions outside of a temperature-stabilized laboratory.