  In a continuous-wave terahertz system based on photomixing, the measured
amplitude of the terahertz signal shows an uncertainty due to drifts of the
responsivities of the photomixers and of the optical power illuminating the
photomixers. We report on a simple method to substantially reduce this
uncertainty. By normalizing the amplitude to the DC photocurrents in both the
transmitter and receiver photomixers, we achieve a significant increase of the
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 non-ideal environmental
conditions outside of a temperature-stabilized laboratory.
