Dispersive mirrors for phase delay variation in THz-homodyne systems

In THz homodyne systems, optical delay lines are the key to time-resolved measurements but they come with a high cost and complexity. They also limit the application of the system in industrial environment, due to their sensitivity to vibrations. Another important point is the scanning speed, for which the mechanical delay line sets severe limitations. Frequency scanning-based systems need a change in THz frequency to recover phase information. Furthermore, there is a tradeoff between phase sensitivity and necessary tuning range. This tradeoff is based on the difference in the length of transmitter and receiver arm in the setup. With our approach, we can introduce a controllable phase shift at 280 GHz by frequency tuning of both lasers. For that purpose, chirped mirrors were designed and introduced into a standard continuous wave Terahertz homodyne system, in order to induce a variable phase shift. In our chirped mirror-based configuration, the phase shift between both optical modes depends on the center frequency of the lasers. Thus, moving the delay stage can be replaced by variation of the center frequency in order to record a THz trace. This means that the measurements are no longer limited by the speed of the delay line. This phase shift is independent of the path length difference in the setup and does not need phase modulators. Simulations show, that these mirrors may achieve a phase shift up to pi inside the C-Band for a difference frequency of 280 GHz. To confirm the calculated behavior of the chirped mirrors, initial characterization measurements were performed. We modified an existing delay stage-based THz system to include the chirped mirrors in front of the receiver. This enables the direct comparison while keeping all other parameters constant.