CW photomixing systems
We have demonstrated performance of cw photomixing systems at the state of the art level in the past, Ref. 11. Further, we have suggested a concept, how to control THz phase in the cw THz photomixing systems with the help of an electro-optical phase modulator, Ref. 12. The concept allows one to get rid of all movable mechanical components in cw THz systems and enables all-in-fiber realization of the systems. That makes tremendous improvement in reliability and simplicity of the THz systems possible. The whole system could be even integrated in one chip. We have demonstrated the concept experimentally, Ref. 12,13. Additionally, the concept enables implementation of a special saw-tooth modulation of the THz phase and that makes possible the measurement of both THz amplitude and phase with a single sampling point per frequency, Ref. 13. As a result, the measurement speed of the system is increased by an order of magnitude without performance degradation. The concept is presently used around the world in the commercial cw photomixing systems.
On-chip THz sub-systems and sensors
We were investigating optically driven on-chip cw THz spectrometers on the basis of coplanar waveguides and photomixers, Ref. 14. We have applied the measurement concept outlined above to such integrated on-chip THz subsystems: we modulate nothing but the THz phase in our measurements. The approach drastically reduces the noise and therefore solves the major difficulty in cw on-chip measurements. In addition, we improve the signal level and signal-to-noise ratio of our on-chip THz transceiver even further by employing finger photomixers. We have improved the dynamic range of the transceiver by several orders of magnitude as compared to standard chopping techniques. This allowed us to extend the frequency range of cw on-chip THz transceiver beyond 1~THz, which is roughly a factor of 5 improvement as compared to previous reports.
We have achieved the record frequency of 460 GHz with p-i-n photodiodes in 2001, Ref. 15. Further on, an analytical model of the uni-travelling-carrier p-i-n photodiodes has been developed by us in 2007, Ref. 16. The different mechanisms of THz-power limitation of the photodiodes were analyzed and we could show that the increase of the output THz power of the photodiodes by an order of magnitude as compared to the present state-of-the-art should be possible.