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.

Schematic: photomixing system based on 2 lasers, featuring 2 photomixers, a delay stage and a mechanical chopper. The THz radiation is guided to the detector via a set of parabolic mirrors.

© Michael Feiginov

Schematic of a photomixing system

Schematic: alternative configuration of the photomixing system. 4 beam splitters are used to create 2 combined optical beams. One beam features an optical phase modulator.

© Michael Feiginov

Schematic of a photomixing system with an optical phase modulator for THz phase control

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.

Picture: integrated chip with 2 photomixers connected by a coplanar waveguide (4mm long). 2 types of photomixers: simple gap and interdigitated type.

© Michael Feiginov

An integrated chip containing two photomixers connected by a coplanar waveguide

Graph: Comparison of measured transmission spectra of gap photomixer and interdigitated finger photomixer. Interdigitated finger photomixer has a higher transmission coefficient.

© Michael Feiginov

Transmission spectra of an integrated photomixer chip

P-i-n photodiodes

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.

Diagram: photodiode THz-emitter module. Photodiode integrated with slot antenna and biasing circuit with sub-mm wave filters, mounted on top of a hemispherical Si lens.

© Michael Feiginov

THz emitter with a p-i-n photodiode