Characterising the Photonic-Assisted Coherent Free-Space Link of Sub-THz Data Transmission

This study demonstrates a measurement setup of an over-the-air (OTA) sub-THz link using a hybrid system of transmission comprising standard optical communications components and propagating the signal over 6 meters. The emitter was a p-i-n photodiode (PIN-PD), and the receiver was a sub-harmonic down-converter. The transmission testbed for the frequencies between 240 GHz to 300 GHz was characterised. Key parameters including the optical frequency, frequency hopping, the direct current (DC) bias of the emitter, the equalisation taps, and the digital signal processing stages were examined to optimise performance. An error-free 25.6 Gbit/s QPSK signal was achieved. A 16-QAM characterisation investigated the power combination from the two optical branches fed to the photodiode (PD) and its influence on the error vector magnitude (EVM) of the in-phase and quadrature (IQ) amplitude linearity and the probability density estimate. An optimum 16-QAM constellation was achieved using power combination of 5 mW and 10 mW for the optical modulator and optical carrier power, respectively. Finally, an image transmission of 24 data packets was performed and achieved error-free QPSK image reception at 25 Gbit/s.