Sensing μm-scale vibrations in the Hz range within a THz communication system

We demonstrate a THz-band integrated sensing and communication (ISAC) system that detects micrometer-scale vibrations of a transmitter in the Hz range within a transmitted free-space signal. Here, we use an optical frequency comb to generate the THz signal. The comb thereby provides higher sensitivity to vibration, as it allows for the suppression of the frequency noise of the THz carrier. Further, we achieve sensing in parallel with high-capacity communications within the same hardware configuration and the same signal. Towards this end, we use the forward transmitted signal directly detected by a receiver. Utilizing the forward transmitted signal has the advantage that the received signal has a much higher power compared to traditional sensing of backscattered signals, where high free-space path losses pose a receiver sensitivity challenge. The arrangement was tested in an experiment. In an anomaly detection scenario involving a 9-meter tower, the detection of an abnormal frequency shift in vibration reproduced using field data was demonstrated within a 220-330 GHz-band ISAC system, which transmitted 2 × 112.8 Gb/s probabilistically shaped 16QAM simultaneously. In this setup, Doppler frequency fluctuations induced by the ±3.8 μm vibrations at 45 Hz have been detected.