The thermal (emitted) infrared frequency bands, from 20–40 THz and 60–100 THz, are best known for applications in thermography. This underused and unregulated part of the spectral range offers opportunities for the development of secure communications. The ‘THz Torch’ concept was recently presented by the authors. This technology fundamentally exploits engineered blackbody radiation, by partitioning thermally-generated spectral noise power into pre-defined frequency channels; the energy in each channel is then independently pulsed modulated and multiplexing schemes are introduced to create a robust form of short-range secure communications in the far/mid infrared.

To date, octave bandwidth (25–50 THz) single-channel links have been demonstrated with 380 bps speeds. Multi-channel ‘THz Torch’ frequency division multiplexing (FDM) and frequency-hopping spread-spectrum (FHSS) schemes have been proposed, but only a slow 40 bps FDM scheme has been demonstrated experimentally. Here, we report a much faster 1,280 bps FDM implementation. In addition, an experimental proof-of-concept FHSS scheme is demonstrated for the first time, having a 320 bps data rate. With both 4-channel multiplexing schemes, measured bit error rates (BERs) of. W ireless ship-to-ship communications, via optical signalling, was implemented by the British Royal Navy in 1867.

This represents an early example of pulsed-banded-noise communications, whereby data is transmitted intentionally within the visible spectral range. Today, the commonwealth navies and NATO forces still use signal lamps when secure radio communications need to be silent or electronic ‘spoofing’ is likely. Signalling at night is usually conducted in the near-infrared part of the spectrum, using filtered incandescent light bulbs and requiring night vision sights, minimising the risks of being intercepted or jammed.

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