Frequency-division multiplexing in the terahertz range using a leaky-wave antenna

The idea of using radiation in the 0.1–1.0 THz range as carrier waves for free-space wireless communications has attracted growing interest in recent years, due to the promise of the large available bandwidth1,2. Recent research has focused on system demonstrations3,4, as well as the exploration of new components for modulation5, beam steering6 and polarization control7. However, the multiplexing and demultiplexing of terahertz signals remains an unaddressed challenge, despite the importance of such capabilities for broadband networks. Using a leaky-wave antenna based on a metal parallel-plate waveguide, we demonstrate frequency-division multiplexing and demultiplexing over more than one octave of bandwidth. We show that this device architecture offers a unique method for controlling the spectrum allocation, by variation of the waveguide plate separation. This strategy, which is distinct from those previously employed in either the microwave8 or optical9 regimes, enables independent control of both the centre frequency and bandwidth of multiplexed terahertz channels. High-bandwidth communications require the ability to multiplex and demultiplex (mux/demux) signal channels carrying independent data streams. Networks can enhance data-throughput capacity by increasing the number of non-interfering channels, which may be distinguished in a variety of ways including orbital angular momentum10, spatial mode11 and (most commonly) carrier frequency12. This latter approach has a long history, both in fibre-optic networks and in radiofrequency (RF) and microwave wireless communications for television and radio broadcasts, and more recently in mobile cellular networks. Future wireless networks operating in the subterahertz range1,2 will also require mux/demux capabilities. The challenges of wireless communication in this frequency range place new demands on the technologies of the physical layer. For example, transmission passbands may not be continuous, but may be segmented into multiple frequency windows due to strong frequency-dependent atmospheric attenuation from, for example, water vapour, which may also vary due to weather conditions13,14. Moreover, dynamic bandwidth allocation will be a crucial aspect of implementing efficient networking protocols in a terahertz network for which the transmission channels can be expected to be both bursty and highly directional15. As a result, it will be important to control both the channel frequencies and their bandwidths. Here, we describe a device architecture that exploits the directional nature of terahertz wireless signals to act as a free-space-to-waveguide mux/demux over a broad frequency range and which offers the flexibility to control the spectral bandwidth of each frequency channel. Our multiplexer uses a leaky-wave antenna based on a metal parallel-plate waveguide (PPWG). Leaky-wave antennas of various types have been in use in the RF community for many years, often in the form of a metal waveguide with a narrow slot opened in one of the waveguide walls to permit some of the radiation to ‘leak’ out and couple to free-space modes. In the terahertz range, leaky-wave devices have not been studied in depth and only a few reports have been published to date16–18. Our approach relies on a PPWG in which two sides are open (thus reducing ohmic losses)19 and where the waveguide is wide enough that the edges do not influence propagation. By opening a single slot in one of the two plates20–23, we can allow the guided wave to leak energy into free space, or to receive energy from a free-space wave (Fig. 1). The multiplexing principle originates from the phase-matching requirement for coupling between the waveguide mode and free