Terahertz waves, which span a frequency range of 300 to 3000 gigahertz, promise a hundred times faster data transmission than today’s cellular and wireless networks. Researchers now face the daunting task of designing and building a communication system with all new components that can work with terahertz radiation. Many efforts have focused on building compact terahertz sources, transmitters and detectors.
Now a group at Brown University in Providence, R.I., has built another key component of any wireless communication system: a multiplexer. Multiplexers combine separate data streams, typically each at a different frequency, into a single combined stream that is sent over optical fibers, TV cables, or telephone lines, making it possible for that medium to carry thousands of phone calls or tens of TV channels at the same time. A demultiplexer separates the signals at the receiver end.
The new device, which acts as both a multiplexer and demultiplexer, is reported in the journal Nature Photonics. It consists of two smooth aluminum plates (any metal should work) placed in parallel, with a few-millimeter gap between them. The plates act like a waveguide for terahertz waves, which travel between them.
To explain how the multiplexer works, it’s helpful to first describe how signals are separated at the receiving end. As the waves travel through the waveguide, some of the radiation leaks out of the slit. The angle at which it emerges depends on the frequency of the wave. So if an entering wave contains multiple frequencies, each carrying a separate data channel, they would each come out a different angle, effectively separated or demultiplexed.
The converse occurs with a multiplexer; the device accepts a signal of a certain frequency only if it comes in at a certain angle.
“We’re envisioning free-space wireless terahertz communication networks,” says Daniel Mittleman, a professor of engineering at Brown University. “But the important difference between a terahertz network and the existing cellular network is that the antenna is broadcasting in all directions. With terahertz, there’s 100 times less diffraction so it’s much more reasonable to think of it as a beam with some directionality.”
The researchers demonstrated a simple multiplexing scheme in which they sent two independent THz-frequency channels to the multiplexer at two different angles. For the source, they used a commercial femtosecond laser, which delivers short pulses of terahertz radiation and is used for spectroscopy. In a practical multiplexer, specially-shaped waveguides and optics could send hundreds of different channels into the multiplexer at different angles.
The device is completely passive right now. But Mittleman says he and his team could make it dynamic by varying the spacing between the plates. The researchers found that this spacing affects the frequency, angle, and bandwidth of the signal that leaks out of the slit. This could, for instance, be used to tune the bandwidth of the channels, Mittleman says.
“In the waveguide, imagine that the lower plate has a trench dug into it parallel to the slot and directly underneath,” he says. “You could use a silicon microelectromechanical switch to dynamically tune the depth of the trench at any location, so that the plate separation varies underneath slot. That would change the bandwidth of that channel.”
The researchers hope to make and demonstrate an active device of that type that changes positions on the timescale of a few milliseconds.