Microwaves for the Last Mile

In the new modulator millimeter wave signals (blue) are received by an antenna and converted into optical signals (red) inside the tiny slot in the middle. The device works without a power supply and is less than a millimeter in size. (Source: ETHZ, J. Leuthold)

The high oscil­lation frequencies of light waves make them ideally suited to fast data trans­mission. They can be sent through optical fibers and easily carry Gigabits per second. The “last mile” from a central fiber optic cable to the internet socket at home, however, is the most difficult and expensive. Some alter­natives, for instance 4/5G mobile telephony, are cheaper, but they cannot provide all users simul­taneously with the extremely high trans­mission rates required by today’s data-hungry appli­cations such as streaming TV.

Jürg Leuthold, professor at the Institute for Electro­magnetic Fields at ETH Zurich, and his colla­borators have now, with support by colleagues at the Univer­sity of Washington in Seattle, developed a novel light modulator that will make it possible in the future to cover the last mile effi­ciently and at a low cost with high-frequency micro­waves and hence high data trans­mission rates. To transfer data encoded in optical fibers through a variation in the light intensity onto milli­meter waves, very fast and hence expensive electronic compo­nents are needed. In the opposite direction, millimeter waves first have to be received by an antenna, then ampli­fied and mixed down to base­band and finally injected into a light modulator, which trans­lates the data contained in the radio waves back into light pulses.

Leuthold and his colleagues have now succeeded in building a light modulator that works entirely without batteries and elec­tronics. “That makes our modulator completely inde­pendent of external power supplies and, on top of that, extremely small so that it can, in principle, be mounted on any lamppost. From there, it can then receive data via micro­wave signals from indi­vidual houses and feed them directly into the central optical fiber”, explains Yannick Salamin, who made crucial contri­butions to the development of the new modu­lator.

The modulator consists of a chip measuring less than a millimeter that also contains the microwave antenna. That antenna receives the millimeter waves and converts them into an electric voltage. The voltage then acts on a thin slot at the centre of the chip. There, a narrow slit, just a few micro­meters long and less than a hundred nano­meters wide, is filled with a material that is particularly sensitive to electric fields. The light beam from the fiber is fed into that slit. Inside the slit, however, the light propa­gates – differently from the fiber optic cable or air – no longer as an electro­magnetic wave, but as a plasmon. Plasmons are hybrid creatures made of electro­magnetic fields and oscil­lations of electric charge at the surface of a metal. Owing to this property, they can be confined much more tightly than light waves.

The electri­cally sensitive, nonlinear material inside the slit ensures that even the tiniest electric field created by the antenna will strongly influence the propa­gation of the plasmons. That influence on the oscil­latory phase of the waves is conserved when the plasmons are converted back into light waves at the end of the slit. In this way, the data bits contained in the milli­meter waves are trans­ferred directly onto the light waves without taking a detour through elec­tronics, and without any external power. In a laboratory experiment with microwave signals at 60 Gigahertz, the researchers were able to demon­strate data transmission rates of up to 10 Gigabits per second over a distance of five metres, and 20 Gigabits per second over one meter.

Besides the tiny size and the negligible energy consumption, the new modulator has a number of further advan­tages. “The direct transfer from milli­meter waves to light waves makes our modulator parti­cularly versatile regarding the frequency and exact format of data encoding”, Leuthold emphasizes. In fact, the modulator is already compatible both with the new 5G technology and with future industry standards based on milli­meter wave and terahertz fre­quencies of 300 Gigahertz and data trans­mission rates of up to 100 Gigabits per second. Moreover, it can be produced using conven­tional silicon tech­nology, and thus at a compara­tively low cost.

Finally, Leuthold can reassure users who might be worried about the electro­magnetic radiation involved. Dif­ferently from the radio waves or microwaves of a WiFi modem, which propa­gate evenly in all direc­tions, millimeter waves can be strongly focused for trans­mission to the outside and only propagate between the roof antenna and a light pole inside a beam that is twenty centi­meters in diameter. This strongly reduces the power needed for trans­mission compared to other wireless tech­nologies. It also elimi­nates the typical problems of WiFi modems, whose signals can get in each other’s way. (Source: ETHZ)

Reference: Y. Salamin et al.: Microwave plasmonic mixer in a transparent fibre–wireless link, Nat. Phot., online 29 October 2018; DOI: 10.1038/s41566-018-0281-6

Link: Institute of Electromagnetic Fields IEF, ETH Zurich, Zurich, Switzerland

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