Millimeter waves for the last kilometer

Researchers at ETH Zurich have developed a modulator by which data transmitted via millimeter waves can be converted directly into light pulses for optical fibers. This could help cover the "last mile" until the Internet takes home considerably faster and cheaper.

Image caption: The last mile to the home Internet connection is also the most demanding. The new modulator is a viable alternative. The data transmitted by the millimeter wave (red arrows) can be converted directly into pulses for the optical fiber (yellow). (Visualizations: Salamin Y et al., Nature Photonics 2018)

The high oscillation frequencies of light waves make them ideal for fast data transmission. They can be sent via fiber optics and easily transport hundreds of billions of bits (Gigabits) per second. The "last mile" between a central fiber optic cable and the home Internet plug is however the most difficult and the most expensive. Some alternative solutions, such as 4 / 5G mobile phones, are cheaper, but they can not simultaneously provide all users with the extremely high transmission rates required by today's data-hungry applications, such as streaming TV.

Jürg Leuthold, professor at the Institute of Electromagnetic Fields of ETH Zurich, and his collaborators have now, with the support of colleagues from the University of Washington in Seattle, developed a new modulator of light that will allow the future to travel the last mile efficiently and low cost with high frequency microwave – called millimeter wave – and therefore high data rates.

Light modulator without electronics

To transfer coded data in optical fibers by a variation of light intensity on millimeter waves, very fast – and therefore expensive – electronic components are needed. In the opposite direction, the millimeter antennas must first be received by an antenna, then amplified and mixed up to the baseband, and then injected into a light modulator, which converts the data contained in the radio waves into light pulses. .

Leuthold and his colleagues have now successfully built a light valve that works entirely without batteries or electronics. "This makes our modulator totally independent of external power sources and, in addition, extremely small, so that it can in principle be mounted on any street lamp. From there, it can then receive data from individual homes via microwave signals and feed them directly into the central fiber optics, "says Yannick Salamin, a PhD student who has made a vital contribution to the development of the new modulator.

Modulation by plasmons

The modulator built by ETH researchers consists of a chip of less than one millimeter also containing the microwave antenna. This antenna receives millimeter waves and converts them into an electrical voltage. The voltage then acts on a thin slit in the center of the chip – the very core of the modulator. There, a narrow slot a few micrometers long and less than one hundred nanometers wide is filled with a material particularly sensitive to electric fields. The light beam of the fiber is introduced into this slot. However, inside the slot, light propagates – differently from the fiber optic cable or air – no longer as an electromagnetic wave, but as a plasmon. Plasmons are hybrid creatures consisting of electromagnetic fields and oscillations of electric charges on the surface of a metal. Because of this property, they can be confined much more closely than light waves.

In the new modulator developed at ETH Zurich, millimeter wave signals (blue) are received by an antenna and converted into optical (red) signals inside the small centrally located slot. The device runs without power supply and measures less than one millimeter. (Visualizations: ETH Zurich / Jürg Leuthold)

The electrically sensitive ("nonlinear") material inside the slot ensures that even the smallest electric field created by the antenna will have a strong influence on plasmon propagation. This influence on the oscillatory wave phase is maintained 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 to the light waves – without detour via the electronics and without any external power supply. In a laboratory experiment with microwave signals at 60 Gigahertz, researchers were able to demonstrate data transmission speeds of up to 10 gigabits per second over a distance of five meters and 20 gigabits per second over one meter.

Cheap and versatile

In addition to its small size and negligible energy consumption, the new modulator has many other advantages. "The direct transfer of millimeter waves to light waves makes our modulator particularly versatile in terms of the exact frequency and format of data encoding," says Leuthold. In fact, the modulator is already compatible with both the new 5G technology and with future industry standards based on millimeter wave and terahertz frequencies of 300 Gigahertz and data rates of up to 39 Gigahertz. at 100 gigabits per second. In addition, it can be produced using conventional silicon technology, and therefore at a relatively low cost.

Finally, Leuthold can reassure users who might be concerned about the electromagnetic radiation involved. Unlike radio waves or microwaves from a WiFi modem, which propagate uniformly in all directions, millimeter waves can be highly focused for transmission to the outside and only propagate when they are transmitted. between the roof antenna and an optical pole inside a beam of twenty centimeters in diameter. . This greatly reduces the power required for transmission compared to other wireless technologies. This also eliminates the typical problems of WiFi modems, whose signals can interfere.