Wireless-to-Optical Receiver Could Speed Data Transmission Over 'last mile'

While light waves sent through optical fibers support fast data transmission, the “last mile,” from the fiber optic cable to the internet socket in your home, can be the most challenging and expensive leg of the journey. In the future a new light modulator developed by researchers at ETH Zürich, with support from the University of Washington, could cover the “last mile” where data must travel efficiently and at a low cost.

The “last mile” to the internet connection at home is also the most demanding. The new modulator is a viable alternative. In it, data transmitted by millimeter waves (red arrows) can be directly converted into pulses for the optical fiber (yellow). Courtesy of Y. Salamin et al.

The modulator consists of a chip that contains the microwave antenna. The antenna receives the high-frequency microwaves, or millimeter waves, used for the data transmission and converts them into an electric voltage.

At the heart of the modulator is a narrow slit, just a few micrometers long and less than 100 nm wide. The slit is filled with a material that is sensitive to electric fields. A light beam from the fiber is fed into the slit. Inside the slit, the light propagates not as an electromagnetic wave, but as a plasmon. Plasmons — hybrids made of electromagnetic fields and oscillations of electric charge at the surface of a metal — 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. The antenna’s 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. The plasmonic mixer directly maps the wireless information onto optical signals. The data bits contained in the high-frequency millimeter waves are transferred directly onto the light waves.

The plasmonic scheme provides a built-in field enhancement in an ultracompact metal-oxide-semiconductor compatible structure. The plasmonic mixer is not limited by electronic speed and is thus compatible with future Thz technologies, said the researchers.

In a laboratory experiment with microwave signals at 60 GHz, the researchers were able to demonstrate data transmission rates of up to 10 Gb/s over a distance of 5 m, and 20 Gb/s over 1 m.

In the new modulator developed at ETH Zürich, 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. Courtesy of ETH Zürich/Jürg Leuthold.

The light modulator works entirely without batteries and expensive 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,” said researcher Yannick Salamin. “From there, it can then receive data via microwave signals from individual houses and feed them directly into the central optical fiber.”

In addition to its compact size and low energy consumption, the new modulator has 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,” said professor Jürg Leuthold.

According to the researchers, the modulator is compatible with the new 5G technology and with future industry standards, based on millimeter-wave and THz frequencies of 300 GHz and data transmission rates of up to 100 Gb/s. It can be built using conventional silicon technology, and thus at a relatively low cost.

In contrast to the radio waves or microwaves of a Wi-Fi modem, which propagate evenly in all directions, millimeter waves can be focused to propagate just between the roof antenna and a light pole inside a beam that is 20 cm in diameter. This could significantly reduce the power needed for transmission, compared to other wireless technologies. It could also eliminate the typical problems of Wi-Fi modems, whose signals can get in each other’s way.

The research was published in Nature Photonics (https://doi.org/10.1038/s41566-018-0281-6).