Merging Plasmonic, Silicon Photonic, and Electronic Circuits for Energy-Efficient Optical Communications

In order to improve the energy efficiency of the optical communication infrastructure, a research team led by ETH Zurich has developed a new platform combining electronic circuits and plasmonic integrated optics on a single chip. To overcome limitations in performance and energy efficiency of conventional approaches, they demonstrate the first plasmonic-electronic transmitter: the monolithic integration of plasmonic silicon-photonic electro-optic converters and advanced bipolar CMOS circuits. The plasmonic-electronic transmitter was published in Nature Electronics.

The challenge of ever-increasing data rates

The next generations of optical communication networks are subject to an exponentially increasing growth in data rates. Particular examples are data centers, which have to keep up with society’s demand for online services such as streaming, storage, computation and more. It is predicted, that by the end of this decade optical links have to provide Terabit per second speeds. Yet, current technologies will not be able to cope with this demand and new paradigms have to be introduced. Companies work on co-packaged optical engines to replace the copper interfaces from the Ethernet switch ICs. The plasmonic-electronic transmitter goes beyond and demonstrates how in future the optical interfaces can be directly processed on the switch ASICs, leading to better power consumption, denser integration, and high-speed interfaces.

The keyword for this paradigm shift is the electronic-photonic co­-integration. Conventionally, the co-integration is realised in a heterogeneous manner with two separate chips. This allows to use two different platforms for electronics and photonics, with almost independent development and testing. Yet, this approach is very costly and delivers non‑ideal performance due to the separation of the electrical and the optical chip at the most critical position. Monolithic integration avoids this by realising an electronic-photonic layer stack on a single substrate. Yet, it faces the challenge of strong interdependence between the underlying technologies, which has so far prevented its use as a high-speed platform.

Monolithic integration of electronics and photonics

Monolithic integration has now been achieved by researchers at ETH Zurich and colleagues. Key to the success was the co‑design of electronic and photonic device performance, including assembly and packaging, thermal optimization, and a new temperature-stable electro‑optic material developed at the University of Washington.

The monolithic transmitter (see Figure) combines electronics (blue) and photonics (red) in a layer stack on a common substrate. The electronic layers perform a 4:1 multiplexing to generate high­-speed electrical signal by mixing of four lower‑speed inputs. The photonic layer uses a plasmonic intensity modulator to convert the electrical signal into the optical domain for transmission via an optical fibre. These layers are connected by on‑chip wires to guarantee shortest distances and best signal quality.

Monolithic Transmitter
Monolithic transmitter existing of an electronic-photonic layer stack using a 4:1 SiGe BiCMOS mulitplexer and a plasmonic Mach-Zehnder modulator. (Picture: Ueli Koch, ETH Zurich)

Research collaboration under Horizon 2020

The unique collaboration under the umbrella of the Horizon 2020 project PLASMOfab has led to a breakthrough in electronic­-photonic co‑integration and the first time demonstration of more than 100 Gb/s data modulation in a monolithic transmitter. The high‑speed‑electronics specialists from the Saarland University and Micram, the high‑speed‑photonics experts from ETH Zurich and Polariton Technologies, and many other collaborators have created a novel BiCMOS-plasmonic platform for highest-speed optical interconnects that is expected to keep up with the datacentre demands of the next decades.

This breakthrough has been published in the article A Monolithic Bipolar CMOS Electronic- Plasmonic High-Speed Transmitter” in Nature Electronics in June 2020 by the authors Ueli Koch, Christopher Uhl, Horst Hettrich, Yuriy Fedoryshyn, Claudia Hoessbacher, Wolfgang Heni, Benedikt Baeuerle, Bertold Ian Bitachon, Arne Josten, Masafumi Ayata, Huajun Xu, Delwin L. Elder, Larry R. Dalton, Elad Mentovich, Paraskevas Bakopoulos, Stefan Lischke, Andreas Krüger, Lars Zimmermann, Dimitris Tsiokos, Nikos Pleros, Michael Möller, and Juerg Leuthold.