July 25, 2023

New research group “Space Division Multiplexing" for efficient digitization

July 25, 2023

The new research group "Space Division Multiplexing" (SDM) has started its work. It was founded as part of the "Fraunhofer Attract" funding program, which provides external researchers with the opportunity to advance their ideas towards industrial application within a Fraunhofer institute. In the "Photonic Networks and Systems" department of the Fraunhofer Heinrich-Hertz-Institute (HHI), the SDM group is working to increase data rates in fiber optic networks. The group was founded in April 2023 and is funded with 2.5 million euros over a period of 5 years.

Fiber optic networks are the backbone of our modern global communications infrastructure. The data volumes in these networks have seen an exponential increase for several years. Conventional fiber optic transmission systems use single-mode fibers, which can no longer sustain this growth increase due to their nonlinear properties. This has led to the development of several novel optical fibers whose capacity is dramatically increased by spatial parallelism. This technology, known as space division multiplexing (SDM), has so far demonstrated an increase in data rates per fiber by a factor of 100.

Commercially successful deployment of this technology in future infrastructure requires the development of an entire SDM ecosystem. This is the goal of the newly established Space Division Multiplexing group. In its SpaceCOMM (Space-Division Multiplexing for classical and quantum optical fiber communications) project, the researchers aim to investigate practical use cases for SDM technology. They are working closely with researchers from other groups and departments at Fraunhofer HHI to pursue four key objectives of increasing efficiency.

The first objective is digital signal processing with multiple inputs and multiple outputs in real-time. To increase the efficiency of spatially limited fiber optic cables, the SDM researchers are investigating digital signal processing (DSP) functions for coupled SDM fibers, such as coupled multi-core optical fibers. This allows each individual fiber core to be used for signal transmission, hence reducing the number of required fibers. Real-time and high-speed DSP functions help to recover multiple spatial channels mixed during transmission in a coupled SDM fiber.

Transmission over optical fiber involves sending data from a transmitter to a receiver. In SpaceCOMM, the researchers are investigating novel transceiver chips. These chips contain several transmitter and receiver modules and thus enable parallel data transmission, which is both more energy-efficient and cost-effective. The second objective is to develop parallel transceiver devices that are optimized for SDM transmission and contain spatial multiplexing functions to connect the transceivers directly to the new SDM fiber.

The third objective is the investigation of energy-efficient optical amplifiers for SDM. Since there is no way to transmit over optical fibers without power loss, erbium-doped fiber amplifiers (EDFAs) are attached to the lines every 50-100 km. Due to the absorption spectrum of EDFAs, different wavelength channels can yield different gains. Based on their investigations, the researchers aim to find the best possible ratio of required electrical power and achievable gain of EDFAs. Such considerations of energy efficiency are particularly important for submarine cables, whose capacity is usually limited by the amount of electrical power that can be delivered from the coast.

The fourth objective is to explore the use of SDM fibers for quantum key distribution (QKD) transmission. These special data transfers do not allow the use of fiber amplifiers, so the transmission distance of conventional QKD transmission is limited by the power loss of optical fibers. One way to increase both the transmission rate and distance is to use high-dimensional quantum keys. The researchers are exploring the potential use of novel SDM fibers to implement these types of high-dimensional quantum keys.

"Given the current power consumption of the internet, we need to find new approaches to increase the efficiency of our infrastructure in order to save energy and costs. Fraunhofer Attract enables us to bring current research findings in the field of SDM closer to market maturity. In addition, our research strengthens Germany as a key location for optical communications: SDM technology is so disruptive that its implementation will generate many innovations," says Prof. Dr.-Ing. Georg Rademacher, who will head the newly established SDM group at Fraunhofer HHI.

Prof. Rademacher has studied SDM technologies in depth and has carried out several very successful projects that have led to breakthroughs in the research of high-capacity optical transmission systems. Simultaneously with the start of the project, he has taken up a position as Professor and Institute Director at the Institute for Electrical and Optical Communications at the University of Stuttgart.