Their system, which uses pulse-amplitude modulation with four discrete levels, significantly improves the cost, space and power efficiency for data center interconnect applications, providing a solution for distances up to 100km.

“The uniqueness of our PAM4-setup is that it enables a cost-effective and low-power high-speed data center interconnect solution by significantly extending the reach of short reach 400G PAM4 solutions, which are currently under standardization at IEEE802.3bs,” said Nicklas Eiselt, a PhD candidate, Department of Photonics Engineering, Technical University of Denmark.

Funded by EU Marie Curie projects ABACUS and CEEOALAN, the researchers will present their work at the Optical Fiber Communication Conference and Exhibition, Anaheim Convention Center, Anaheim, California, USA.

The PAM4-based transmission system is an application of DWDM (dense wavelength division multiplexing) technology, which combines multiple streams of data – in this case, eight – onto a single optical fiber. Each signal is carried at the same time on its own separate wavelength, enabling multiples of 400Gbit/s over one strand of fiber.

For distances of up to 100km at this speed the current industry-standard modulation format NRZ (non-return to zero) with two signal levels is no longer effective due to the large required bandwidth.

The key to transmitting a PAM4 signal at these data rates over 100 kilometers in real-time was a prototype transceiver consisting of an equalizer on the transmitting and on the receiving side, in order to compensate for limited bandwidth and residual dispersion.

The emulation of a data center interconnect transmission link was done by using conventional 80km and 100km of SSMF (standard single mode fiber), a multiplexer and de-multiplexer to combine and separate the eight channels and optical amplifiers to ensure sufficient power into the transmission link.

According to Eiselt, these results confirm PAM4 as a promising approach for building low-cost, high-speed data center interconnect solutions. This concept would also eventually allow for link capacity greater than 4Tbit/s at distances up to 100km. Future research interests for Eiselt and his colleagues include increasing transmission speeds up to 56GBaud, while tightening integration at the component-level, thus enabling a lower power consumption and cost.