Institute of Photonics and Quantum Electronics (IPQ)

Optical Communications

The Optical Communications Group aims at high-speed data transmission (100 Gbit/s to several Tbit/s) exploiting novel system concepts and components. The advancement of digital signal processing algorithms towards increased functionality and performance at reduced complexity is an integral part of that work. Current research includes multi-carrier communication systems based on optical frequency combs, coherent transceiver optimization by digital signal processing, Kramers-Kronig reception for data center interconnects, passive optical networks and high-capacity free-space transmission systems.

Artist’s view of an on-chip multi-channel optical transmitter setup comprising a laser source, frequency comb generator, an array of modulators which are connected with photonic wirebonds.

While optical communications was closely related to long-haul transmission systems in the past, more than 50 % of the global data traffic is attributed to datacenter interconnects nowadays. For energy-efficient and fast communication between the servers and clients, fiber-optic solutions need to replace electrical connections and shrink down in size. As a result, the fiber-optics market at present is mainly driven by the development of photonic-integrated circuits and high-capacity short-reach transmission systems.
Under the lead of Prof. Randel, the Optical Communications Group is working towards data transmission beyond data rates of 1 Tbit/s exploiting coherent modulation formats with high spectral efficiency. In that context, novel system architectures with low power consumption are investigated. An example is the recently proposed Kramers-Kronig receiver. Since high-throughput transceivers are susceptible to noise, transmitter impairments and nonlinear effects, their optimization requires merging knowledge from a variety of research areas like coding, signal processing, and nonlinear optics. Following a recent trend in the communications community, researchers at IPQ explore whether machine learning can be employed for optimization of such systems, especially for the compensation of nonlinear channel effects.
Another research topic is the investigation of coherent architectures for passive optical networks (PONs) that provide broadband access to end-customers and usually span a few kilometers. Such networks feature a point-to-multipoint topology and require low-cost hardware at the customer side. While coherent systems allow achieving higher data rates, frequency synchronization between the lasers at the optical line terminal (OLT) and the optical network units (ONUs) is a challenge as cheap lasers tend to drift over a large wavelength range. Coherent systems that employ frequency combs instead of single-wavelength lasers are promising candidates to solve that problem.
The Optical Communications Group also advances high-capacity wireless transmission systems in the terahertz (THz) or optical regime. In this context, the scientists are in close collaboration with the Teratronics Signal Processing Group , which allows combining state-of-the art components with novel THz circuits developed at IPQ. Recently, the group achieved some groundbreaking system demonstrations that may pave the way towards high-speed short-distance point-to-point links in urban or peripheral areas, where fiber deployment would cause massive costs. Free-space optical (FSO) transmission systems are an alternative to THz systems when larger distances need to be bridged. However, optical signals are very susceptible to adverse weather conditions and turbulent atmosphere. At IPQ, researchers examine to which extend the effect of turbulence can be digitally mitigated at the receiver side of the transmission link.
In the DFG project INTERFERE, the Optical Communications Group investigates a novel interferometric structure that allows arbitrary optical signal manipulation and opens up new possibilities concerning add/drop multiplexing in superchannels. It relies on conversion of the optical signal to the electronic baseband, followed by digital signal processing and subsequent electro-optic conversion to optical frequencies.
For system-level experiments, the group can resort to one of the best-equipped optical transmission laboratories worldwide. It comprises multi-format transmitters to generate high-speed electrical data that is modulated onto optical carriers using either commercial Lithium Niobate or in-house designed modulators, (for details read more about the Hybrid Photonic Integrated Circuit Group ). For reception, dual-polarization integrated coherent receivers or high-speed photodiodes can be combined with real-time oscilloscopes capturing frequencies up to 100 GHz. For transmission experiments, the lab is equipped with reconfigurable optical switches, multiplexers, and hundreds of kilometers of single-mode fiber (SMF). The signal processing algorithms are implemented either in simulation environments or on an FPGA platform for real-time measurements.