Research

The rapid growth of the Internet creates enormous technical challenges. Global data traffic is expected to increase by a factor 2.5 in the next 5 years. A major source of growth for optical communications is the connectivity from server to server inside the data centers of major internet service providers. Today, data center architects are demanding optical interconnects that reach 500 meters in order to allow a fully interconnected network across an entire datacenter with a single technology. Combined with rising data rates (100 GE) and demands for lower power consumption motivated by economical, ecological as well as technical reasons – it is becoming increasingly challenging to manage the cooling of modern datacenters – this poses tremendous technical difficulties and creates a real need for breakthrough optical technologies.

The realization of optical nano-devices in Silicon chips and the integration of complete optical systems in a single Silicon chip, Silicon Photonics, is the frontrunner amongst the emerging optical technologies posed to solve these problems. Integration in semiconductor chips allows ultra-compact, densely integrated systems. Monolithic co-integration with electronics has allowed lowering receiver noise and reducing power consumption. Fabrication out of a comparatively cheap material with IC fabrication technology could result in an extremely competitively priced technology. The primary drawback of Silicon Photonics, its reliance on single mode optics, could become much less of an issue as Datacom in general is moving towards longer distances at higher data rates and other single mode solutions are being deployed.

Micrographs of (a) a Resonantly Enhanced Mach Zehnder Modulator (RE-MZM) and (b) a Coupled Resonator Optical Waveguide (CROW) based wideband tunable optical filter. (c) Micrograph of a complete system chip and (d) testing in an optically enabled probe station allowing coupling to fibers embedded in a fiber array. (e) Assembly with chip-scale modulator drivers, transimpedance amplifiers (TIAs), temperature sensor and flip-chip integrated laser. (f) Assembly with a flip-chip integrated laser and (g) complete module with heat sink, 2 QSFP connectors connecting 8 full duplex 25 Gbps links and fiber connectivity.

Another trend consists in the deployment of optical technologies at ever smaller distances. Today, the biggest number of optical transceivers is already deployed in data centers to bridge distances on the order of ~10 meters. As data rates grow and the cost of transceivers drops, these distances will continue to shrink to service links within individual server racks, for example to service data throughput bottlenecks generated by novel hyper scale data center architecture trends such as memory desegregation. As the number of cores of high performance CPUs continues to grow, increasing demand for low latency data throughput to external RAM might push optical interconnects to even smaller length scales, in between and within systems-in-a-package. Here, the potential for ultra-low cost of Silicon Photonics plays a very important role. An outstanding challenge remains in finding a good solution for ultra-low cost laser integration, since, to date, the laser cannot be monolithically integrated.

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Left: Conceptual rendering of an optically enabled silicon interposer allowing for intra-package, as well as package-to-package optical connectivity. Right: Corresponding optical system diagram.

Over 10 years of intense activities in Silicon Photonics have seen the development of integrated modulators, Germanium photodetectors, multiplexers and coupling structures. However, many important challenges remain. At the IPH, we are exploring the on-chip generation of multi-wavelength sources and their utilization in complex communications systems, the development of low drive voltage and low insertion loss optical modulators, the integration of ultra-high quality factor cavities in fully functional Silicon Photonics chips as well as the development of novel coupling structures that enable automatized assembly of optical systems with pick-and-place machinery. Activities reach from fundamental material science problems such as the quantum mechanical modeling of the nonlinear properties of strained silicon to the design and implementation of novel system architectures that are specifically tailored to leverage the strengths of Silicon Photonics. Recent focus has shifted to the development of SiN based Photonic Integrated Circuits for visible wavelength life science applications as well as the development of SiGeSn monolithically integrated light sources and modulators.

Left: Scanning Electron Microscope (SEM) image of an optically pumped SiGeSn microdisk laser. Right: (a) Optical excitation dependent emission spectrum measured at cryogenic temperatures, (b) Light In – Light Out curve featuring a clear threshold (S-shaped on a log-log scale) and (c) above threshold spectrum collapse.

Selected Research Topics

SiGeSn based lasers and modulators: GeSn has recently been experimentally shown to be a direct bandgap material at sufficiently high Sn contents and to support optically pumped lasing at cryogenic temperatures. Together with our research partners at the Forschungszentrum Jülich, we are working towards room temperature and electrically pumped lasing, as well as  on the development of supporting optical devices (modulators, passive waveguide platform) for the realization of a complete, active, group IV photonic platform.

Low Voltage Modulators: Rapid progress in high-speed DSPs and FPGAs has enabled programmable optics, advanced architectures and increased flexibility in high-speed communications. We are currently working on low voltage Silicon Photonics modulators with the objective of directly interfacing Silicon Photonics with D/A converters.

Comb Laser based Communication Systems: We are currently exploring advanced communication system architectures leveraging comb light sources for the realization of compact and densely integrated WDM transceivers.

Strained Silicon Modulators: The application of a strain gradient to silicon breaks the centro-symmetry of the silicon crystal lattice and generates a second order nonlinear coefficient – thus allowing the implementation of Pockels type modulators in silicon. We are currently working on the theory of strained silicon as well as on strained silicon modulators.

Integrated Optical High-Q Cavities: Ultra-high Q silicon dioxide optical cavities have been demonstrated with a variety of methods. We are working on monolithically integrating such resonators with mainstream Silicon Photonics devices and systems.

Optical Interfaces for Pick-And-Place Assembly: Single mode alignments typically still require significant operator involvement. In order to make single mode optics more effective for mass-market applications, we are working on optical interfaces enabling machine vision guided pick-and-place assembly.

Silicon Nitride Optics for Biosensing: Silicon nitride is a material that allows guiding of light at visible wavelengths compatible with fluorescent biosensors. We are working on enabling efficient optical coupling and optical power distribution in silicon nitride based optics.