Long-haul optical transmission
For our work on high spectral efficiency communication, we implemented a circulating loop test-bed allowing us to analyse signal propagation over many thousands of km, and demonstrated transmission over 12,000 km using a single optical carrier with 43 Gb/s data. In addition, we proposed and analysed new modulation formats with higher sensitivity, thus reducing impairments caused by fibre nonlinearities.
Our research on phase-sensitive optical amplifiers was expanded through funding from the European Research Council and the Wallenberg Foundation. We seek to demonstrate improved performance in optical transmission by utilizing the unique feature of such amplifiers; producing nearly no excess noise. We demonstrated the first use of such amplifiers in real transmission links and showed a clear improvement over existing solutions. In addition, we explored the use of these amplifiers in non-telecom applications such as high-sensitivity detection in spectroscopy.
Datacom lasers and optical interconnects
Our latest 850 nm vertical cavity surface emitting lasers (VCSEL) enabled transmission over multimode fibre at record bit-rates of 47 Gbit/s at room temperature and 40 Gbit/s at 85ºC. Techniques for reducing the spectral width of VCSELs, thereby extending the reach of optical interconnects, were also implemented, resulting in the transmission of data at bit-rates exceeding 20 Gbit/s over more than 1 km of multimode fibre. In a collaborative effort with HP Labs we continued the development of multi-wavelength VCSEL arrays for wavelength multiplexed optical interconnects.
On a systems level, we investigated the use of multi-level modulation formats as a mean of increasing the data throughput of short reach optical links. 4-PAM (pulse amplitude modulation) was compared to OOK (on-off keying) in terms of sensitivity and inter-symbol interference.
Wide bandgap optoelectronics
Epitaxial layers and heterostructures for wide bandgap optoelectronics are grown in-house using two molecular beam epitaxy (MBE) systems. One of the systems was refurbished to allow for the growth of hybrid oxide/nitride structures.
State-of-the-art III-nitride layers and the first hybrid ZnO/GaN structures were demonstrated.
In our efforts on III-nitride-based VCSELs emitting in the blue we proposed new designs for reduced optical loss and reduced threshold currents. Graphene was investigated as a transparent, intra-cavity contact material.
A new project on ultra-violet light emitting diodes (UV-LEDs) was launched. The project aims at the development of efficient InGaN/AlGaN UV-LEDs for solid-state-lighting as such LEDs, when coated with a multi-colour phosphor blend, may enable efficient generation of white light with superior colour rendering properties.
Novel photonic materials and applications
This research deals with a variety of new materials for various applications. The materials are grown by MBE. Notable achievements include high performance InAs/GaSb type-II superlattice detectors for mid-infrared detection (with IRnova), thick stacks of coupled InAs quantum dots with strain compensating GaNAs barriers for intermediate band solar cells, high mobility Bi2Te3 thin films as topological insulators, and pristine graphene sheets deposited at low temperature as transparent conductors on light emitting devices.