Fiber Optical Communication
Advanced modulation formats in optical communication
Our research on phase-sensitive optical amplifiers was focused on demonstrating improved transmission link performance. An essential technique explored was injectionlocking that allows very efficient recovery of the pump wave needed for proper in-line phase-sensitive amplification.
Extensive work was conducted on evaluating the so-called polarization-switched QPSK format in long-haul dense WDM transmission, and its merits versus traditional formats were quantified. Also, the use of chirped fiber gratings for optical dispersion compensation in coherent systems was extensively evaluated with emphasis on the aspect of group delay ripple in the gratings. In the area of short reach applications, we demonstrated new modulation formats with sensitivity superior to conventional formats which results in an improved power link budget.
Optoelectronic Devices and Materials
Major efforts were devoted to the development of high-speed and high-efficiency vertical cavity surface emitting lasers (VCSELs) for short reach communication. The efficiency and dynamics of our 40 Gbit/s VCSELs were investigated with respect to the cavity photon lifetime and current induced self-heating. The integration of a mode filter for reduced spectral width was shown to be effective for extending the transmission distance on multimode fiber up to 500 m at 25 Gbit/s. In a collaborative effort with HP Labs (Palo Alto, CA, USA) we developed the fabrication process for high contrast gratings in (Al)GaAs as such gratings can be used as highly reflective mirrors in VCSELs for transverse and polarization mode control as well as wavelength setting. In addition, the high speed characteristics of our electro-thermal MEMS tunable VCSEL design was improved, resulting in 5 Gbit/s modulation over 16 nm of tuning.
The work on optically pumped semiconductor disk lasers continued with the development of a new technique for accurate measurements of the spectral reflectance of the gain element in such lasers. The technique enabled, for the first time, the spectral gain characteristics to be investigated and quantified under optical pumping.
Funded by Swedish Research Council, an effort on III-nitride based blue VCSELs was launched, and we initiated collaboration with EPFL in Lausanne (Switzerland) on this topic. The main challenges, such as high quality mirrors and current spreading and confining structures, are addressed. In collaboration with the Quantum Device Physics Laboratory at MC2 we explored the use of graphene as a transparent current spreading layer. Graphene was grown directly on p-type GaN and the structural quality and resistivity were investigated.
New photonic materials
The work on rebuilding an MBE system into a unique system that will allow growth of both nitrides and oxides progressed rapidly throughout the year. Hybrid oxide/nitride structures will be produced during the beginning of 2012.
In collaboration with the Swedish company IRnova, we demonstrated single pixel InAs/GaSb type-II superlattice photodetectors with a very low leakage current at a cut-off wavelength of 3.8 μm at 77 K. A novel strain engineering scheme was proposed to effectively tune the overall strain in such superlattices with more than 500 InAs/GaSb periods.
Dilute InGaSbBi thin films, for potential infrared applications, were grown by MBE for the first time and revealed unexpected lattice contraction due to vacancies caused by Bi segregation. It was also found that Bi atoms act as excellent surfactants for InGaAs grown on GaAs, resulting in strong emission up to 1275 nm at room temperature.
Thin films of Bi2Te3, for thermoelectric and topological insulator applications, were successfully grown on Si substrates by MBE. AFM measurements showed smooth surfaces with atomic steps and XRD revealed high crystal quality.
A new technique for growing graphene at lower temperatures, using CBr4 as a precursor and Ga as a catalyst, was proposed and tested in collaboration with a research team at SIMIT, Chinese Academy of Sciences. Results from MBE growth on Ge substrates indicate successful, nanoscale graphitization. The technique may enable the growth of graphene on a large variety of semiconductors.