16 Jan 2008
The use of microring laser technology could yield more compact optical transmitters and multiplexers for optical communications.
A European collaboration has demonstrated a new technique for fabricating complex InP chips that could result in much smaller optical components for wavelength division multiplexing (WDM).
The new approach, developed by the €2.1 m ($3.2 m) WAPITI project, exploits a novel wafer bonding technique to integrate three microring lasers made from indium phosphide onto a gallium arsenide substrate. By connecting multiple microrings on single transparent waveguide, the team has been able to demonstrate a monolithic wavelength-multiplexed ring resonator transceiver for the first time.
Although the project – known as WAPITI (Wafer Bonding and Active Passive Integration Technology and Implementation) – officially finished in September 2007, project leader Helmut Heidrich says these demonstration experiments have only been performed within the past month.
The three InP microrings, which have radii increasing from 50 to 60 µm, were fabricated on chips measuring 900 µmx400 µm. In contrast, existing multiplexers based on InP arrayed waveguide gratings are millimeters in length, while silicon versions come in centimeter dimensions.
The output power from the WAPITI components is currently below 0.1 mW, but Heidrich says this should be enough for short-range transmission at 10 Gbit/s data rates and is potentially able to reach 100 Gbit/s.
"We see two fields of applications for these microrings: low output power WDM sources for short-reach data links, and for exploitation of non-linear phenomena," Heidrich told compoundsemiconductor.net.
In the fabrication method developed by WAPITI, the waveguide, spacer and laser layer stacks are first grown on an InP substrate. These structured upper laser layers are then attached to a GaAs transfer wafer. The original InP substrate is then chemically etched down to the waveguide layer before further processing.
The advantage of the technique is that it allows multiple microrings to be integrated onto a single transparent optical waveguide, which enables optical multiplexing. Microrings with different radii produce different wavelengths of light, which can each carry different data streams within a multiplexing protocol.
Heidrich, who has worked for 25 years as a project manager at the Berlin-based Heinrich-Hertz Institute, is now hoping to secure funding to follow up the 40-month WAPITI project.
Other research institutions participating in the project include the University of Cambridge, the University of Athens, the German Max-Planck Institute of Microstructure Physics and the Romanian National Insitute for R&D in Microtechnologies in Bucharest. Industrial partner EV Group also adds further wafer processing expertise and equipment to the project team.• A second European microring laser project, called IOLOS, that is looking to explore the non-linear phenomena for all-optical computing purposes is ongoing, and scheduled to run until the end of 2009 (see Optical memory project builds on ring laser).