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technology  PICs


is possible to have wavelength-selective-feedback, leading to single-mode operation. We have helped to build this type of structure in an EU project called HELIOS, working in partnership with CEA-Leti and III-V lab, France, who provided silicon waveguide circuits and III-V processing, respectively (see Figure 3).


A ring resonator and a grating-based mirror in the silicon layer form the laser cavity, and the III-V layer provides the needed gain. The device delivers up to 10 mW, has threshold currents below 40 mA, and features a side-mode suppression in excess of 30dB. Tuning the ring resonator – in this case via the thermo-optic effect – enables tuning of the wavelength by typically between 5 nm and 10 nm.


...and detectors We can also integrate InGaAs-based detectors with our silicon photonic circuits. Our involvement in the EU-project BOOM has required this, because this programme requires an optical label detector. One of the goals of this project is to label optical data packets with an inband wavelength code, which has to be extracted from the packet and sent to a routing unit. This allows full optical routing of IP- packets, without decreasing the spectral efficiency of the system. The label extractor consists of an optical demultiplexer with very high resolution – 12.5 GHz – fabricated on our silicon photonics platform and integrated with high efficiency photodetectors.


It has been very challenging to reach the required resolution, with success only possible after an in- depth study of silicon micro-ring resonators. This


analysis revealed that the required specifications could be reached with single ring-resonator-based filters. These resonators have integrated resistors, which allow fine-tuning of the wavelength channels (bottom electrodes) through the thermo-optic effect. They are connected to evanescently coupled InGaAs photodetectors using heterogeneous integration technology developed by INTEC, imec’s associated laboratory at Ghent University.


The efficiency of these detectors is almost 1 A/W, and they can operate at a speed of 5 Gbit/s, considerably higher than the 1 Gbit/s specification set forward at the start of the project. Engineers at Fraunhofer IZM group in Berlin have helped to package this device, which is now ready for operation in a system test bed.


Heading further into the infrared Our efforts at developing PICs started with the development of devices operating at telecom wavelengths. We are now expanding the spectral range of the devices that can be made, because this allows circuits to serve different applications. For example, devices operating in the short-wave (SWIR) and mid-wave (MWIR) infrared wavelength ranges, which span 2 µm to 8 µm, could be used for spectroscopic sensing. That’s because there are numerous gas, liquid and solid absorption features in this spectral domain.


To date, there have been very few, if any, complex PICs that have been built to work in this spectral range. Progress has been hampered by difficulties associated with developing an integration process that is fully monolithic – in other words, one that is


Figure 4 a) Hybrid DFB-laser.b) Schematic of III-V silicon tunable laser.c) Spectral response of III-V silicon tunable laser,for different values of tuning power on an integrated ring resonator


March 2013 www.compoundsemiconductor.net 59


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