European silicon-photonics project demos CMOS-compatible laser and 10 Gb/s modulator

Grenoble, France--CEA-Leti, coordinator of the European Helios project to accelerate commercialization of silicon photonics, has announced that the project partners demonstrated a 10 Gbit/s silicon modulator fabricated using a process that is compatible with complementary metal-oxide semiconductor (CMOS) processing, in addition to the project's CMOS-compatible laser.

Why CMOS compatibility?
In the rapidly developing area of silicon photonics, making active photonic devices fully CMOS compatible is one of the biggest goals, as it allows standard wafer-fabrication facilities to fabricate photonic integrated circuits. Otherwise, manufacturers would have to rely on the more-specialized and expensive processes used to fabricate conventional communications-wavelength diode lasers and modulators.

Silicon photonics is an exciting technology that can enable low-cost optical communications, chip-to-chip and rack-to-rack connections, data-center cables, optical signal processing, optical sensing, and biological applications. In particular, inexpensive chip-to-chip communications would be an interesting addition to desktop computing, tying CPUs and GPUs together in new and important ways.

Standard processes
Laurent Fulbert, photonics programs manager at Leti and coordinator of Helios, gives a few details on the CMOS-compatible process and devices. "We use the same equipment and the same processes as in a microelectronics clean room," he says. "The modulator is a Mach-Zehnder interferometer made of silicon. A p-n junction is made by implantation in the waveguide. As for the laser, a piece of III-V material (for example indium phosphide) is heterogeneously integrated on silicon by bonding. The laser cavity is then processed on the silicon wafer. The Helios consortium is developing CMOS compatible processes (such as indium phosphide etching, and fabrication of metal contacts) for fabricating such laser sources."

The 10 Gbit/s silicon modulator has an extinction ratio of 7 dB; a 40 Gbit/s version has already been designed by the consortium and is under fabrication, with first characterization results expected next year. "The average length of a modulator is 4 mm with a width dependant on the metallization integration process (less than 500 µm)," says Fulbert. "The modulator is one of the key building blocks of silicon photonics."

Research on many fronts
Numerous novel concepts for light emission and modulation are being investigated at Helios. "The hybrid InP-silicon laser is quite innovative by having light traveling in both materials," notes Fulbert. "More innovative proofs-of-concept based on III-V/Si 2.5D laser microsources are also investigated in the project. They consist of combining photonic-crystal cavity mirrors with the III-V/Si heterogeneous integration."

The group is pursuing nanotech-based devices as well. "Direct emission of erbium-doped silicon nanocrystals is being investigated," says Fulbert. "As for modulators, slow-wave Mach-Zehnder modulators are being studied in order to reduce the size and power consumption of the devices. Innovative concepts based on the use of amorphous silicon instead of crystalline silicon are also being investigated."

Other recent results of the project include:

--demonstration of high-responsivity (0.8 to 1 A/W), low-dark-current, and high-bandwidth (up to 130 GHz) photodiodes

--efficient passive waveguides (mux/demux, polarization diversity circuit, fiber coupling, and rib/strip transition)

--establishment of a photonics design flow

Additional info
For those wanting to know more about silicon photonics, the Helios consortium has developed a training course addressing all aspects of the topic. The 21 hour free course is available on the Helios website at: http://www.helios-project.eu/Download/Silicon-photonics-course

In addition to Leti, the many HELIOS partners are:
--Imec (Belgium)
--CNRS (France)
--Alcatel Thales III-V lab (France)
--University of Surrey (England)
--IMM (Italy)
--University of Paris-Sud (France)
--Technical University of Valencia (Spain)
--University of Trento (Italy)
--University of Barcelona (Spain)
--3S Photonics (France)
--IHP (Germany)
--Berlin University of Technology (Germany)
--Thales (France)
--DAS Photonics (Spain)
--Austriamicrosystems AG (Austria)
--Technical University of Vienna (Austria)
--Phoenix BV (Netherlands)
--Photline Technologies (France)