WDM components

Research Area: Silicon photonics for telecom, datacom and interconnect

Main Researcher: Pieter Dumon

Silicon-on-insulator technology allows for a dramatic reduction of the footprint of integrated optical filters. Light can be forced to turn, split and interact over distances of just a few thousandths of a millimetre. The key to this size reduction is the very high refractive index contrast of silicon-on-insulator (SOI) waveguide structures. These waveguides have submicrometre dimensions and confine light very tightly within the waveguide core.

Wafer-scale integration of ultra-compact WDM devices in silicon-on-insulator.

The size reduction of high index-contrast SOI technology however comes at the cost of a very strong sensitivity to small dimensional variations, which exist due to processing imperfections. Also,the only way to use silicon technology in the real world, and at the same time the main reason to use it, is to build on the wafer-scale technology developed for fabrication of electronic device, and more specifically CMOS devices. Integration with CMOS on the same chip is then even a possibility.
However, photonic and electronic integrated circuits have very different requirements with respect to fabrication technology, at least for general high-contrast integrated structures. The wafer-scale, high throughput CMOS technology was not developed to support features varying from fully isolated to dense and perfectly periodic structures in the same processing step. Due to the alignment tolerances of photonic circuits, this is needed for photonic ICs. This has a large impact on the development of fabrication processes and the performance of optical filters. Small, nanometre-scale variations of the feature size within a structure give rise to imperfect imaging or suboptimal interference, resulting in loss and crosstalk between wavelength channels.

Racetrack resonator

We have demonstrated ring resonators with bend radii of just 1 to
5 ìm. Single-ring channel drop filters with a large free spectral range, low insertion loss and down to 20 dB extinction in the throughput channel were demonstrated. Lower loss and lower distributed scattering would allow for higher-order filters to be designed and fabricated.

Mach-Zehdner lattice filters dropping one channel out of 4 or 8 were designed and fabricated with WDM channel spacings of 100 to 400 GHz.
These filters can be designed with filter synthesis methods well-known from the digital filter world. The fabricated devices showed a low loss, bandwidths as designed and an extinction of the order of 10 dB (down to 15 dB in the dropped channel port). Analysis showed the crosstalk is due to both coupler variations and phase errors in the delay lines. These phase errors were already reduced by broadening the waveguides in the straight sections of the delay lines.

Wavelength routers based on arrayed waveguide gratings were designed and fabricated. Devices with 4 to 16 wavelength channels and 100 to 400GHz channel spacings, showed an insertion loss of down to 2.5 dB
and a channel isolation level down to 15 to 17 dB. To reach this, the refractive index contrast was lowered around the star couplers through a double-etch process, and the arrayed waveguides were made less sensitive to small dimensional variations by broadening them.

The components were fabricated with wafer-scale processes that are developed for fabrication of a wide range of structures. Despite the present non-uniformity between and within structures, a component performance decent enough for first application tests was demonstrated, with optical filters with WDM channel spacings fitting on a footprint of the order of just 0.01 to 0.1 mm2.

Other people involved:

• Pieter Dumon
• Shibnath Pathak
• Eva Ryckeboer
• Ahmed Bayoumi
• Ying Tan
• Geert Morthier

Related Research Projects

• FWO: G013815N-ScaleModel (Scalable behavioral models for large-scale silicon photonics)
• AFOSR: PhotonMolecule (Programmable Photonic Molecules)