Bert Luyssaert

Low-loss SOI photonic wires and ring resonators fabricated with deep UV lithography

We demonstrate single-mode photonic wires in silicon-on-insulator with propagation loss as low as 2.4 dB/cm, fabricated with deep ultraviolet lithography and dry etching. We have also made compact racetrack and ring resonators functioning as add-drop filters, attaining Q values larger than 3000 and low add-drop crosstalk.

Basic structures for photonic integrated circuits in Silicon-on-insulator

For the compact integration of photonic circuits, wavelength-scale structures with a high index contrast are a key requirement. We developed a fabrication process for these nanophotonic structures in Silicon-on-insulator using CMOS processing techniques based on deep UV lithography. We have fabricated both photonic wires and photonic crystal waveguides and show that, with the same fabrication technique, photonic wires have much less propagation loss than photonic crystal waveguides. Measurements show losses of 0.24dB/mm for photonic wires, and 7.5dB/mm for photonic crystal waveguides. To tackle the coupling to fiber, we studied and fabricated vertical fiber couplers with coupling efficiencies of over 21%. In addition, we demonstrate integrated compact spot-size converters with a mode-to-mode coupling efficiency of over 70%.

Fabrication of photonic crystals in silicon-on-insulator using 248-nm deep UV lithography

We demonstrate wavelength-scale photonic nanostructures, including photonic crystals, fabricated in silicon-on-insulator using deep ultraviolet (UV) lithography. We discuss the mass-manufacturing capabilities of deep UV lithography compared to e-beam lithography. This is illustrated with experimental results. Finally, we present some of the issues that arise when trying to use established complementary metal-oxide-semiconductor processes for the fabrication of photonic integrated circuits.

Analysis of butt coupling in photonic Crystals

We present a detailed analysis of butt coupling from conventional dielectric waveguides into photonic crystal waveguides. Closed-form expressions for the reflection and transmission matrices based on an eigenmode expansion technique are derived and validated by means of simulations. We use them to investigate butt-coupling losses in two kinds of photonic crystal structures: one formed by rods with a higher refractive index than the surrounding medium and the other formed by air holes inserted in a high-refractive-index medium. The origin and difference of coupling losses between the two photonic crystal structures is analyzed and discussed. We show that, although the coupling efficiency is much worse in the former structure, it can be significantly improved by choosing the optimum interface position that minimizes the mode impedance mismatch. Furthermore, the dependence of coupling efficiency on frequency is also analyzed. Finally, we also relate some traditionally used approximate formulas to our rigorous expressions.

Efficient nonadiabatic planar waveguide tapers

We report taper designs with high transmission efficiencies and with lengths shorter than those needed for adiabatic operation. The tapering occurs between rectangular optical waveguides with the same vertical silicon-on-insulator layer structure, but with different horizontal widths, namely 0.5 and 2.0 /spl mu/m, and for taper lengths between 0.5 and 3.0 /spl mu/m. After a comparison between two different optimization methods in a two-dimensional calculation scheme, one of these is repeated using three-dimensional calculations. The results show that, also in the length region where conventional linear and parabolic tapers are not yet adiabatic, tapers with a high efficiency can be designed by applying complex taper structures with more degrees of freedom.

Large-scale production techniques for photonic nanostructures.

Nanophotonic ICs promise to play a major role in the future of opto-electronic signal processing and telecommunications. But for these devices, which consist of large numbers of wavelength-scale photonic components, to be successful, reliable and cost-effective mass-fabrication technology is needed. Photonic components, and among them photonic crystals, require a high degree of accuracy, which translates to low fabrication tolerances. Today, similar demands are made for high-end CMOS components, made of Silicon, for which a large manufacturing base is installed. We demonstrate the fabrication of nanophotonic components, like photonic crystal waveguides and photonic wires, using state-of-the-art CMOS processing tools. The foremost of these is deep UV lithography at 248nm and 193nm, combined with dry-etch processes. To maintain compatibility with standard CMOS processes, we use Silicon-on-Insulator (SOI) as our material system. SOI is transparent at telecom wavelengths and provides a good substrate for high-index contrast optical waveguides. Moreover, recent studies have shown that nanophotonic components in SOI are less sensitive to surface roughness than similar components made in III-V semiconductor. Although deep UV lithography cannot attain the resolution of e-beam lithography, this can be compensated with thorough process characterization, and the technique offers more speed because of its parallel nature. We will illustrate this with experimental results, and will also discuss some of the issues that have arisen in the course of this project.

A compact photonic horizontal spot-size converter realized in silicon-on-insulator

We present a compact planar coupler connecting two optical waveguides with highly different widths. The coupler consists of various nonperiodic waveguide sections, whose dimensions are determined using a genetic optimization algorithm. Efficiencies that exceed those of the more conventional designs with similar lengths, like gradual linear tapers, were obtained in silicon-on-insulator using 248-nm-deep ultraviolet lithography.

Silicon-on-insulator based nano-photonics: Why, How, What for?

Silicon-on-insulator is rapidly emerging as a versatile platform for a variety of integrated nano-photonic components. This paper discusses the variety of merits offered by this system. The key technological challenges are discussed as well as the potential in multiple application fields.

Silicon-on-insulator nanophotonics

Nanophotonics promise a dramatic scale reduction compared to contemporary photonic components. This allows the integration of many functions onto a chip. Silicon-on-insulator (SOI) is an ideal material for nanophotonics. It consists of a thin layer of silicon on top of an oxide buffer. In combination with high-resolution lithography, one can define a high refractive index contrast both in horizontally and vertically, resulting in a tight confinement of light. Moreover, SOI can be processed with industrial tools now used for silicon microelectronics. There are two candidates for nanophotonic waveguides. Photonic wires are basically conventional waveguides with reduced dimensions and a high refractive index contrast. These waveguides with submicron dimensions can have bend radii of only a few micrometres. The alternative is to use photonic crystals, which confine light by the photonic band gap effect. Introducing defects in a photonic crystal creates waveguides and other functional components. To make nanophotonics commercially viably, mass-manufacturing technology is needed. While e-beam lithography delivers the required accuracy for nanophotonic structures, it is too slow. We have used deep-UV lithography, used for advanced CMOS fabrication, to make nanophotonic waveguides. The fabrication quality is very good, which translates to low propagation losses. E.g. a 500nm (single-mode) photonic wire has a propagation loss of only 0.24dB/mm. Using these low-loss waveguides, we have implemented a variety of nanophotonic components, including ring resonators and arrayed waveguide gratings.

Silicon-on-insulator platform for integrated wavelength-selective components

The silicon-on-insulator (SOI) platform allows to make ultra-compact photonic integrated circuits by means of standard processes used for silicon CMOS. Basic properties of SOI waveguides and the coupling to fiber are briefly discussed. Afterwards, various WDM-components (filters and demultiplexers) based on high-contrast nanophotonic waveguides in SOI are reported.