Päivi Heimala

Sub-/spl mu/s switching time in silicon-on-insulator Mach-Zehnder thermooptic switch

We have demonstrated both rise and fall times below 1 /spl mu/s with 10%-90% modulation in a silicon-on-insulator thermooptical Mach-Zehnder switch. The switch is based on 9-/spl mu/m-thick and 10-/spl mu/m-wide single-mode rib waveguides. Very fast switching was achieved by using a differential control method. The switch was driven with a digital signal processor accompanied by simple electronic circuitry.

Erbium-doped waveguides fabricated with atomic layer deposition method

Atomic layer deposition was used in preparing erbium (Er)-doped waveguides. Ridge-type Er-doped Al/sub 2/O/sub 3/ waveguides were patterned on silica-coated silicon wafers using photolithography and wet etching. Optical absorption, emission, fluorescence lifetime, and signal enhancement measurements were performed. Polarization dependence of the absorption spectrum and birefringence of the waveguide were measured. The material showed strong absorption and wide emission spectrum around 1530 nm with full-width at half-maximum of 52 nm. Signal enhancement of 6 dB was measured for a 3.9-cm-long waveguide.

Low-loss converters between optical silicon waveguides of different sizes and types

Two types of low-loss converters between different optical waveguides on silicon-on-insulator are demonstrated. A vertical taper between 9.4- and 3.8-/spl mu/m-thick single-moded rib waveguides gives an excess loss of 0.7/spl plusmn/0.2 dB with negligible polarization dependency. The second structure converts a 9.7-/spl mu/m-thick rib waveguide into an equally thick and highly multimoded strip waveguide with a negligible loss (<0.07 dB) for the fundamental mode. The fabrication of both structures is based on a simple two-step etch process with a relaxed mask alignment tolerance and no need for epitaxy.

Development of multi-step processing in silicon-on-insulator for optical waveguide applications

Multi-step processing for a silicon-on-insulator (SOI) platform was developed. It allows the incorporation of additional grooves and steps into the basic optical waveguide structures, so that light can be adiabatically coupled between waveguides with different cross-sections. The processes were based on simple fabrication methods easily scalable for mass production. Two options for the fabrication sequence were tested, both having one silicon etch step with an oxide mask and another etch step with a resist mask. The applicability of the developed processes was tested with different waveguide structures. An additional groove etched beside a bent 10xa0µm thick rib waveguide suppressed the bend losses to below 1xa0dB/90° with a 5xa0mm bending radius. A waveguide mirror exhibited optical losses below 1xa0dB/90°. The excess losses of a vertical taper between 10 and 4xa0µm thick rib waveguides were 0.7xa0dB. A converter between a rib and a strip waveguide showed negligible losses, below 0.07xa0dB.

Dry-etched silicon-on-insulator waveguides with low propagation and fiber-coupling losses

Optical rib waveguides with various widths and heights were fabricated on silicon-on-insulator (SOI) substrates. Silicon etching was based on dry etching with inductively coupled plasma (ICP)-type reactive ion etcher. The etching process was developed to ensure low optical losses. Propagation loss of 0.13/spl plusmn/0.02 dB/cm was measured for the fundamental mode at the wavelength of 1550 nm in a curved 114-cm-long waveguide. The reflection losses were suppressed by applying atomic layer deposition (ALD) in the growth of antireflection coatings (ARCs).

Efficient Bragg waveguide-grating analysis by quasi-rigorous approach based on Redheffer's star product

We introduce a computationally efficient quasi-rigorous method for the analysis of corrugated planar waveguide structures. The method is based on rigorous diffraction theory of gratings. The computational efficiency is achieved by using Redheffers star product and the so-called binary method for the involution of the transfer matrix. The developed method enables efficient rigorous analysis of corrugated waveguide structures without any limitations for the corrugation depth. Comparison with the thin-film stack method shows that the proposed method gives similar results for Bragg grating for the fundamental mode when the corrugations are shallow, but the results differ significantly when the corrugations are deep. Furthermore, the quasi-rigorous method also facilities the analysis of the coupling of light from the fundamental mode into the higher waveguide modes.

Development of silicon-on-insulator waveguide technology

An overview of the present silicon-on-insulator (SOI) waveguide technology is given and supplemented with an extensive set of theory and simulation results. Characteristics of slab-, rectangular- and ridge waveguides in SOI are explained. In particular, the number of modes and the single-mode conditions are carefully analyzed. Experimental work with straight and bent 8 to 10 μm thick SOI ridge waveguides and a very fast thermo-optical switch are reported. Propagation loss in a very long spiral waveguide down to 0.3 dB/cm, waveguide birefringence below 10-4, and a switching frequency up to 167 kHz were obtained. A very promising multi-step patterning principle for SOI waveguides is described together with many practical application examples.

Fabrication of photonic crystal waveguide elements on SOI

Photonic crystal angle elements fabricated in silicon-on-insulator (SOI) are reported. These elements are modelled using three-dimensional finite difference time domain (FDTD) method. Photonic crystals have a two-dimensional trigonal lattice structure with cylindrical air columns. The period of the crystal is approximately 420 nm and the cylinder diameter is about 330 nm. Defect creation is performed by removing air columns from certain lattice sites. The SOI-layer is one micron thick and it also defines the column height. The FDTD modelling results imply that photonic crystal angle elements with lower height do not exhibit proper light transmission at the telecommunications wavelength window, 1550 nm. FDTD modelling results give higher transmission for TE-polarised light than for TM-polarisation. For better light coupling a taper element with widened waveguide end is designed.

Design and fabrication of arrayed waveguide grating multiplexers on silicon-on-insulator platforms

We report on the design, fabrication, and optical characteristics of arrayed waveguide grating (AWG) devices on silicon-on-insulator (SOI) platforms to act as multiplexers in a hybridly integrated wavelength division multiplexing (WDM) transmitter for telecommunications and datacom applications. In order to achieve efficient coupling to laser diodes, SOI layers with 4-µm-thick Si were used to form rib waveguides. The AWG devices comprised eight channels with a channel spacing of 200 GHz around a center wavelength at 1550 nm. AWG integration with variable optical attenuators is demonstrated to add channel equalization ability.

Stimulation of human embryonic stem cell-derived cardiomyocytes on thin-film microelectrodes.

We describe successful long‐term stimulation of human embryonic stem cell‐derived cardiomyocyte clusters on thin‐film microelectrode structures in vitro. Interdigitated electrode structures were constructed using plain titanium on glass as the electrode material. Titanium rapidly oxidizes in atmospheric conditions to produce an insulating TiOχ layer with high relative permittivity. Capacitive coupling to the incubation medium and to the cells adherent to the electrodes was still efficient, and the dielectric layer prevented electrolysis, allowing a wider window of possible stimulation amplitudes to be used, relative to conducting surfaces. A common hypothesis suggests that to achieve proper differentiation of electroactive cells from the stem cells electrical stimuli are also needed. Spontaneously beating cardiomyocyte clusters were seeded on the glass‐electrode surfaces, and we successfully altered and resynchronized a clearly different beat interval. The new pace was reliably maintained for extended periods of several tens of minutes.