Sami Ylinen

Dramatic size reduction of waveguide bends on a micron-scale silicon photonic platform

We demonstrate theoretically and experimentally how highly multimodal high index contrast waveguides with micron-scale cores can be bent, on an ultra-broad band of operation, with bending radii below 10 µm and losses for the fundamental mode below 0.02 dB/90°. The bends have been designed based on the Euler spiral and fabricated on 4 µm thick SOI. The proposed approach enabled also the realization of 180° bends with 1.27 µm effective radii and 0.09 dB loss, which are the smallest low-loss bends ever reported for an optical waveguide. These results pave the way for unprecedented integration density in most semiconductor platforms.Even though submicron silicon waveguides have been proposed for dense integration of photonic devices, to date the lightwave circuits on the market mainly rely on waveguides with micron-scale core dimensions. These larger waveguides feature easier fabrication, higher reliability and better interfacing to optical fibres. Single-mode operation with large core dimensions is obtained with low lateral refractive index contrast. Hence, the main limitation in increasing the level of integration and in reducing the cost of micron-scale waveguide circuits is their mm- to cm-scale minimum bending radius. Fortunately, single-mode rib waveguides with a micron-scale silicon core can be locally transformed into multi-mode strip waveguides that have very high lateral index contrast. Here we show how Euler spiral bends realized with these waveguides can have bending radii below 10 {\mu}m and losses below 0.02 dB/90{\deg} for the fundamental mode, paving way for a novel densely integrated platform based on micron-scale waveguides.

Dual SOA-MZI Wavelength Converters Based on III-V Hybrid Integration on a

We report on the simultaneous wavelength conversion operation of a dual-element semiconductor optical amplifier-Mach-Zehnder interferometer (SOA-MZI) array hybridly integrated on a 4-μm silicon-on-insulator (SOI) waveguide platform through thermocompression bonding. The SOAs are part of a six-element SOA array with both facets coupled on SOI through vertical and horizontal alignments. The device achieves almost two orders of magnitude reduction in footprint compared with state-of-the-art hybridly integrated SOA-MZI structures. We present for the first time experimental proof of the successful operation of a dual-element SOA-MZI device based on III-V technology on SoI that serves as a wavelength converter, with one SOA-MZI yielding error-free performance with a 0.8-dB power penalty at 12.5 Gb/s and the second SOA-MZI operating error-free at 10 Gb/s with a 2-dB power penalty.

\mu{\rm m}

Abstract. We present a method to etch vertical and smooth silicon sidewalls. The so-called modified Bosch process combines a passivation step, a breakthrough step, and an anisotropic etch step within one repeatable loop. Thanks to the sidewall protection from the passivation step and the anisotropic nature of the etch step, this method can etch smoother silicon sidewalls than either a typical Bosch process or a continuous anisotropic etch process. The silicon sidewall of a 4-μm-deep waveguide structure etched by this method has a root mean square roughness below 10 nm and a peak-to-valley (P-V) roughness below 60 nm. Comparisons between silicon waveguides etched by this process and by another deep reactive-ion etching process showed remarkable improvement in propagation loss at the wavelength of 1550 nm.

-Scale Si Platform

Thermocompression bonding of InP lasers to 4 mum thick SOI waveguides has been demonstrated. Good horizontal alignment is achieved by using active alignment. Excellent passive vertical alignment is reached by using a easily controlled fabrication process.

Smooth silicon sidewall etching for waveguide structures using a modified Bosch process

A novel guided-wave optical power coupler is presented, based on two 2x2 50/50 multimode interference splitters connected with tapered waveguides that play the role of a phase shifter. By simply changing the length of this phase shifter, these double-MMI couplers can be easily designed to get any desired splitting ratio. Results of simulations are discussed and compared with the characterizations of devices fabricated on micron-scale SOI wafers, to highlight pros and cons of the proposed solution. The fabricated splitters have been found to have average losses about 0.4 ± 0.5 dB and splitting ratios ranging from 56/44 to 96/4.

Hybrid integration of InP lasers with SOI waveguides using thermocompression bonding

We have recently characterized spirals waveguides based on 4 μm thick strip waveguides with suitably designed lowloss micron scale bends. Different bends and different lengths have been tested to extrapolate propagation losses and bending losses. In particular lowest bending losses have been found to be smaller than 0.01 dB per turn, while propagation losses are about 0.15 dB/cm. Very small footprint is achieved thanks to a novel bend concept combined to waveguide pitches of a few microns. The unique properties of our waveguides make our platform the ideal one for low loss long spiral waveguides with very small footprint.

Unconstrained splitting ratios in compact double-MMI couplers

Compound semiconductors provide state-of-the-art performance in optoelectronics, while silicon-on-insulator (SOI) is an ideal platform for many passive functions in integrated optics. By combining them one can realise optical devices with high performance and low cost. This paper discusses the various applications and technologies for integrating InP chips with SOI waveguides. Bonding of lasers, SOA arrays and detectors for practical applications is described. Experimental results are given for visually aligned thermo-compression bonding and self-aligned flip-chip bonding with Indium bumps. Flip-chip bonding is reported directly on SOI chips, as well as on a separate silicon-optical-bench.

Low-loss spiral waveguides with ultra-small footprint on a micron scale SOI platform

We report, to the best of our knowledge, the first experimental proof of MMI-based resonators. The resonators have been designed and fabricated on a micron-scale silicon photonics platform and are based on different reflectors suitably placed on two of the four ports of 2x2 MMIs with uneven splitting ratios, namely 85:15 and 72:28. The reflectors are either based on aluminum mirrors or on all-dielectric MMI mirrors. Performances of the different designs are compared with each other and with numerical simulations. Finesse values as high as 13.1 (9.9) have been measured in best aluminum (all-dielectric) resonators, corresponding to a quality factor of 5.8·10(3) (12.5·10(3)) and mirror reflectivity exceeding 92% (88%).

Integration of InP-based optoelectronics with silicon waveguides

We present our recent breakthrough for high density integration in micron-scale thick semiconductor platforms. The novel bend concept is presented from a theoretical point of view and supported by experimental results on silicon strip waveguides, including the smallest low-loss bends ever reported for an optical waveguide. Some experimental example applications to resonators, spirals, and Mach-Zehnder interferometers are also presented, along with envisaged applications to other semiconductor platforms. A special focus will be dedicated to potential applications in III-V platforms, where the novel bend could lead to unprecedented dense integration of devices as well as to novel concepts for active components.

MMI resonators based on metal mirrors and MMI mirrors: an experimental comparison.

Silicon photonics is a rapidly growing R&D field where universities, institutes and companies are all involved and the business expectations for the next few years are high. One of the key enabling elements that led to the present success of silicon photonics is ePIXfab. It is a consortium of institutes that has together offered multi-project wafer (MPW) runs, packaging services, training, and feasibility studies. These services have significantly lowered the barrier of various research groups and companies to start developing silicon photonics. Until now the MPW services have been offered by the ePIXfab partners IMEC, CEA-Leti and IHP, which all use CMOS-type silicon photonics technology with a typical silicon-on-insulator (SOI) waveguide thickness of 220 nm. In November 2013 this MPW offering was expanded by the ePIXfab partner VTT that opened the access to its 3 μm SOI waveguide platform via ePIXfab MPW runs. This technology platform is complementary to the mainstream silicon photonics technology (220 nm) and it offers such benefits as very low losses, small polarization dependency, ultrabroadband operation and low starting costs