Kai Wu

Bio: Kai Wu is an academic researcher from Delft University of Technology. The author has contributed to research in topics: Actuator & Photonic integrated circuit. The author has an hindex of 2, co-authored 4 publications receiving 16 citations.

Bimorph actuators in thick SiO 2 for photonic alignment

Abstract: This paper proposes and tests a design of electro-thermal bimorph actuators for alignment of flexible photonic waveguides fabricated in 16 µm thick SiO2. The actuators are for use in a novel alignment concept for multi-port photonic integrated circuits (PICs), in which the fine alignment is taken care of by positioning of suspended, mechanically flexible waveguide beams on one or more of the PICs. The design parameters of the bimorph actuator allow to tune both the initial relative position of the waveguide end-facets, and the motion range of the actuators. Bimorph actuators have been fabricated and characterized. The maximum out-of-plane deflection of the bimorph actuator (with 720 μm-long poly-Si) can reach 18:5 μm with 126:42mW, sufficient for the proposed application.

Photonic hybrid assembly through flexible waveguides

Abstract: Fully automated, high precision, cost-effective assembly technology for photonic packages remains one of the main challenges in photonic component manufacturing. Next to the cost aspect the most demanding assembly task for multiport photonic integrated circuits (PICs) is the high-precision (±0.1 μm) alignment and fixing required for optical I/O in InP PICs, even with waveguide spot size conversion. In a European research initiative - PHASTFlex - we develop and investigate an innovative, novel assembly concept, in which the waveguides in a matching TriPleX interposer PIC are released during fabrication to make them movable. After assembly of both chips by flip-chip bonding on a common carrier, TriPleX based actuators and clamping functions position and fix the flexible waveguides with the required accuracy.

Post-Release Deformation and Motion Control of Photonic Waveguide Beams by Tuneable Electrothermal Actuators in Thick SiO₂.

Abstract: Photonic packaging, which includes high-precision assembly of photonic sub-systems, is currently a bottleneck in the development of commercially-available integrated photonic products. In the pursuit of a fully-automated, high-precision, and cost-effective photonic alignment scheme for two multi-channel photonic chips, this paper explores different designs of the on-chip electrothermal actuators for positioning mechanically-flexible waveguide structures. The final alignment goal is ∼100 nm waveguide to waveguide. The on-chip actuators, particularly for out-of-plane actuation, are built in a 16 μm-thick SiO 2 photonic-material stack with 5 μm-thick poly-Si as an electrothermal element. A major challenge of out-of-plane positioning is a 6 μm height difference of the waveguides to be aligned, due to different built-up material stacks, together with a misalignment tolerance of 1 μm⁻2 μm from the pre-assembly (flip-chip) process. Therefore, the bimorph-actuator design needs to compensate this height difference, and provide sufficient motion to align the waveguides. We propose to exploit the post-release deformation of so-called short-loop bimorph actuator designs to meet these joint demands. We explore different design variants based on the heater location and the integration of actuator beams with waveguide beams. The actuator design (with 30 μm poly-Si and 900 μm SiO 2 in length) has ∼8 μm out-of-plane deflection and is able to generate ∼4 μm motion, which meets the design goal.

In-plane positioning of flexible silicon-dioxide photonic waveguides

Abstract: This paper describes the design, fabrication and characterization of an in-plane positioning system within a thick (16 micrometers) silicon dioxide photonic-material stack. This is part of a proposed novel photonic alignment scheme, targeting at highly-automated assembly and high-precision alignment of multi-port photonic chips. Creating such functionality in thick silicon dioxide is challenging because of its low coefficient of thermal expansion, as well as the stresses present in the material. A design is proposed which addresses both challenges, and which in fact makes positive use of the present stress. The in-plane positioning system combines an electrothermal chevron actuator and a lever mechanism, aiming to achieve several micrometer displacement. The lever structure is proposed to amplify the motion of the chevron actuator. The shuttle of the chevron actuator and the lever are provided with a set of hooks. The hooks engage during the fabrication of the structure, because of the stress-induced retraction of the chevron actuator. With this design, a robust fabrication yield was achieved. The characterization work includes analyzing the engagement between the hook and chevron actuator, and the in-plane displacement with the lever enhancement.

LTCC technology for planar microwave antenna systems

Abstract: Die keramische Mehrlagentechnologie LTCC (Low Temperature Cofired Ceramic) hat sich uberall dort bewahrt, wo hohe Anforderungen an die Zuverlassigkeit, insbesondere auch an die mechanische und klimatische Belastbarkeit, gestellt werden. Das sind zum Beispiel Anwendungen im Automobil, in der Medizin, in der Sicherheitstechnik und in der Luft- und Raumfahrt. Die Verfugbarkeit von verlustarmen und hochfrequenzgeeigneten Materialsystemen und eine verbesserte Fertigungstechnologie erweitern den Anwendungsbereich fur LTCC-Applikationen bis zu Millimeterwellenfrequenzen (bis 250 GHz). Die dreidimensionale Aufbau- und Verbindungstechnik der LTCC-Substrate umfasst neben passiven Komponenten (Kondensatoren, Widerstande und Spulen) auch planare Wellenleiter wie Mikrostreifenleiter, Koplanarleitungen und geschirmte Streifenleiter. Bei der in dieser Arbeit vorgestellten Anwendung in der Satellitenkommunikation im Ka-band (Uplink 27.5 GHz - 31 GHz) gelten enge Fertigungstoleranzen fur die LTCC-Module, da die Genauigkeitsanforderungen fur die Dimensionen von Leitungsbauelementen auf die Wellenlange im Dielektrikum bezogen werden. Im Verlauf der SANTANA-Projekte wurde eine Reihe von Sende- und Empfangsmodulen mit zunehmender Integrationsdichte entwickelt. Dabei wurden die o. g. Integrationstechniken eingesetzt und die Eignung der LTCC-Technologie fur hochkomplexe Satellitenterminals im Ka-band demonstriert. Die Systemfunktionalitat (d.h. die Breitbanddatenubertragung und elektronische Strahlschwenkung) wurde durch eine erfolgreiche Kommunikationsverbindung zwischen einer Funkbake und einem mobilen Terminal bewiesen. Das SANTANA Antennenmodul ist ein zirkular polarisiertes Antennenarray mit 8 × 8 Elementen. Dieses Sub-Array ist der Grundbaustein fur eine wesentlich grosere Antennenapertur. Das Antennenmodul selbst besteht aus 17 LTCC-Ebenen und beinhaltet die Antennenelemente mit Branchline-Kopplern, das Kalibrier-Netzwerk, die aktiven Mikrowellenschaltungen, die Leistungsteiler fur das LO-Signal, die Spannungsversorgung und die Mikrokanale fur die Flussigkeitskuhlung. Diese komplexen und hochintegrierten LTCC-Antennenmodule stellen die Substrattechnologie vor eine Reihe neuer Herausforderungen, die masgeschneiderte Losungen erfordern. Die vorliegende Arbeit fuhrt aus, worin die spezifischen strukturellen und technologischen Anforderungen dieser keramischen Mehrlagenschaltungen bestehen und wie sie im Rahmen der SANTANA-Projekte erfolgreich gelost und umgesetzt wurden.

Photonic hybrid assembly through flexible waveguides

Abstract: Fully automated, high precision, cost-effective assembly technology for photonic packages remains one of the main challenges in photonic component manufacturing. Next to the cost aspect the most demanding assembly task for multiport photonic integrated circuits (PICs) is the high-precision (±0.1 μm) alignment and fixing required for optical I/O in InP PICs, even with waveguide spot size conversion. In a European research initiative - PHASTFlex - we develop and investigate an innovative, novel assembly concept, in which the waveguides in a matching TriPleX interposer PIC are released during fabrication to make them movable. After assembly of both chips by flip-chip bonding on a common carrier, TriPleX based actuators and clamping functions position and fix the flexible waveguides with the required accuracy.

Advanced photonic signal processing and hybrid integrated microwave photonic systems

Abstract: In this thesis, we address the interdisciplinary research field known as “microwave photonics” (MWP), which has attracted considerable interest in scientific and industrial communities, and we investigate how integrated photonic technologies can be exploited to realize integrated microwave photonic systems. In brief, microwave photonics explores and develops methods and technologies to generate, process and distribute microwaves, millimeter waves and terahertz radiation via a photonic approach, i.e. in the optical domain. The main motivation for this approach is that systems based on microwave photonic technology can benefit from several advantages that are inherent to optical systems, such as high speed, low and frequency-independent propagation loss and reduced electromagnetic interference. However, although there is an indisputable and significant potential of microwave photonics, it has not yet been applied in real contexts. The main reason is that microwave photonics so far had to rely mostly on discrete components, which render according microwave photonic systems bulky, unstable and fragile. In order to overcome these limitations, while still taking advantage of the named, great opportunities, research and technology have begun targeting integration of microwave photonic systems, with the goal to enable the processing of microwave and millimeter waves via photonic chips.