Embedding of optical interconnections in flexible electronics

P

Novel printing technologies for nanophotonic and nanoelectronic devices

textAs optical interconnects make their paces to replace traditional electrical interconnects, implementing low cost optical components and hybrid optic-electronic systems are of great interest. In the research work described in this dissertation, we are making our efforts to develop several practical optical components using novel printing technologies including imprinting, ink-jet printing and a combination of both. Imprinting process using low cost electroplating mold is investigated and applied to the waveguide molding process, and it greatly reduces the surface roughness and thus the optical propagation loss. The imprinting process can be applied to photonic components from multi-mode waveguides with 50[mu]m critical dimension down to photonic crystal structures with 500nm hole diameter. Compared to traditional lithography process, imprinting process is featured by its great repeatability and high yield to define patterns on existing layers. Furthermore we still need an approach to deposit layers and that is the reason we integrate the ink-jet printing technology, another low-cost, low material consumption, environmental friendly process. Ink-jet printing process is capable of depositing a wide range of materials, including conductive layer, dielectric layer or other functional layers with defined patterns. Together with molding technology, we demonstrate three applications: proximity coupler, thermo-optic (TO) switch and electro-optic (EO) polymer modulator. The proximity coupler uses imprinted 50[mu]m waveguide with embedded mirrors and ink-jet printed micro-lenses to improve the board-to-board optical interconnects quality. The TO switch and EO modulator both utilize imprinting technology to define a core pattern in the cladding layer. Ink-jet printing is used to deposit the core layer for TO switch and the electrode layers for EO modulator. The fabricated TO switch operates at 1 kHz with less than 0.5ms switching time and the EO modulator shows V[pi][middle dot]L=5.68V[middle dot]cm. To the best of our knowledge, these are the first demonstrations of functional optical switches and modulators using printing method. To further enable the high rate fabrication of ink-jet printed photonic and electronic devices with multiple layers on flexible substrate, we develop a roll-to-roll ink-jet printing system, from hardware integration to software implementation. Machine vision aided real time automatic registration is achieved when printing multiple layers.Electrical and Computer Engineerin

Optoelectronic devices and packaging for information photonics

This thesis studies optoelectronic devices and the integration of these components onto optoelectronic multi chip modules (OE-MCMs) using a combination of packaging techniques. For this project, (1×12) array photodetectors were developed using PIN diodes with a GaAs/AlGaAs strained layer structure. The devices had a pitch of 250μm, operated at a wavelength of 850nm. Optical characterisation experiments of two types of detector arrays (shoe and ring) were successfully performed. Overall, the shoe devices achieved more consistent results in comparison with ring diodes, i.e. lower dark current and series resistance values. A decision was made to choose the shoe design for implementation into the high speed systems demonstrator. The (1x12) VCSEL array devices were the optical sources used in my research. This was an identical array at 250μm pitch configuration used in order to match the photodetector array. These devices had a wavelength of 850nm. Optoelectronic testing of the VCSEL was successfully conducted, which provided good beam profile analysis and I-V-P measurements of the VCSEL array. This was then implemented into a simple demonstrator system, where eye diagrams examined the systems performance and characteristics of the full system and showed positive results. An explanation was given of the following optoelectronic bonding techniques: Wire bonding and flip chip bonding with its associated technologies, i.e. Solder, gold stud bump and ACF. Also, technologies, such as ultrasonic flip chip bonding and gold micro-post technology were looked into and discussed. Experimental work implementing these methods on packaging the optoelectronic devices was successfully conducted and described in detail. Packaging of the optoelectronic devices onto the OEMCM was successfully performed. Electrical tests were successfully carried out on the flip chip bonded VCSEL and Photodetector arrays. These results verified that the devices attached on the MCM achieved good electrical performance and reliable bonding. Finally, preliminary testing was conducted on the fully assembled OE-MCMs. The aim was to initially power up the mixed signal chip (VCSEL driver), and then observe the VCSEL output

Novel Photostructurable Polymer for On-Board Optical Interconnects Enabled by Femtosecond Direct Laser Writing

Die integrierte Optik hat sich als vielversprechende Lösung für elektronische Verbindungen erwiesen, die eine hohe Bandbreitendichte und einen geringen Stromverbrauch ermöglicht. Seit kurzem ist es möglich photochemische und physikalische Reaktionen auf ein Mikrovolumen zu begrenzen. Dies hat der optischen Verbindungstechnik unter Verwendung von Glas oder Polymer eine zusätzliche Dimension verliehen. Dreidimensionale Wellenleiter können das optische Signal zwischen Blöcken aller Dimensionen verbinden, kombinieren oder aufteilen. Die Erhöhung des Brechungsindex ist jedoch immer noch eine Herausforderung für die Herstellung stabiler Freiform- und monomodaler Wellenleiter mit dreidimensionaler Ausdehnung, welche sich innerhalb der Platine befinden. Diese Dissertation stellt ein neues Konzept vor, um dieser Herausforderung zu begegnen, indem direktes Femtosekunden-Laserschreiben in Polymer und externe Diffusion eines gasförmigen Monomers verwendet wird. Direktes Laserschreiben mit Zwei-Photonen-Absorption wurde verwendet, um die Vernetzung entlang eines vorher definierten Pfades zur Bildung des Wellenleiterkerns zu initiieren. Es wurde ein ausreichender Brechungsindexkontrast erzeugt, um gaußförmige Strahlen mit einem Modus zu führen. Feature-Größen konnten durch Variieren der Scangeschwindigkeit und der Laserintensität linear angepasst werden. Dieses Herstellungsverfahren erfordert nur eine Schicht eines einzelnen Materials ohne Masken-, Kontakt- oder Nassbearbeitung. Durch Verwendung dieser neuartigen Methode wurden dreidimensionale optische Wellenleiter-Arrays, Fan-in/Fan-out- und Splitter-Strukturen hergestellt. Dreidimensionale freiforme Wellenleiter haben ein hohes Potential zur Verbesserung der Packungsdichte und Flexibilität optischer Verbindungen auf Platinenebene

Modeling and simulation of surface profile forming process of microlenses and their application in optical interconnection devices

Free space micro-optical systems require to integrate microlens array, micromirrors, optical waveguides, beam splitter, etc. on a single substrate. Out-of-plane microlens array fabricated by direct lithography provides pre-alignment during mask fabrication stage and has the advantage of mass manufacturing at low cost. However, this technology requires precise control of the surface profile of microlenses, which is a major technical challenge. The quality control of the surface profile of microlenses limits their applications. In this dissertation, the surface forming process of the out-of-plane microlenses in UV-lithography fabrication was modeled and simulated using a simplified cellular automata model. The microlens array was integrated with micromirrors on a single silicon substrate to form a free space interconnect system. The main contributions of this dissertation include: (1) The influences of different processing parameters on the final surface profiles of microlenses were thoroughly analyzed and discussed. A photoresist etching model based on a simplified cellular automata algorithm was established and tested. The forming process and mechanism of the microlens surface profile were explained based on the established model. (2) Microlens arrays with different parameters were designed, fabricated, and tested. The experiment results were compared with the simulation results. The possible causes for the deviation were discussed. (3) A microlens array based beam relay for optical interconnection application was proposed. A sequence of identical microlens array was fabricated on a single silicon substrate simultaneously and its optical performance was tested. A fast replication method for the microlens optical interconnects using PDMS and UV curable polymer was developed. A selective deposition method of micro-optical elements using PDMS ‘lift-off’ technique was realized. No shadow mask was needed during deposition process. With the continuous advances in the integration of micro-optical systems, direct lithography of micro-optical elements will be a potential technology to provide both precision alignment and low cost in manufacturing process. Microlenses and microlens array with precisely controlled surface profiles will be an important part in the micro-optical system

Design, measurement and analysis of multimode light guides and waveguides for display systems and optical backplane interconnections

The aim of the research in this thesis was to design and model multimode lightguides for optimising visible light for liquid crystal display systems and to design, model and experimentally test infrared light propagation within polymer multimode waveguides as board-to-board interconnects for high data rate communication. Ray tracing models the behaviour of a novel LCD colour separating backlight to optimize its efficiency by establishing the optimum dimensions and position for a unique micro-mirror array within the light guide. The output efficiency increased by 38.2% compared to the case without the embedded mirror array. A novel simulation technique combined a model of liquid crystal director orientation and a non-sequential ray tracing program was used first time to compute the reflected intensity from a LCOS device for a rear projection TV system. The performance of the LCOS display was characterised by computing the contrast ratio over a ±15° viewing cone. Photolithographically manufactured embedded multimode waveguides made from acrylate Truemode® polymer are characterized by measuring the optical transmission loss of key waveguide components including. straight, bend and crossing. Design rules derived from the experimental measurement were used to optimize optical PCB (OPCB) layout. A most compact and complex optical interconnects layout up-to-date for data centres, including parallel straight waveguide sections, cascaded 90° bends and waveguide crossing other than 90° angles, was designed, tested and used in an optic-electrical demonstration platform to convey a 10.3 Gb/s data. A further new method for reducing the end facet roughness and so the coupling loss, by curing a thin layer of core material at the end of the waveguide facet to cover the roughness fluctuations, was proposed and successfully demonstrated giving the best results reported to date resulting in an improvement of 2.8 dB which was better than the results obtained by using index matching fluid

Multimode optical waveguides and lightguides for backplane interconnection and laser illuminated display systems

The aim of the research in this thesis was to design, model, analyse and experimentally test multimode optical waveguides and lightguides for manipulating infrared light for optical backplane interconnections and visible light for laser illuminated display systems. Optical Input/Output Coupling loss at the entry and exit of polymer waveguides depends on optical scattering due to end facet roughness. The input/output coupling loss was measured for different end facet roughness magnitudes and the waveguide surface profiles due to different cutting methods (dicing saw and three milling routers) were compared. The effect of the number of cutting edges on the router, the rotation rate and translation (cutting) speed of the milling routers on the waveguide end facet roughness was established. A further new method for reducing the end facet roughness and so the coupling loss, by curing a layer of core material at the end of the waveguide to cover the roughness fluctuations, was proposed and successfully demonstrated giving the best results reported to date resulting in an improvement of 2.8 dB, even better than those obtained by use of index matching fluid which is impractical in commercial systems. The insertion loss due to waveguide crossing having various crossing angles was calculated using a beam propagation method and ray tracing simulations and compared to experimental measurements. Differences between the results were resolved leading to an understanding that only low order waveguide modes at no more than 6 degrees to the axis were propagating inside the waveguide. Several different optical designs of multimode waveguide for the light engine of a 3D autostereoscopic laser illuminated display system were proposed. Each design performed the functions of laser beam combining, beam shaping and beam homogenizing and the best method was selected, designed, modelled, tested, and implemented in the system. The waveguide material was inspected using spectroscopy to establish the effect of high power optical density on the material performance showing an increased loss particularly in the shorter wavelengths. The effect of waveguide dimensions on the speckle pattern was investigated experimentally and the speckle contrast was reduced to below the threshold of human perception. Speckle contrast was also recorded for the first time along the axis of the 3D display system and normal to it in the viewing area and the speckle characteristics at each stage were investigated. New algorithms for analysing speckle were used and the perceptual ability of human eyes to detect speckle size and contrast were taken into account to minimise perceived speckle patterns. The effect of the core diameter of optical fibres on the speckle pattern was investigated and it was shown that the speckle spot diameter is reduced by increasing the fibre core diameter. Based on this experiment, it was suggested that speckle reduction is more effective if the optical fibre used in the display system has larger diameter. Therefore, a slab waveguide of 1 mm thickness and 20 m width was used for laser beam combining, homogenising and beam shaping and a uniformity of 84% was achieved with just 75 mm length. The speckle was also completely removed at the output of the waveguid

Flexible Polymer Waveguides for High-Speed Short-Reach Links

This dissertation presents a detailed study of flexible polymer waveguides for used in short-reach communication links. Flexible polymer waveguides enable a wider range of applications as compared to the rigid polymer waveguides, especially for the rack-to-rack links and data bus systems in autonomous car and avionic industry. However, their optical performance including bending and twisting loss, crosstalk, bandwidth and mode coupling behaviour hasn’t been studied in detail when the flexure is applied to the waveguide. This research quantifies those performances when flexible polymer waveguides are being flexed and provides a useful guideline when designing those flexible polymer links in the real world. In addition, some suggestions have been discussed, which can be used to improve waveguide loss performance. A lot simulation work has been done to support the observations from the experimental results. The flexible polymer waveguides are proven to be robust and have low propagation loss (0.03 dB/cm) and high temperature resistance (up to 350 °C). They can be bent down to 2 mm without cracks and the resultant excess bending loss can be less than 2 dB. The excess twisting loss is also shown to be very low, around 0.02 dB for 4 × 360° full twisting turns as long as lateral tension is carefully reduced. The crosstalk results reveal their values are < -25 dB under any launch conditions when waveguides are flexed. The dynamic behaviour study shows that flexible polymer waveguides are robust and can work dynamically for a long-time horizon. In addition, a new design of flexible polymer waveguides has been proposed which can reduce the excess bending loss to around 0.5 dB at 2 mm bending, which is a big improvement. As for the mode coupling behaviour, both simulation and experimental works have been done to investigate how optical modes will evolve due to the small bends, micro bends and rough sidewalls. A better understanding of propagation mechanism inside waveguide is given and discussed. Then, the ultra-short laser pulse measurements are carried out to get the bandwidth length products (BLP) of the waveguides under flexure. The results indicate polymer waveguide can support over 100 GHz×m BLP and small bends have the ability of improving bandwidth performance further. At last, a 40 Gbps transmission over 1-m spiral flexible polymer waveguides under different bending radius has successfully been demonstrated