An important challenge that remains to date in board level optical interconnects is the coupling between the optical
waveguides on printed wiring boards and the packaged optoelectronics chips, which are preferably surface mountable on
the boards. One possible solution is the use of Ball Grid Array (BGA) packages. This approach offers a reliable
attachment despite the large CTE mismatch between the organic FR4 board and the semiconductor materials.
Collimation via micro-lenses is here typically deployed to couple the light vertically from the waveguide substrate to the
optoelectronics while allowing for a small misalignment between board and package. In this work, we explore the
fabrication issues of an alternative approach in which the vertical photonic connection between board and package is
governed by a micro-optical pillar which is attached both to the board substrate and to the optoelectronic chips. Such an
approach allows for high density connections and small, high-speed detector footprints while maintaining an acceptable
tolerance between board and package. The pillar should exhibit some flexibility and thus a high-aspect ratio is preferred.
This work presents and compares different fabrication methods and applies different materials for such high-aspect ratio
pillars. The different fabrication methods are: photolithography, direct laser writing and deep proton writing. The
selection of optical materials that was investigated is: SU8, Ormocers, PU and a multifunctional acrylate polymer. The
resulting optical pillars have diameters ranging from 20um up to 80um, with total heights ranging between 30um and
100um (symbol for micron). The aspect-ratio of the fabricated structures ranges from 1.5 to 5.
In this paper we describe the measurement of optical propagation losses in polymer waveguide structures. The
fabrication of single mode waveguides using direct laser writing is also presented in the paper. Simulated and
experimental results of coupling between two single mode waveguides at varying coupling length are also presented.
Finally the concept of using a herringbone structure in a non destructive technique of loss measurement in waveguides
is presented with simulation results.
Fill-factor of microlens arrays (MLAs) is one of the most important performance criteria of microlens arrays (MLA), especially in imaging applications. Low fill-factor lenses suffer greatly from spurious light and diffraction affects and result in low contrast in a beam steering system. Contrast ratio of low fill-factor circular shaped microlens arrays is nearly one-fourth of that of the system with high fill-factor square shaped microlens arrays. In this study performance of various types of nearly 100% fill-factor spherical MLAs in beam steering applications are compared. Design and fabrication of the MLAs are studied. A new hybrid method for design and fabrication of 100% fill-factor MLAs--by combining refractive-diffractive lenses, is suggested and tested.
In this paper we describe the direct laser writing of complex structures in a multifunctional acrylate polymer system using fan-out and grey-scale diffractive optical elements (DOE). The effective writing speed of waveguides was successfully increased using a fan-out DOE element. The DOEs to produce greyscale outputs (such as an intensity ramp) and other complex outputs (such as Heriot-Watt University logo) were used to fabricate greyscale and complex structures in polymer. Quantitative fabrication results of these structures are presented in this paper.
In this paper, laser ablation (at UGent), deep proton writing (at VUB) and laser direct writing (at HWU) are presented as versatile technologies that can be used for the fabrication of coupling structures for optical interconnections integrated on a printed circuit board (PCB). The optical layer, a highly cross-linked acrylate based polymer, is applied on an FR4 substrate. Both laser ablation and laser direct writing are used for the definition of arrays of multimode optical waveguides, which guide the light in the plane of the optical layer. In order to couple light vertically in/out of the plane of the optical waveguides, coupling structures have to be integrated into the optical layer. Out-of-plane turning mirrors, that deflect the light beam over 90°, are used for this purpose. The surface roughness and angle of three mirror configurations are evaluated: a laser ablated one that is integrated into the optical waveguide, a laser direct written one that is also directly written onto the waveguide and a DPW insert that is plugged into a cavity into the waveguiding layer.
A qualitative comparison is made between laser direct writing and laser ablation as enabling technologies for the structuring of multimode waveguides (50x50μm2) and 45° micro-mirrors into an optical layer. A small demonstrator is fabricated that allows us to couple light vertically from a transmitter into an optical layer and from the optical layer to a receiver. The optical layer, a multifunctional acrylate-based photo-polymer, is applied on an FR4-substrate. Multimode waveguides, that carry signals in the plane of the optical layer, are fabricated by means of laser direct writing, a technology that is available at HWU. The 45° micro-mirrors, that provide out-of-plane coupling, are ablated with the laser ablation set-up available at UGent. This set-up contains a KrF-excimer laser (248nm) that can be tilted, which eases the definition of angled facets. Surface roughness measurements are performed on both the optical layer and the micro-mirrors with a non-contact optical profiler. Loss measurements are performed on both the waveguides and the micro-mirrors.
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