Peter Geerinck

MT-compatible laser-ablated interconnections for optical printed circuit boards

Integration of optical interconnections on a printed circuit board (PCB) is very challenging, since compatibility should be maintained with standard PCB manufacturing technology. This paper describes the use of laser ablation, a technique already used in PCB manufacturing for drilling microvias, as a suitable technique for the fabrication of multimode polymer waveguides, micromirrors, alignment features, and microlenses. A frequency-tripled Nd-YAG laser and a KrF excimer laser are used, both mounted on the same stage, resulting in very high alignment accuracies. This paper demonstrates a parallel optical link over approximately 5-cm-long PCB integrated waveguides, fully connected using a standard MT-based connector. This proves that laser ablation can be a key technology in optical board manufacturing to reach the stringent coupling tolerances.

Laser cleaving of glass fibers and glass fiber arrays

An ultraviolet (UV)-excimer-laser-based cleaving procedure for silica fiber has been developed that enables automated cleaving for high-volume production of fiber-optic assemblies. A selective ablation of the glass in the form of a small rectangular cavity serves as a fracture initiator when the fiber is put under stress. The position of the UV-excimer-laser-induced scratch is very precise. The system provides high-quality cleaves on single-fiber and ribbon configurations. The end angle of the cleaved optical fiber is measured using a noncontact optical interferometer system and was 0.85/spl deg/ for perpendicular cleavages. Insertion loss after splicing is in the range of 0.03 dB, which is compatible with mechanical-cleaved fibers.

Laser Ablation as Enabling Technology for the Structuring of Optical Multilayer Structures

In this paper, laser ablation is presented as a versatile technology that can be used for the fabrication of all building blocks and functional elements required for an optical interconnection, integrated in printed circuit boards (PCBs). The integration of optical interconnections in PCBs is an emerging field in which interest worldwide is rapidly growing. The limiting factor is mainly the compatibility of new technologies, used to define and fabricate the optical interconnections, with standard FR4-processing steps, temperatures and lamination pressures. Laser ablation, which is currently frequently used for the drilling of electrical micro-vias in PCBs, has proven to be fully compatible with standard PCB manufacturing. An optical two layer structure is studied that can make full use of the functionalities of 2D elements such as VCSEL or photodiode arrays.

Laser ablated coupling structures for stacked optical interconnections on printed circuit boards

Laser ablation is presented as a versatile technology that can be used for the definition of arrays of multimode waveguides and coupling structures in a stacked two layer optical structure, integrated on a printed circuit board (PCB). The optical material, Truemode BackplaneTM Polymer, is fully compatible with standard PCB manufacturing and shows excellent ablation properties. A KrF excimer laser is used for the ablation of both waveguides and coupling structures into the optical layer. The stacking of individual optical layers containing waveguides, that guide the light in the plane of the optical layer, and coupling structures, that provide out-of-plane coupling and coupling between different optical layers, is very interesting since it allows us to increase the integration density and routing possibilities and limit the number of passive components that imply a certain loss. Experimental results are presented, and surface roughness and profile measurements are performed on the structured elements for further characterization. Numerical simulations are presented on the tolerance on the angle of the coupling structures and the influence of tapering on the coupling efficiency of the waveguides.

Embedding of Optical Interconnections in Flexible Electronics

This paper describes the development of a flexible substrate or foil in which optical waveguides, light sources, detectors, and electronic circuitry are embedded. The generic technology offers an integrated solution to the increasing demand for flexible optical sensors and creates a technology platform for the establishment of flexible high-speed optical data connections, based on optical wave guiding layers. Patterning of the optical waveguiding is done using both a standard photolithography process and laser ablation. The resulting opto-electrical foil shows very good flexible behavior and low optical propagation and bending losses.

Deep Lithography with Protons to prototype pluggable micro-optical out-of-plane coupling structures for multimode waveguides

We present a pluggable micro-optical component fabricated with Deep Lithography with Protons, incorporating a micro-mirror for the out-of-plane coupling of light to or from polymer multimode waveguides integrated on a printed circuit board (PCB). This millimeter-sized mass-reproducible component can then be readily inserted into laser ablated cavities. The roughness of the optical surfaces of the component is measured using a non-contact optical profiler, showing a local average RMS roughness around 30nm. Non-sequential ray-tracing simulations are performed to predict the optical performance of the component, showing coupling efficiencies up to 78% and a rigorous study on misalignment tolerances is performed. These results are then experimentally verified using piezo-motorized positioning equipment with submicron accuracy. As a first step, we characterize the component in a multimode fiber-to-fiber coupling scheme, showing coupling efficiencies up to 56%. As a second testbed, we use multimode waveguides patterned by UV-exposure in Truemode™ polymer, incorporating excimer laser ablated cavities. The size and depth of the cavities can be easily adapted on the design of the coupling structure, whereas alignment marks can be defined in the same processing step. Due to the multimode character of the waveguides, the total internal reflection condition is not always fully satisfied. Therefore, we investigate the application of a metal reflection coating on the micro-mirrors to improve the coupling efficiency. The fabricated coupling components are suitable for low-cost mass production since the compatibility of our prototyping technology with standard replication techniques, such as hot embossing and injection molding, has been shown before.

Laser ablated coupling structures for optical printed circuit boards

We report on the cost effective fabrication of 45° micromirror couplers within single-mode polymer waveguides for achieving fully embedded board-level optoelectronic interconnections. Compatibility with existing board manufacturing technology is achieved by making use of polymers with high thermal stability. The sol-gel polymers behave as negative photo resist and waveguides are patterned by UV exposure. Micromirrors are fabricated using excimer laser ablation, a very flexible technology that is particularly well suited for structuring of polymers because of their excellent UV-absorption properties and highly non-thermal ablation behavior. A coupling structure based on total internal reflection (TIR) is enhanced by developing a process for embedding a metal coated 45° mirror in the optical layers. The mirrors are selectively metallized using a lift-off process. Filling up the angled via without the presence of air bubbles and providing a flat surface above the mirror is only possible by enhancing the cladding deposition process with ultrasound agitation. Surface roughness of both the mirrors and the upper cladding surface above the mirrors is investigated using a non-contact optical profiler. Initial loss measurements at 1.3 μm show a propagation loss of 0.62 dB/cm and an excess mirror loss of 1.55 dB. During most recent experiments mirror roughness has been reduced from 160 nm to 20 nm, which will seriously reduce the mirror loss.

Development of a fabrication technology for integrating low cost optical interconnects on a printed circuit board

We present a fabrication technology for integrating polymer waveguides and 45° micromirror couplers into standard electrical printed circuit boards (PCBs). The most critical point that is being addressed is the low-cost manufacturing and the compatibility with current PCB production. The latter refers to the processes as well as material compatibility. Multimode waveguides are patterned by KrF excimer laser ablation in acrylate polymers with 0.13 dB/cm propagation loss at 850 nm. Single mode waveguides using inorganic-organic hybrid polymers show an attenuation loss of 0.62 ± 0.08 dB/cm at 1.3 μm. A process for embedding metal coated 45° micromirrors in optical waveguiding layers is developed. Mirrors are selectively metallized using a lift-off process. Filling up the angled via without the presence of air bubbles and providing a flat surface above the mirror is only possible by enhancing the cladding deposition process with ultrasound agitation. Initial single mode coupling loss measurements at 1.3 μm show an excess mirror loss of 1.55 dB. Multimode coupling loss measurements will improve this excess loss, because of the lower surface roughness of the mirrors using the acrylate polymers for multimode waveguides.

Optical connections on flexible substrates

Optical interconnections integrated on a flexible substrate combine the advantages of optical data transmissions (high bandwidth, no electromagnetic disturbance and low power consumption) and those of flexible substrates (compact, ease of assembly...). Especially the flexible character of the substrates can significantly lower the assembly cost and leads to more compact modules. Especially in automotive-, avionic-, biomedical and sensing applications there is a great potential for these flexible optical interconnections because of the increasing data-rates, increasing use of optical sensors and requirement for smaller size and weight. The research concentrates on the integration of commercially available polymer optical layers (Truemode BackplaneTM Polymer, Ormocer®) on a flexible Polyimide film, the fabrication of waveguides and out-of plane deflecting 45° mirrors, the characterization of the optical losses due to the bending of the substrate, and the fabrication of a proof-of-principal demonstrator. The resulting optical structures should be compatible with the standard fabrication of flexible printed circuit boards.