Nina Hendrickx

Discrete Out-of-Plane Coupling Components for Printed Circuit Board-Level Optical Interconnections

We propose discrete out-of-plane coupling components as a versatile alternative to current approaches used to couple light in and out of the propagation plane in waveguide-based printed circuit board (PCB)-level optical interconnections. The out-of-plane couplers feature a 45deg micromirror and are fabricated using deep proton writing as a rapid prototyping technology. Their fabrication is compatible with replication techniques and shows all the potential of low-cost mass fabrication. In a first configuration, we use the component in a fiber-to-fiber coupling scheme. Coupling losses as small as 0.77 dB were achieved. In a second configuration, the out-of-plane coupler is plugged into a laser ablated cavity in optical waveguides integrated on a PCB. Here a total link loss between out-of-plane fiber and in-plane fiber of 3.00 dB was achieved when using it at the transmitter side and 5.69 dB when using it at the receiver side.

Embedded Micromirror Inserts for Optical Printed Circuit Boards

We present the use of an embedded 45deg micromirror, which is patterned in polymer photoresist using deep proton writing. The micromirror is metallized and inserted in a laser ablated cavity in the optical layer and in a next step covered with cladding material. Surface roughness measurements confirm the excellent quality of the mirror facet. Loss measurements have been carried out to evaluate the behavior of the embedded micromirror. These measurements indicate an average loss below 0.35 dB, measured in a receiver scheme, which is the most stringent configuration.

Laser Ablated Micromirrors for Printed Circuit Board Integrated Optical Interconnections

Optical interconnections offer a possible solution to the bandwidth problems associated with future electrical interconnections. Optics has proven its potential for long-haul communication networks, where it is today a well accepted standard. The integration towards shorter distances is challenging. Compatibility with technologies used in printed circuit board manufacturing is required to implement optical interconnections on board-level in the near future in a cost-effective way. Especially coupling structures, which are used to deflect the light beam over 90deg, pose problems. We propose the use of metallized 45deg micromirrors which are fabricated with the use of laser ablation. This letter gives an overview of the fabrication process and shows experimental results. The root-mean-square surface roughness of the mirror facet is 70 nm or better, depending on the used polymer material. The 45deg angle can be ablated with an accuracy of plusmn1 deg and has a high reproducibility. The mechanical properties of the micromirrors were maintained after a Telcordia 85/85 stability test

Tolerance Analysis for Multilayer Optical Interconnections Integrated on a Printed Circuit Board

Polymer multilayer optical interconnections have gained interest over the past few years in view of their ability to increase the integration density, increase the routing flexibility, and make full use of the characteristics of 2D optoelectronic elements. The alignment between the functional elements in the different optical layers has to be sufficiently accurate in ensuring a high overall efficiency of the system. Numerical simulations have been used as a tool to determine whether laser ablation can be used as an alternative technology for the structuring of the functional elements of optical interconnections into a polymer optical layer. The experimentally achievable alignment accuracies are compared to tolerance ranges for an excess loss ¿ 1 dB obtained from the numerical study. The experimental achievements show that the alignment accuracies fall within the numerical tolerance ranges and have a good reproducibility. Experimental realizations of a two-layer multimode waveguide and inter- and out-of-plane- coupling structures are discussed and shown.

Efficient and tolerant resonant grating coupler for multimode optical interconnections.

More than 60% overall coupling efficiency is achieved in the demonstrator of an optical interconnect comprising an input grating coupler, a multimode slab waveguide section and an output grating coupler. The grating coupling strength is enhanced by means of a leaky mode resonance. The efficiency of the resonant grating coupler compares favourably with the performancs reported on mirror inserts.

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.

High-efficiency diffraction grating coupler for multimode optical interconnect

The device presented in this paper is designed for coupling a free space optical wave under quasi-normal incidence in and out of a highly multimode waveguide with high efficiency. It uses two resonant diffraction gratings at the substrate-waveguide interface that are made of a shallow metal grating, covered with a high refractive index layer. It is shown that the resonant structure can theoretically diffract up to 90% of the incident energy in and out of the waveguiding layer. The geometrical parameters of the structure and the tolerances can easily be achieved by conventional technology means.

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.

Low-cost micro-optics for PCB-level photonic interconnects

One of the grand challenges in solving the interconnection bottlenecks at the Printed Circuit Board (PCB) and Multi-Chip-Module (MCM) level, is to adequately replace the PCB and intra-MCM galvanic interconnects with high-performance, low-cost, compact and reliable micro-photonic alternatives. Therefore we address the following components in this paper: 1) out-of-plane couplers for optical waveguides embedded in PCB, 2) peripheral fiber ribbons and two-dimensional single- and multimode fiber connectors for high-speed parallel optical connections, and 3) intra-MCM level optical interconnections via free-space optical modules. For the fabrication of these micro-optical interconnect modules, we are focusing at the Vrije Universiteit Brussel on the continuous development of a rapid prototyping technology, which we call Deep Proton Writing (DPW). The special feature of this prototyping technology is that it is compatible with commercial low-cost mass replication techniques such as micro injection moulding and hot embossing. Laser ablation is used at Ghent University for the fabrication of PCB-embedded waveguides and integrated micro-mirrors. The main advantage of this technology is that it is compatible with present-day PCB manufacturing. For the free-space MCM-level optical interconnect module, we furthermore give special attention to the optical tolerancing and the opto-mechanical integration of the components. We use both a sensitivity analysis to misalignment errors and Monte Carlo simulations. It is our aim to investigate the whole component integration chain from the optoelectronic device to the micro-opto-mechanical components constituting the interconnect module.

Laser ablation and laser direct writing as enabling technologies for the definition of micro-optical elements

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.