Optical interconnections have gained interest over the last years, and several approaches have been presented
for the integration of optics to the printed circuit board (PCB)-level. The use of a polymer optical waveguide
layer appears to be the prevailing solution to route optical signals on the PCB. The most difficult issue is the
efficient out-of-plane coupling of light between surface-normal optoelectronic devices (lasers and photodetectors)
and PCB-integrated waveguides. The most common approach consists of using 45° reflecting micro-mirrors.
The micro-mirror performance significantly affects the total insertion loss of the optical interconnect system, and
hence has a crucial role on the system's bit error rate (BER) characteristics.
Several technologies have been proposed for the fabrication of 45° reflector micro-mirrors directly into waveguides.
Alternatively, it is possible to make use of discrete coupling components which have to be inserted into
cavities formed in the PCB-integrated waveguides. In this paper, we present a hybrid approach where we try to
combine the advantages of integrated and discrete coupling mirrors, i.e. low coupling loss and maintenance of the
planararity of the top surface of the optical layer, allowing the lamination of additional layers or the mounting
of optoelectronic devices.
The micro-mirror inserts are designed through non-sequential ray tracing simulations, including a tolerance
analysis, and subsequently prototyped with Deep Proton Writing (DPW). The DPW prototypes are compatible
with mass fabrication at low cost in a wide variety of high-tech plastics. The DPW micro-mirror insert 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. An average mirror
loss of 0.35-dB was measured in a receiver scheme, which is the most stringent configuration. Finally, the
configuration is robust, since the mirror is embedded and thus protected from environmental contamination,
like dust or moisture adsorption, which makes them interesting candidates for out-of-plane coupling in high-end
boards.
We present a peripheral fiber connector to seamlessly interface optical fiber ribbons with polymer optical waveguides
integrated on a Printed Circuit Board (PCB). Laser ablation was used to fabricate the optical board, featuring alignment features which enable an accurate positioning of an alignment plate carrying MT-compatible guide pins. These allow for a high-precision in-line mating of commercial MT ferrules with the board-integrated
waveguides. The alignment plate is fabricated through deep proton writing (DPW) and is mounted on the PCB by means of steel micro-balls, and is compatible with mass replication technologies.
The increasing demand and use of optical fibers for sensor-applications and the increasing use of short distance optical
communication on backplanes has been witnessed because of their many advantages. The work described in this paper
provides a technology platform to increase the integration and compactness of these optical applications. We present the
establishment of a bendable package of optical interconnections and opto-electronic components. Standard commercially
available GaAs VCSEL's and GaAs photodetectors are thinned down to a thickness of 30 µm and embedded into a stack
of cladding-, core- and Polyimide layers. Multimode waveguides are patterned in the core layer to connect the VCSEL
and photodiode array's. Laser ablated 45 degrees micro-mirrors couple the light from the embedded opto-electronic
components into- and out of the waveguides. The final layer-stack with embedded active optical interconnections is
highly flexible and shows no warpage due to a symmetrical layer build-up. Galvanic fan-out of the contact pads of the
VCSEL's and PD's is realized by laser ablated via's and sputtered copper tracks in between the layers.
The stand-alone optical foil is only 160 μm thick and can reach a minimum bending radius of 0.5 cm. Optical bending
losses of the flexible waveguides are lower than 0.25 dB per cm for a 8 mm bending radius at a wavelength of 850 nm.
We present an enhanced out-of-plane coupling component for Printed Circuit Board-level optical interconnections.
Rather than using a standard 45° micro-mirror to turn the light path over 90° we introduce a curvature in the
mirror profile and incorporate an extra cylindrical micro-lens for beam collimation. Both modifications enable
an increase in coupling efficiency and are extensively investigated using non-sequential ray tracing simulations in
combination with Matlab optimization algorithms. The resulting design is fabricated using Deep Proton Writing
and experimental characterization of the geometrical properties and measured coupling efficiencies are presented.
Coupling structures are critical building blocks that have a big influence on the performance of board-level optical
interconnections. 45° micro-mirrors deflect the light beam over 90° and are used for out-of-plane coupling in single
layer structures and out-of-plane and inter-plane coupling in multilayer structures. Two different approaches are being
presented: a micro-mirror that is directly integrated with the multimode waveguides and a discrete coupling element that
can be plugged into a cavity in the optical layer. The advantage of the integrated micro-mirror is the high achievable
alignment accuracy. The discrete couplers on the other hand have the advantage that they can be characterized and
measured prior to the insertion into the optical layer. Both mirror configurations are discussed and the performance is
evaluated at wavelength 850nm.
Multilayer optical interconnects offer high interconnection densities and flexible routing schemes. Signals can be routed
between the different layers, which can limit the number of cross-overs. In addition, the characteristics of 2D opto-electronic
elements can be fully exploited. The alignment between the different optical elements is critical for the
performance of the system. Efficient coupling structures are required to couple the light signals between two layers. We
propose the use of laser ablated 45° mirrors, which are integrated with the waveguides. Two mirror configurations are
proposed: one based on total internal reflection (TIR) and a metallized 45° mirror. A two layer optical structure is
presented that contains multimode waveguides and micro-mirrors. The achievable alignment accuracy between the
multimode waveguides and micro-mirrors in the two layers is in accordance with the results obtained from a numerical
study. Experimental realizations of the mentioned structures and the first results on the loss measurements on the mirrors
are presented.
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.
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.
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.
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.
In the fist part the waveguide fabrication technology is discussed, both photo lithography and laser ablation are
proposed. It is shown that a frequency tripled Nd-YAG laser (355 nm) offers a lot of potential for defining single mode
interconnections. Emphasis is on multimode waveguides, defined by KrF excimer laser (248 nm) ablation using acrylate
polymers. The first conclusion out of loss spectrum measurements is a 'yellowing effect' of laser ablated waveguides,
leading to an increased loss at shorter wavelengths. The second important conclusion is a potential low loss at a
wavelength of 850 nm, 980 nm and 1310 nm. This is verified at 850 nm by cut-back measurements on 10-cm-long
waveguides showing an average propagation loss of 0.13 dB/cm. Photo lithographically defined waveguides using
inorganic-organic hybrid polymers show an attenuation loss of 0.15 dB/cm at 850 nm. The generation of debris and the
presence of microstructures are two main concerns for KrF excimer laser ablation of hybrid polymers.
In the second part a process for embedding metal coated 45° micromirrors in optical waveguiding layers is described.
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 loss measurements indicate an excess mirror loss of 1.5 dB.
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.
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.
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.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.