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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.
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