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The integration of optical interconnections in printed circuit boards (PCBs) is an emerging field that arouses rapidly
growing interest worldwide. At present the key issue is to identify a technical concept, which allows for the realization of
optical interconnections that are compatible to existing PCB manufacturing processes. Above all, the material in which
the optical interconnections are embedded has to withstand increased temperatures and lamination pressures as well as
various wet chemistry processes.
AT&S uses so-called two-photon absorption (TPA) laser structuring - a rather new and innovative technology - to realize
optical circuits in a special polymer layer. In this case a near infrared laser is applied working in the femto-second
regime. The high photon density that can be reached in the laser's focus results in a modification of the optical polymer,
which is usually photosensitive in the UV-spectrum of light only. In our particular case, the refractive index of the
optical polymer is increased. Choosing the right laser intensity and focus propagation speed one achieves a waveguide
well embedded within the polymer layer, which has not been affected by the laser. In contrast to one-photon absorption,
which only allows a two dimensional respectively lateral modification of a polymer, this technology allows a
modification within the volume resulting in 3D-microstructures inside the polymer layer. Apart from the possibility to
realize structures in three dimensions, this TPA-technique has additional advantages. First of all, it allows one step
fabrication, which reduces costs and production time compared to etching procedures or conventional UV lithography
processes. Moreover, this technique allows varying the waveguide's cross section geometry and diameter simply varying
size and form of the structuring laser focus.
Whereas the realization of optical waveguides is not challenging anymore the coupling of waveguides with
optoelectronic components is rather delicate. That is, the waveguide's ends have to be accurately positioned close to the
emitting surface of the signal source and the sensing area of the light detector, respectively. Using the TPA technology to
structure optical waveguides AT&S has successfully evaluated a powerful method to solve this interface problem for the
realization of integrated optical interconnections (IOIs) on PCBs.
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