Gbit/s. It is, however, well known from basic physical laws that electrical interconnections will suffer from high transmission losses and severe signal integrity
problems at such data rates. In order to overcome the evident high-speed interconnection bottleneck, optical interconnects are considered as the preferred option
[1].
In the optical communication field, polymethylmethacrylate (PMMA), polystyrene (PS), polycarbonate (PC), epoxy, polyimide, perfluorocyclobutyl (PFCB)based polymers, one-component polysiloxane and UV-cured polysiloxane have been
widely used as multimode waveguide materials for short-distance datacom applications, e.g. in combination with polymer optical fibres (POFs) in filtering, routing
devices and others. However, their implementation in applications for the datacom
market is still critical and restricted due to various factors such as high cost,
shrinkage, thermal and environmental instability, etc., despite the fact that all these
materials do not show the required optical transparency in the datacom and telecom
regions (600–1600 nm) [2–5]. Recently, polysiloxane materials, which were produced, for example, by addition thermal curing from two-component PDMS,
received much interest for application in waveguide-based devices due to simple and
low cost ( 50 €/L) thermal casting techniques, avoiding by-product formation
(except for the catalyst) and material shrinkage [5–9]. In addition, such
siloxane-based polymers, in which the thermally robust molecular SiO units are the
material backbone granting thermal stability up to 300 °C, show refractive indices
that range from 1.39 to 1.60 for a wavelength of 589 nm (20 °C) [7]. Therefore, these
polymers can be employed as core and cladding materials for optical waveguides.
The bands in the spectral VIS-NIR region are primarily overtones and combinations
of the fundamental bands found in the classical mid-infrared region. Most chemical
bonds have negligible harmonic absorption compared with so-called X–H subgroup
bonds, i.e. C–H, N–H and O–H. Among the latter, the absorption losses from N–H and
O–H subgroups are usually 10–20 times larger than those arising from C–H vibrations
[2, 3], so for the pre-selection of optical siloxane-based polymers, N–H and O–H bonds
should not exist. With the growing interest in two-component siloxane-based materials
with addition thermal curing for optical datacom applications, however, until now only
very few reports have been devoted to the characterization of their optical characteristics
in the visible and near-infrared (VIS-NIR) region.
In this chapter, PDMS materials were selected for this study with regard to their
bandwidth applicability. For this, refractive indices and the absorption as attenuation in dB/cm related to the power budget have been determined. An empirical
equation for estimating the most significant intrinsic absorption losses due to the
corresponding vibration bands was formulated. It provides a relationship between
the integral band strength and the intrinsic absorption loss, thus furnishing us with
estimates of the loss limit in such materials for implementation in datacom and
telecom devices. For such purpose, multimode waveguides were fabricated and
their respective optical insertion loss was measured at 850 nm with tests simulating
also accelerated ageing, as well their thermal stability verified. Tolerances with
regard to mould inner surface roughness and waveguide interlayer from the fabrication have also been investigated.
402
D. Cai and H. M. Heise
problems at such data rates. In order to overcome the evident high-speed interconnection bottleneck, optical interconnects are considered as the preferred option
[1].
In the optical communication field, polymethylmethacrylate (PMMA), polystyrene (PS), polycarbonate (PC), epoxy, polyimide, perfluorocyclobutyl (PFCB)based polymers, one-component polysiloxane and UV-cured polysiloxane have been
widely used as multimode waveguide materials for short-distance datacom applications, e.g. in combination with polymer optical fibres (POFs) in filtering, routing
devices and others. However, their implementation in applications for the datacom
market is still critical and restricted due to various factors such as high cost,
shrinkage, thermal and environmental instability, etc., despite the fact that all these
materials do not show the required optical transparency in the datacom and telecom
regions (600–1600 nm) [2–5]. Recently, polysiloxane materials, which were produced, for example, by addition thermal curing from two-component PDMS,
received much interest for application in waveguide-based devices due to simple and
low cost ( 50 €/L) thermal casting techniques, avoiding by-product formation
(except for the catalyst) and material shrinkage [5–9]. In addition, such
siloxane-based polymers, in which the thermally robust molecular SiO units are the
material backbone granting thermal stability up to 300 °C, show refractive indices
that range from 1.39 to 1.60 for a wavelength of 589 nm (20 °C) [7]. Therefore, these
polymers can be employed as core and cladding materials for optical waveguides.
The bands in the spectral VIS-NIR region are primarily overtones and combinations
of the fundamental bands found in the classical mid-infrared region. Most chemical
bonds have negligible harmonic absorption compared with so-called X–H subgroup
bonds, i.e. C–H, N–H and O–H. Among the latter, the absorption losses from N–H and
O–H subgroups are usually 10–20 times larger than those arising from C–H vibrations
[2, 3], so for the pre-selection of optical siloxane-based polymers, N–H and O–H bonds
should not exist. With the growing interest in two-component siloxane-based materials
with addition thermal curing for optical datacom applications, however, until now only
very few reports have been devoted to the characterization of their optical characteristics
in the visible and near-infrared (VIS-NIR) region.
In this chapter, PDMS materials were selected for this study with regard to their
bandwidth applicability. For this, refractive indices and the absorption as attenuation in dB/cm related to the power budget have been determined. An empirical
equation for estimating the most significant intrinsic absorption losses due to the
corresponding vibration bands was formulated. It provides a relationship between
the integral band strength and the intrinsic absorption loss, thus furnishing us with
estimates of the loss limit in such materials for implementation in datacom and
telecom devices. For such purpose, multimode waveguides were fabricated and
their respective optical insertion loss was measured at 850 nm with tests simulating
also accelerated ageing, as well their thermal stability verified. Tolerances with
regard to mould inner surface roughness and waveguide interlayer from the fabrication have also been investigated.
402
D. Cai and H. M. Heise