EOCBs, we combined general PCB processing and quality test methods (e.g.
IPC-TM-650 and IEC 68) together with standard test methods for optical components
in telecommunications (Telcordia Standards GR-1220 and GR-1209). From these
standards, the following main test conditions have been selected:
• Lamination (180 °C for 2 h);
• Soldering process (300 °C for 1 min in soda bath);
• Dry heat: 150 °C for 4 days (depending on the specific applications);
• Damp heat: 40 °C, 93% humidity, 96 h; and
• Thermal shock: −50–125 °C, 30 °C/min, 100 cycles.
13.3 Characterization of the Optical PDMS Materials
13.3.1 Refractive Index and Bandwidth
Guided wave optics confines radiation in the optical waveguides through the
phenomenon of total internal reflection (TIR), where the core material is surrounded
by a cladding material with a lower refractive index. The coupling and propagation
characteristics of the waveguide are thus defined by the core–cladding index
difference.
The refractive index of polymers mainly depends on the molecular polarizability,
the molecular volume and the difference between the used optical wavelength and
the absorption band wavelength of polymers [12]. In terms of the PDMS polymer
described in this chapter, the core material has an intrinsic higher refractive index
than the cladding material through the introduction of phenyl groups that are partly
substituting methyl groups in the side chain of the PDMS backbone. The ratio of
methyl to phenyl units can be varied to obtain a broad refractive index interval.
With the refractive indices measured at several wavelengths, there are various
dispersion formulas available that could be used for the fitting refractive index data,
e.g. equations provided by Hartman, Conrady, Herzberger, Schott Optical Glass,
Inc. and Sellmeier [13]. Some of the formulas, e.g. by Conrady, Schott or
Herzberger, are linear in their fitting constants, which means they can be fitted
easily using a classical least squares procedure, but it should be clear that the fitting
accuracy should be the dominant aspect in choosing the most appropriate dispersion
formula. From this point of view, the Sellmeier equation is especially suited for the
coverage of the refractive index in the wavelength range from the UV through the
visible to the mid-IR range (up to 2.3 µm) due to its high accuracy and applicability
[7]. The Sellmeier formula—see Eq. (13.1)—is derived from the classical dispersion theory and allows the description of the progression of refractive index
throughout the total transmission region with one set of parameters and the calculation of accurate interpolated values. Presently, it has been found that a
three-term Sellmeier formula is generally a necessary and sufficient descriptor for
406
D. Cai and H. M. Heise
IPC-TM-650 and IEC 68) together with standard test methods for optical components
in telecommunications (Telcordia Standards GR-1220 and GR-1209). From these
standards, the following main test conditions have been selected:
• Lamination (180 °C for 2 h);
• Soldering process (300 °C for 1 min in soda bath);
• Dry heat: 150 °C for 4 days (depending on the specific applications);
• Damp heat: 40 °C, 93% humidity, 96 h; and
• Thermal shock: −50–125 °C, 30 °C/min, 100 cycles.
13.3 Characterization of the Optical PDMS Materials
13.3.1 Refractive Index and Bandwidth
Guided wave optics confines radiation in the optical waveguides through the
phenomenon of total internal reflection (TIR), where the core material is surrounded
by a cladding material with a lower refractive index. The coupling and propagation
characteristics of the waveguide are thus defined by the core–cladding index
difference.
The refractive index of polymers mainly depends on the molecular polarizability,
the molecular volume and the difference between the used optical wavelength and
the absorption band wavelength of polymers [12]. In terms of the PDMS polymer
described in this chapter, the core material has an intrinsic higher refractive index
than the cladding material through the introduction of phenyl groups that are partly
substituting methyl groups in the side chain of the PDMS backbone. The ratio of
methyl to phenyl units can be varied to obtain a broad refractive index interval.
With the refractive indices measured at several wavelengths, there are various
dispersion formulas available that could be used for the fitting refractive index data,
e.g. equations provided by Hartman, Conrady, Herzberger, Schott Optical Glass,
Inc. and Sellmeier [13]. Some of the formulas, e.g. by Conrady, Schott or
Herzberger, are linear in their fitting constants, which means they can be fitted
easily using a classical least squares procedure, but it should be clear that the fitting
accuracy should be the dominant aspect in choosing the most appropriate dispersion
formula. From this point of view, the Sellmeier equation is especially suited for the
coverage of the refractive index in the wavelength range from the UV through the
visible to the mid-IR range (up to 2.3 µm) due to its high accuracy and applicability
[7]. The Sellmeier formula—see Eq. (13.1)—is derived from the classical dispersion theory and allows the description of the progression of refractive index
throughout the total transmission region with one set of parameters and the calculation of accurate interpolated values. Presently, it has been found that a
three-term Sellmeier formula is generally a necessary and sufficient descriptor for
406
D. Cai and H. M. Heise