Page 354 - System on Package_ Miniaturization of the Entire System

328 Cha pte r S i x

Normalized absorption (–dB/cm) 1.5

1.0

0.5

800 900 1000 1100 1200 1300 1400 1500 1600
Wavelength (nm)

FIGURE 6.3 Representative spectral absorption common to most organic optical quality
polymers.

There are four basic power-loss mechanisms associated with organic polymer
waveguides.

1. Electronic absorption is mostly due to the absorption of light by hydrogen
atoms and is effective in the 2000- to 4000-angstrom (Å) band [14] and
contributes mostly to absorption in the visible range.
2. Absorption by overtones of fundamental molecular vibrations (mostly stretch
vibrations) happens to fall in the range of 1100 to 1600 nm. This includes O—H,
C—H, and C O stretch vibrations. By introducing fluorine or other atoms
into the bond, the frequency of the fundamental stretch absorption is shifted
along with a corresponding shift in the overtones.
3. Light scattering from a number of sources, both intrinsic and extrinsic, includes
compositional or density fluctuations, mostly due to nonuniform solvent
evolution, particulate contamination in the original liquid monomer, processing
imperfections such as bubbles, cross-section nonuniformity, and sidewall
roughness (generally associated with reactive ion etching processes).
4. Finally, stress-induced birefringence is due primarily to film stresses that arise
because of differences in the thermal expansion coefficients of the various
polymer layers on the PCB and the board core material itself as well as by actual
board bending and warping during thermal cycling, and scales as the elastic
modulus of the waveguide material and the coefficients of its isotropic piezo-
optic tensor. Generally, birefringence in multimode waveguides contributes to
power loss by rotating the polarization of a guided mode into the polarization
of an unsupported mode.
Table 6.1 lists commonly used polymer materials, suppliers, optical properties, basic
integration processes, thermal properties, and references for further reading and
investigation.