Maria Bayard Dühring - Solid Mechanics

44 Chapter 6 Design of acousto-optical interaction [P3]-[P7]
Figure 6.9 Results for the buried waveguide where the color bar indicates ∆n11/ √ P
close to the optical waveguide. The time averaged power flow in the x3-direction of the
fundamental mode is indicated by the contour lines with an arbitrary scale.
in the SiO2 layer such that their upper surface is leveled with the substrate surface.
The waveguides still support the two first order modes and for the first one polarized
in the x1-direction the interaction is ∆neff,1 = 7.16 · 10 −5 W −1/2 . This is 8 times
bigger than for the original waveguides on top of the surface. The power flow of
this optical mode is plotted together with ∆n11/ √ P in figure 6.9 and the entire
waveguide is now influenced by the change of the refractive index. For the optical
mode polarized in the x2-direction the interaction is ∆neff,1 = 3.89 · 10 −5 W −1/2 .
From an experimental point of view it is difficult to couple the optical mode to
a waveguide if the height is small, and it is more complicated to fabricate a buried
waveguide than a ridge waveguide. An alternative design is therefore to introduce a
layer of Si next to the waveguide with the height l as seen in figure 6.10(a). This can
simply be fabricated by stopping the etching of the Si on the surface before reaching
the SiO2 layer. In this way the optical mode comes closer to the stresses at the
surface, but the original height of the Si layer is kept such that the optical mode can
be coupled to the waveguide. The interaction ∆neff,ν as function of l is illustrated
in figure 6.10(b) for the two first order modes that the waveguide supports. The
interaction is increasing for increasing l and it is bigger for the mode polarized in the
x1-direction with ∆neff,1 = 1.11 · 10 −4 W −1/2 for l = 2.5 µm. This is 12 times bigger
than for the original geometry and the increase has the same size as for the improved
height and width. For increasing l the mode that is polarized in the x2-direction
is not supported anymore. The power flow of the optical mode and ∆n11/ √ P are
plotted in figure 6.10(a) for l = 0.2 µm. The mode is still confined to the waveguide
area and the Si layer, but now the center is below the waveguide where the stresses
are big. However, the drawback of this design is, that as the mode has a bigger
extension for increasing l the dimensions of other parts in the geometry, as the
input waveguides, taperings and splitters, must be correspondingly bigger.