Edwin Jan Klein - Universiteit Twente

Chapter 6
that the TEOS has a very high material stress which may cause cracking at a larger
thickness.
The coupling between the resonator and port waveguide is largely determined by the
lateral offset s, the radius R of the resonator, and the thickness hsep of the separation
layer. As shown in Table 6.1 a thickness of 1 µm was chosen for the separation layer.
Although this layer can be thinner, experiments on a number of fabricated devices
have shown that this thickness usually gives the best MR performance in the range of
lateral offsets that are also given in the table. In addition, if for some reason the
coupling of turns out to be lower than expected then it is still possible to increase the
coupling by reducing hsep and no new mask design is required.
6.2.2 Dimensions of the resonator and port waveguides
As discussed in Chapter 3.5.1 the design of the waveguide dimensions in a vertically
coupled resonator starts with the design of the resonator and then tries to match this
resonator to a port waveguide with well chosen dimensions. This design strategy
assumes that the dimensions of the port waveguide can be adjusted such that its
effective refractive index can be made to match that of the ring resonator.
Neff
1.58
1.56
1.54
1.52
1.50
1.48
1.46
0.5 1.0 1.5 2.0 2.5 3.0 3.5
Guide Width (µm)
134
h = 0.110
h = 0.120
h = 0.130
h = 0.140
h = 0.150
h = 0.160
h = 0.170
h = 0.180
h = 0.190
h = 0.200
h = 0.220
h = 0.260
Figure 6.5. Port waveguide Neff versus the waveguide height
and width. The indicated areas show the dimensions for best
phase matching and the best dimensions when tolerances are
included.
1 st order cut-off
Best design dimensions
when including tolerances
Figure 6.6. Waveguide
stitching.
Figure 6.5 shows that, in theory, it is indeed possible to design a monomodal
waveguide for a wide range of effective refractive indices. In practice, however, the
useful design range was much more limited for several reasons:
• Most of the devices that have been fabricated use masks made at the MESA+
institute for the lithography. One problem of these masks is that stitching in
the waveguides may occur, as shown in Figure 6.6. Because the additional
waveguide losses induced by this stitching are proportional to the waveguide
width, the waveguides should not be chosen too narrow. Another reason for
doing so is the fact that, for widths in the order of 1.5 µm, defects sometimes
occurred in the masks which can be avoided for larger dimensions.
• Bi-modality in the waveguides needed to be avoided at all cost. For fabricated
waveguides a deviation in width and height with respect to the original design