Edwin Jan Klein - Universiteit Twente

67
Design
step to define the vertical slope of the waveguide as will be explained in more detail
in Chapter 5.
For both taper types it is important that the taper itself does not add to the coupling
losses. The tapers therefore have to be designed to be adiabatic. The taper angle θt for
which this can be achieved [112] is given by
ρ(
s).(
N eff − n
θ t ( s)
≤
λ
c
clad
)
(3.13)
where nclad is the index of the cladding layer, Neff the effective index of the
waveguide at position s, and ρ the half width of the waveguide at that position. The
definition of these parameters is also given in Figure 3.29.
Figure 3.29. Definition of taper parameters.
3.7.3 Optimal port waveguide bend radius
The minimum size of some devices, like the OADM, is largely dictated by the fiber
array. In other devices, however, especially those made in low-contrast materials, the
minimum size is directly related to the minimum achievable bend radius of the port
waveguides. That this radius can affect the design size in quite a significant way is for
instance shown in Figure 3.30a. In this layout of a single micro-resonator device
nearly two thirds of the device is taken up by the return bend in the drop port of the
device. While this particular layout can be improved by inserting an S-bend as shown
in Figure 3.30b this might not be possible in many other cases.
Figure 3.30a. Standard single resonator
layout.
s
ρ(s)
θt(s)
Figure 3.30b. Improved layout with reduced
height.
From a design perspective it is therefore critical to know what the minimum bend
radius of a port waveguide is to achieve the best possible layout. Not only the
minimum radius is of importance, however, but also the maximum radius should be
determined. At small bend radii the waveguide losses in a, for instance, 180-degree
bend are largely determined by the pure bend losses. For large radii, however, these
losses are relatively insignificant while the scatter and material losses become more