Maria Bayard Dühring - Solid Mechanics

56 Chapter 7 Concluding remarks
designs when optimizing a new problem.
Another disadvantage is that the optimized designs in general are complicated
with details comparable to the size of the wavelength. By collaborating with researchers
during the project, who work with the fabrication and testing of the SAW
structures and optical devices, the experimental limitations could be taken into account.
This has led to the study of simplified designs inspired by the optimized
results. Their performance were in general not as good as for the optimized designs,
but improvements compared to the initial designs were obtained.
In the case of interaction between SAWs and optical waves in waveguides, both
mechanical and optical factors have an influence on the performance and can enhance
or counteract each other’s effects. The simulations with the coupled model are thus
crucial in order to understand the acousto-optical interaction and to design the
devices. The interaction has been improved by topology optimization based on
the piezoelectric model, which resulted in detailed designs that are complicated to
fabricate. In contrast to topology optimization, experimental aspects and fabrication
issues can be taken directly into consideration when optimizing the interaction by
parameter studies of the geometry. The improvements gained by the parameter
studies were significant for both thin and high aspect ratio electrodes and the design
changes can be introduced in the experimental setups by the existing fabrication
techniques.
7.1 Future work
The studied simulation and optimization methods can be exploited further either to
improve the studied problems additionally or to extend the methods to other types
of wave propagation phenomena.
The problems in this work are simplified in different ways and the focus has
in general been on 2D problems. Extensions to 3D problems should be exploited
in order to investigate if the optimized structures are similar in 3D or if they are
fundamentally different and take advantage of other physical effects. Many types of
problems have to be studied by a 3D model as for instance the effects of the optical
waves propagating along the photonic-crystal fiber or the waveguide. In order to
design structures that guide and control surface acoustic waves in the out of plane
direction, as focusing IDTs, a 3D model is necessary as well. The computation time
and the use of memory increase significantly with the addition of the third dimension
and even though parallel computing is now possible within Comsol Multiphysics, the
simulations are still limited to smaller 3D problems. The effects of other physical
phenomena as the temperature increase in the SAW devices, the mechanical deformation
of the optical waveguides, the sound wave and structure interaction as well
as the effect of bending the optical fiber, can be incorporated into the models and
the influence on the device performance can be studied.
It is relevant to test the method of topology optimization for other expressions
of the objective functions. First of all it can be investigated if the performance of