Silicon photonics: Are smaller devices always better?

There is a worldwide trend towards miniaturising silicon photonic waveguides for both performance and cost reasons. It is clear than in many circumstances the shrinking of the device dimensions provides advantages in terms of cost and packing density, modulation bandwidth, improved performance in resonant structures, and an increase in optical power density within the devices. However, the size reduction comes at some costs in increased difficulty in maintaining single mode operation of the waveguides whilst controlling the polarisation properties of the device. Furthermore, the difficulty of coupling into and out of these devices is increased due to the mismatch in size and refractive index with an optical fibre.