Optical switching allows us to make the photonic network flexible and expandable. We have been studying two types of optical switches incorporating ferro-electric liquid crystals (FLCs). One type of them is a polarization-controlled free-space optical switch whose operation principle is based on the polarization switching by FLC cells. It has a novel integrated structure consisted with FLC polarization control devices (FLC-PCDs), thin-film beam-splitters and mirrors. The FLC-PCD with a metal mirror electrode is the key element in this switch. The design theory for the FLC-PCD to achieve an accurate 90-degree polarization switching for the oblique incident light has been developed and verified by experiments. 2x2 and 4x4 optical switches were fabricated for optical communication wavelengths of 1300 and 1550nm and their feasibility was demonstrated.
The other switch type is a waveguide switch composed of an optical waveguide having FLC claddings. The FLCs in the cladding layer change their effective refractive index corresponding to the applied voltage polarity, providing the phase shift of the travelling lightwave in the waveguide. The operation principle of the switch has been confirmed by an experimental Si waveguide Mach-Zehnder interferometer (MZI) having FLC claddings.
This paper describes a four-channel optical switch module prototype for a reconfigurable optical interconnection network using a polarization-switched architecture and liquid crystal polarization control devices. Compact size and low insertion loss have been achieved by using integrated optical elements. The successful switching of 1 Gbps optical signals with little skew has been demonstrated.