Contents I. Introduction
Protection switches are key devices for allowing an optical system to be reconfigured when a problem occurs in an optical network system. Several approaches have been presented to realize 2×2 DOSs for protection switching [1]–[5]. Among them, microelectromechanical systems DOSs have shown a very good performance [1]; however, a few shortcomings related to their mechanical reliability exist. Therefore, planar lightwave circuit-type thermo-optic DOSs based on a silica or polymer material have been of interest to researchers owing to their mass reproducibility with easy packaging and long-term reliability [2]–[8]. Silica DOSs, which have a Mach–Zehnder structure or directional coupler operated using phase control, are generally sensitive to wavelength and polarization and require high power consumption for switching [2], [3]. Thus, polymer-based DOSs are more attractive owing to their capability to overcome the above-mentioned problems [4]–[8].
