Polarization grating-based wavelength selective switches

Wavelength selective switches (WSS) play a key role in current optical networks, and are the core components of most reconfigurable optical add-drop multiplexers (ROADM), that enable fast, inexpensive, automated reconfiguration in wavelength division multiplexing (WDM)-based systems. In a routing node, a WSS can selectively forward certain wavelengths at its outputs, allowing optical broadcasting or multicasting solutions, with variable bandwidth. In a 1×N configuration, the incoming WDM signal is angularly dispersed by a static grating onto a liquid crystal on silicon (LCoS)-based spatial light modulator (SLM), which independently directs and filters the different frequency channels to the N output ports. In most configurations, light travels twice through the diffractive grating, doubling the intrinsic losses; in addition, the input polarization state should be carefully controlled, and two WSSs are required in each ROADM for polarization-multiplexed transmission. We present two other WSS architectures, with larger efficiency, that act simultaneously as polarization beam splitter and frequency demultiplexer, using polarization gratings (PG), that periodically vary the polarization state of the input light. The PGs are used either to frequency and polarization disperse the input light into two sets of diffraction orders, with orthogonal polarization states, replacing the static grating. Otherwise, the LCoS array is used in a PG configuration, to steer the frequency channels for each state of polarization toward 2N output ports.