Planar waveguide echelle gratings: an embeddable diffractive element for photonic integrated circuits

We have demonstrated a 40-channel echelle grating demultiplexer with a 100 GHz channel spacing, using silica-on-silicon planar waveguides. Figure 3 shows the output spectrum of several channels, measured using a tunable laser source. The measured cross-talk is better than -35 dB, the insertion loss is approximately 5 dB including fibre-to-waveguide coupling, and the polarization birefringence has been eliminated using a modified compensator design which eliminates losses due to mode mismatch. The demultiplexer itself occupies only about 1 cm² of chip area. The small size of this EG device means that high channel count demultiplexers and other dispersive elements can be easily embedded along with other components on a single chip of practical size. At this time, echelle gratings provide the only viable technology for integrating dispersive elements into photonic integrated circuits. Planar waveguide EG devices are also easily scalable to higher channel counts, and the device described here is currently being adapted to a 256-channel demultiplexer chip.