Photonic WDM switch using fixed transmitters and tunable receivers

We discuss experimental results on a photonic switch architecture using dense WDM where laser signals are transmitted at distinct but close frequencies and then summed in a star coupler. The switching function is performed by selecting a particular frequency at each star coupler output. Two types of receivers were implemented: a tunable optical filter followed by a direct-detection receiver and a heterodyne receiver with a tunable laser as local oscillator. To avoid performance degradation due to frequency drifts, the laser transmitters are frequency-locked to a fixed reference frequency comb using a dithering technique with distinct tones. At the receiver side, the tunable filter is computer controlled and locks to the selected channel using the channel dithering tone. Likewise, the heterodyne receiver uses a frequency discriminator in combination with the dithering tone to lock the local oscillator at the appropriate distance from the selected channel.