Tunable acoustooptic filters and equalizers for WDM applications

We describe a high performance acoustooptic tunable filter for add-drop application and for signal equalization in WDM telecommunication crossconnects. It results from a thorough investigation in TeO₂ of bulk collinear interaction, the geometry of which, particularly the direction of propagation of the acoustic wave, has been chosen in order to obtain the best compromise between the spectral resolution of the device and the acoustooptic figure of merit. Less than 40 mW of electric power is needed either to deviate 100% of a selected light wavelength λ at resonance, or to induce a 30-dB attenuation of its intensity. The sidelobes practically vanish for this configuration and the resolution is equal to 0.75 nm (or 94 GHz) for λ=1.55 μm. Polarization splitters combined with half-wave plates allow to completely get rid of polarization sensitivity problems. The use of optical fibers to collect the signal at the filter outputs, actually contributes to the high performance of the device as a whole. Experiments have been performed by multiplexing three signals in the input fiber, separated by 4, 2, and 1 nm. The transmission of the filter has been examined through the bar and cross state.