Crosstalk in a lossy directional coupler switch

Crosstalk due to material absorption in a two-waveguide, symmetric directional coupler switch is investigated. In a material with absorption, it is not possible to completely eliminate the crosstalk by adjusting the coupling length. The coupling length for minimum crosstalk differs from that of lossless systems. Theoretical limits of the lowest achievable crosstalk and the corresponding coupling lengths are calculated. The results show that the effect of absorption on crosstalk is more severe when the devices are designed for low crosstalk. The increase in crosstalk due to absorption can be as high as 20 dB. The material absorption is thus a critical parameter in designing low crosstalk devices.< >