The influence of aperture size and shape on crosstalk level in free-space grating demultiplexers for WDM networks

A simple method is presented to calculate crosstalk caused by aberrations and diffraction at the aperture edges in free-space grating demultiplexers for wavelength-division multiplexed (WDM) communication systems. The influence of circular and square aperture sizes is described for ideal and aberrated optical systems. It is shown that aperture enlargement is not critical to aberrations whilst providing significant crosstalk reduction. The minimum crosstalk value is obtained when the dispersion plane is parallel to the diagonal of the square aperture. -60 dB crosstalk is attainable for moderately aberrated systems with 80 channels (0.5 nm spacing) in the range 1530-1570 nm.