Crosstalk reduction in N×N WDM multi/demultiplexers bycascading small arrayed waveguide gratings (AWG's)

This paper shows a new scheme which improves the crosstalk performance of large optical multi/demultiplexers, a key component in wavelength division multiplexing (WDM) systems. This scheme uses arrayed waveguide gratings (AWG's) of various sizes and requires no additional equipment. It is well known that a large multi/demultiplexer can be constructed by cascading small multi/demultiplexers. We have studied the impact of the number and size of AWG stages on crosstalk performance. This paper proves that to obtain a multistage multi/demultiplexer with minimum crosstalk, the total channel number of each AWG stage must be minimized. For example, cascading 10-channel AWG's and 11-channel AWG's improves the crosstalk performance of a 110-channel multi/demultiplexer by about 7.5 dB. Furthermore, the crosstalk performance degradation due to fabrication error is theoretically investigated taking channel bandwidth into account. Optimum design parameters of multistage AWG's are introduced: When the AWG suppression ratio is 30 dB and the ratio of channel bandwidth to channel spacing is about 0.24, the degradation in crosstalk performance due to fabrication error is minimized. The tradeoff between the crosstalk performance and the efficiency in terms of hardware and wavelength are also discussed. It is discovered that this simple scheme can yield a crosstalk-free WDM router. Crosstalk reduction obtained by this scheme allows the realization of flexible multiwavelength networks based on wavelength routing