Arrayed waveguide gratings (AWG) play a key role in dense wavelength division multiplexing (DWDM) systems. While
the standard channel count (up to 40) and standard channel spacing (100 GHz or 50 GHz) AWGs feature very good
transmission characteristics, increasing the channel counts and narrowing the channel spacings leads to a rapid increase
in the AWG size and this, in turn; causes the deterioration in optical performance like higher insertion loss and, in
particular, higher channel crosstalk. Channel crosstalk is a result of amplitude errors of the far field profile at the end of
the input coupler and phase errors appearing in the phased array as a result of possible effective index and geometrical
irregularities of the arrayed waveguides.
I our work we show that keeping the length of arrayed waveguides short and using specially-shaped couplers, both phase
and amplitude errors can be minimized and, as such, the channel crosstalk strongly improved. To demonstrate this effect,
we designed 256-channel, 25 GHz AWG with both, standard and also with specially-shaped couplers. The far field
distribution at the end of standard input coupler features strong amplitude errors causing the high channel crosstalk and
particularly very high background crosstalk. Applying the specially-shaped couplers led to elimination of amplitude
errors in the far field distribution and this had a positive effect on the transmission characteristics. The adjacent crosstalk
was improved by ~ 4 dB, non-adjacent crosstalk was improved by ~ 5 dB and background crosstalk by about 10 dB.