An arrayed waveguide grating (AWG) device (1) comprising a substrate having a first array of waveguides (8) optically coupled between first and second slab couplers (3, 4) and a second array of input-output waveguides (10) optically coupled at first ends thereof to an input/output side (5) of the second slab-coupler (4). The input/output waveguide are tapered at first end portions (13) thereof so as to increase in width towards the second slap coupler, and the width (W) of the first ends of the input/output waveguides varies across the second array (10) in a manner so as to increase uniformity of at least on performance parameter, for example adjacent crosstalk (AXT). In the described embodiment, the width (W) of the tapered waveguide ends increases from one side to the array of input/output waveguides (10) to the other, so as to keep the separation (s) between the input/output waveguides.