Abstract: An arrayed waveguide grating [WDG] comprises I/O slabs (53a, 53b) interconnected by a grating region (51). The grating region (51) comprises many, e.g. at least 25 and preferably 50 to 500, individual cores (61,62,63), which have different lengths so as to provide wavelength selectivity by causing phase changes in light conveyed between the I/O slabs (53a, 53b). There are bends (64) for providing the different lengths and, to improve optical guidance round the bends, empty (R.I.=1) grooves (71) are located adjacent to the inside (71.1) and outside radii (71.2) so that the evanescent fields extend into the grooves (71). In order to keep the dimensions small, which improves the optical accuracy, tight bends (preferred radii of curvature of less than 2 mm and especially less than 500 &mgr;m) are desirable. The grooves conveniently have tapered ends and they extend into straight portions adjacent to the bends.