In an embodiment, an N×N integrated optical interconnection apparatus capable of switching, multiplexing or demultiplexing a large number of input and output wavelength channels achieves low levels of crosstalk and insertion loss. Two substantially identical N×M star couplers are connected by an optical diffraction grating comprising M unequal length waveguides spaced from one another by predetermined amounts. The waveguides of the grating consists of an array of curved waveguides of different lengths. The waveguides are closely spaced at their ends and widely spaced and strongly curved in the central region. The curves or bends of the planar waveguides here disclosed have sharper bends, lower losses and increased tolerance to fabrication defects. The improved bends are realized by selecting a width and radius of curvature for the waveguide which is large enough to force the fundamental mode of an optical signal to propagate away from the inner edge of the bend, thus causing negligible illumination of this edge. Moreover, the curvature of the bend is large enough to effectively cut off modes above the fundamental mode. In the new waveguide here disclosed, the mode propagation constant is effectively independent of the waveguide width thus avoiding prior art phase error problems caused by variations in the width of the waveguide.
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