Monolithic 4*4 single-mode fused coupler

The fabrication and performance of a monolithic 4*4 single-mode fused coupler is reported. Measurements show that the device exhibits excellent coupling uniformity together with low excess loss. Two propagation paths are seen to be wavelength-flat between 1.2 mu m and 1.6 mu m, and two paths have a small coupling variation which may be minimised by fabrication technique.