Optical wavelength division multiplexer/demultiplexer having patterned opaque regions to reduce optical noise

A patterned opaque layer is applied between adjacent filters mounted on a common optical block surface in a wavelength division multiplexer and demultiplexer. The patterned opaque layer covers the optically transparent regions between adjacent filters mounted on the common surface of the optical block. The patterned opaque layer is also applied around the perimeter of the footprint of the array of filters mounted on the common surface of the optical block. The patterned opaque layer reduces cross-talk between the filters and reduces optical noise.