Apparatus for demultiplexing a wavelength division multiplexed optical signal comprising a plurality of signals each having a different wavelength. The apparatus comprises a demultiplexer sub-assembly and a multi-channel receiver optical sub-assembly. The demultiplexer sub-assembly comprises a filter block sub-assembly for splitting the input optical signal. The receiver optical sub-assembly comprises an array of photo detectors for converting the optical signals from the filter block sub-assembly into electrical signals. The electrical signals are amplified by amplifier ICs and transmitted through electrical leads. The demultiplexer sub-assembly and the multi-channel receiver optical sub-assembly are actively aligned and fixed together.