Designs of multiplexing/demultiplexing modules configured for N channels are disclosed. One of the features in the designs is to provide at least one concave mirror for compenstating filter tilting errors. The concave mirrors is placed where an incident traveling distance of a light beam is substantially similar or equal to the reflected traveling distance of the beam such that the filter tilting errors successively introduced over a number of optical filters can be successively reduced, compensated or removed over a subsequent number of optical filters as the light beams travels.