Embodiment of present invention provide a wavelength division multiplexing (WDM) module. The WDM module includes a substrate having a first side and a second side opposing the first side, wherein the first side includes a transpassing region coated with an anti-reflective (AR) film and a reflective region coated with a high-reflective (HR) film, and the second side includes multiple ports of optical paths; multiple WDM filters attached to the multiple ports at the second side of the substrate, wherein surfaces of the WDM filters attached to the substrate are coated with WDM films; and at least one reflector attached to the second side of the substrate in a space between the multiple WDM filters, wherein the reflector has a first surface attached to the substrate and a second surface, opposing the first surface, that has a convex shape and coated with a high-reflective (HR) coating.