A wavelength division multiplexer (WDM) and a type of fiber arrangement for the WDM are provided. In the WDM according to the invention, a filter having dielectric coating is used to allow lights within a specific band of wavelength to pass but reflect lights not within the specific band of wavelength. A reflection mirror is then used to reflect the lights passing through the filter to pass through the filter for a second time. Because of the two filtering, the WDM provided by the invention has very high adjacent channel isolation. In particular, the WDM contains a fiber pigtail having a fiber bundle and a ferrule with a square aperture in the center. By carefully arranging the common input fiber, the reflection output fiber, and the transmission output fiber inside the aperture, the lights in the specific band of wavelength have identical wavelength bandwidth when passing through the filter twice, and the insertion loss for both the transmission output and the reflection output is minimum. Having identical wavelength bandwidth implies that the wavelength bandwidth of lights emitted out of the transmission output is not reduced because the lights pass through the filter twice and cancel out each other. On the other hand, minimum insertion loss for both the reflection output and the transmission output implies that the performance of the WDM is optimal.