The theory, production process, and properties of wavelength division multiplexers (WDM) designed on the beam splitting principle are described. Typically these modules require no lenses, and the edge interference filter applied to the oblique polished single-mode or multimode fiber end face exhibits only minimal polarizing effects. Whereas all the fibers of multimode modules are identical in type, single-mode modules require a large-diameter core fiber for coupling out light. The characteristics of the couplers depend on the given type of fiber (single-mode or multimode), the light sources (laser diodes or LEDs), the channel separation, and the channel used (transmission or reflection). Insertion losses vary between 0.7 and 2 dB, far-end cross talk attenuation varies between 11 and 22 dB, and near-end cross talk attenuation exceeds 40 dB.
G. Winzer, H. F. Mahlein, and A. Reichelt, "Single-mode and multimode all-fiber directional couplers for WDM," Appl. Opt. 20, 3128-3135 (1981)