Design of a wavelength-division multiplexing filter for multimode optical waveguide is discussed. The filter is a dielectric multilayer thin-film filter with a periodic structure, which is called an edge filter. Its unit cell consists of two materials. Filters are designed for two kinds of waveguides. The one of them uses BCB and PFCB as a core and a cladding material, respectively, and the other uses NOA72 and NOA73 as a core and a cladding material, respectively. Two materials of each filter are chosen so that reflection loss is as low as possible for wavelength between 845nm and 855nm. SiC and $Ta_2O_5$ are used for the former waveguide, while ZnS and SiO are used for the latter. The characteristics of the designed filters are as follows: the loss of each filter in that wavelength range is less than 0.085dB and 0.34dB, respectively, the 3dB bandwidth of each filter is 146nm and 132nm, respectively, and the crosstalk of each filter is less than -15 dB in the wavelength range, (950nm, 1119nm) and (950nm, 1208nm), respectively. In addition to the filter design, the influence of both translation and tilt of the filter are also investigated by using the plane-wave expansion method. To maintain additional loss less than 1dB, tilt of the filter should be less than 1°.
