A ten-channel single-mode wavelength division demultiplexer operating at 740, 750, 760, 770, 780, 790,800,810,820 and 830 nm with diffraction angles varying from 16° to 52° and using a graded index (GRIN) polymer waveguide is reported for the first time. Diffraction efficiency up to 60% was measured. The wavelength spreading of the Ti:Al2O3 laser (~ 4 nm, 3 dB bandwidth) causes an average crosstalk figure of -21 dB. The beamwidth of the diffracted signal as a function of the input beam width, the grating interaction length, and the diffraction angle are considered. Occurrence of maximum values are discussed. A waveguide lens is needed to efficiently couple the diffracted light into an output fiber whenever the diffracted beam size is beyond the core diameter of the fiber involved.