There are N+1 gradient-index-rod lenses (GRINs) in a 1×N wavelength division multi/demultiplexer, forming a 1-to-N collimating system. Excess coupling losses arise due to the varying separations between the GRINs. The lengths of the GRINs need to be properly chosen to achieve a lowest average coupling loss. Numerical results show that the input GRIN is ΔZ0 longer than quarter pitch with ΔZ0=20～50 μm, depending on N. There is an upper limit for ΔZ0. The lengths of N output GRINs are differently shorter (ΔZi<0) or longer (>0) than quarter pitch. ΔZi increases from negative to positive starting from the near end to the far end GRINs, also depending upon N. The optimized matching GRINs assure the lowest coupling loss at N≤8, especially negligible at N≤6.