An optical coupler for measuring mean wavelength of a broadband optical source is described. Sensitivity of the optical coupler to variations of refractive index and coupler length is minimized. The sensitivity to the variations of the refractive index is minimized by making a difference between the group refractive indices of the symmetric and asymmetric propagation modes approximately constant and independent of the refractive index. The sensitivity to the variations of the coupler length is minimized by selecting the coupler length that makes output powers from two waveguides to the first order insensitive to its length. This selection is possible for broadband optical sources.