A new method for characterizing the amplitude and phase distributions of the light propagating through arrayed-waveguide gratings is presented. The new method is based on the IFT. A very good agreement of the results obtained from the IFT method in comparison with OLC method has been achieved. The new method has been applied to an AWG with a small .DELTA.L of 22 .mu.m where the other methods fail. The precision of this measurement is demonstrated by a good agreement between the transmittance obtained from direct measurement and from calculation using the amplitude and phase distributions. Finally we want to point out, that although the results are based on a FTS measurement, the method can be applied to any experimental technique that provides a measurement of the complex transmittance of the AWG.