The effect of the group propagation of waves on the spectral behaviour of a multilayer Ar coating

As the monitoring wavelength detected by an optical monitor (photometer) is inevitably of a considerable spectral width, it should be travelling as a group of waves rather than a monochromatic wave. In addition, the photometer is influenced simultaneously by the waves’ combined effect. The phases of the disturbance of different wavelengths travelling with the group velocity are in good agreement. Therefore, the phase thickness(es) of the monitored layer(s) should be modulated by the group factor. The effect of the new thickness(es) is investigated and the close correlation between theoretically predicted and experimental results is presented.