Alongside the ever-increasing demands imposed on optical coating technology, in-situ spectral broadband monitoring has been developed to a versatile process control tool during the last two decades. In this context, the group around Michel Lequime reports on a novel approach for the determination of the optical constants of dielectric layers based on in-situ transmission measurements. Within their scientific cooperation, the authors designed and implemented an optical in-situ control device that combines optical broad band monitoring with a high precision single-wavelength system. With this device, the periodic variation of the transmittance of a single layer with its thickness can be recorded for each wavelength and analyzed focusing on its typical modulation time. In conjunction with a proper low-pass filtering and a further derivation of the modulation time behavior this approach results in a clean curve from which the increase of the quarter-wave optical thickness order number k as a function of the deposition time can be directly evaluated. Finally, the time dependence of the order number k can be plotted obtaining a direct picture of the layer thickness growth. This data reduction procedure can be performed for each wavelength separately; the combination of all data provides a very precise value of the layer thickness. In the same way, this novel concept for data reduction also allows for a precise determination of the refractive index and the extinction coefficient of the single layer. In summary, the presented algorithm is independent of any a priori information on the layers, which is needed for most other competing techniques, is completely deterministic as well as exceptionally precise and opens new ways for the deterministic production of complex optical coatings.
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