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Optical interference filters are key components for space applications with more and more stringent specifications. As of today, it is common to design and fabricate filters with several hundreds of layers associated with several tens of microns of total thicknesses. This is particularly the case for narrow bandpass filters, notch filters, dichroic… Despite the significant improvement in the stability of the deposition machines and processes, precisely controlling the thickness of each layer of the stack remains one of the limiting step to achieve close to theoretical performances. This can be achieved through in-situ monitoring of the optical performances during deposition at a single wavelength (monochromatic optical monitoring) or over a broad spectral range (broadband optical monitoring). As of today, it is not clear what technique must be used depending on the type of filter and what are the limits of each of them. In this paper, we will review the different advantages and limits of each technique and how to implement them and improve their sensitivities according to the type of components. We will demonstrate various examples of thin-film components fabricated by Plasma Assisted Reactive Magnetron Sputtering (Bühler/Leybold optics HELIOS) optically monitored using Bühler broadband and monochromatic optical monitoring systems. We will conclude on the actual capabilities in terms of spectral performances of complex multilayer filters.
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