High accuracy measurement of the residual air gap thickness of thin-film and solid-spaced filters assembled by optical contacting

Optical contacting is a powerful tool for assembling solid-spaced filters in order to form a wavelength division multiplexing (WDM) multiple-cavity filter. In this article, we propose a method able to characterize the optical quality of such assembling with a high accuracy. We use localized spectral transmittance measurements to map the thickness of the residual air gap existing at the adhesion interface with a few nanometers precision. Tests on thick (2 mm) and thin (100 μm) substrates coated by Dual Ion Beam Sputtering are performed. Thus, we show that our 25 mm-diameter samples are strictly contacted over more than 80% of their surface, with no detectable residual air gap.