A method for thickness monitoring and turning-point prediction during deposition of narrow band pass optical filters (NBPF) for dense-wavelength-division-multiplexing (DWDM) applications is proposed. The method is based on a recurrent approach, with relative transmittance fitting, and includes partial coherence and monochromator bandpass effects. We show that the partial coherence effects in thin film structures are significant and can not be neglected. The proposed method is applicable for precise thickness monitoring and deposition control of any complex multilayer coating.
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