Turning Value Monitoring of Narrow-band All-dielectric Thin-film Optical Filters

The effects, on the performance of all-dielectric interference filters, of errors in the turning value method of film-thickness monitoring, are studied theoretically by both an accurate and an approximate method. It is shown that, although the potential accuracy of the method in monitoring a single layer is very poor, the theoretical accuracy of peak frequency and the overall performance of complete narrow-band filters are extremely good. Poor performance of real production filters is therefore unlikely to be due to an inherent limitation of the method, but rather to a faulty application of it, or to other factors unconnected with the monitoring method.