optically coherent , two - port , serially cascaded - form optical delay line circuits can realize arbitrary signal processing functions identical to those of fir digital filters with complex filter coefficients whilst maintaining a maximum optical transmission characteristic of 100 %. the invention provides an iterative process for transitioning in a step - wise manner a filter function of an optical delay line circuit filter from a start filter function to a target filter function . the invention also describes a dynamic gain equalizer incorporating an optical delay line circuit filter .