Another limitation consists in the limited speed of the tuning mechanism, based on the piezoelectric effect, which has response times that are longer than several milliseconds. Acoustic-optic filters on lithium-niobate and optical filters based on Mach-Zender interferometers made by silicon oxide on silicon technology have both higher manufacturing costs and far greater power consumption than optical filters which are the object of the present invention. Other characteristics and advantages of the invention will appear from the following description of a technique of cre ating the invention given as a non-limiting example in the figures and by the description of the chosen illustrations that follow.