 The optical filtering devices used have conventionally been etalons (stable solid-state single-frequency Fabry–Pérot interferometers in the form of thin-film-coated optical glass).As there are three different WDM types, whereof one is called "WDM", the notation "xWDM" is normally used when discussing the technology as such.The concept was first published in 1978, and by 1980 WDM systems were being realized in the laboratory.