A WDM system uses a at the to join the several signals together and a at the to split them apart. With the right type of fiber, it is possible to have a device that does both simultaneously and can function as an. The optical filtering devices used have conventionally been (stable solid-state single-frequency in the form of. Glass optical fibers are almost always made from, but some other materials, such as,, and as well as crystalline materials like, are used for longer-wavelength infrared or other specialized applications. Silica and fluoride glasses usually have refractive indices of about 1.5, but some materials such as the can have indices as high as 3. Typically th.