Infrared radiation (IR) is electromagnetic radiation (EMR) with longer wavelengths than those of visible light, and is therefore generally invisible to the human eye (although IR at wavelengths up to 1050 nm from specially pulsed lasers can be seen by humans under certain conditions). It is sometimes called infrared light.
An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal.
Optical communication, also known as optical telecommunication, is communication at a distance using light to carry information.
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.