The laser light used in fiber-optic communications operates within a narrow band on the electromagnetic spectrum. Radiation (such as TV signals and light) on the electromagnetic spectrum can be measured by both frequency (the number of wave cycles per second, or Hertz) and wavelength (in meters). Frequency and wavelength are inversely proportional (that is, the higher the frequency, the shorter the wavelength), and either can be used to describe communications signals. For example, radio broadcasts are denoted in frequency—a 100 megahertz (MHz) frequency on the FM dial corresponds
to approximately a three meter wavelength. In contrast, signals on fiber-optic cables operate at much higher frequencies, and have tiny wavelengths—only 850 to 1,625 nanometers
(billionths of a meter).
In scientific literature, a wavelength often is denoted as lambda (l). Individual wavelengths also are referred to as
colors—an analogy to frequencies within the visible light spectrum.
One of the more important objectives of fiber designers has been to design fiber that has a wider "window" or range of usable frequencies for light signals. The wider the usable band, the more distinct signals can be transmitted. This is determined in part by the composition of the fiber itself. Hence, some recent designs have extended the low attenuation window at 1550 nm (now called the C-band) to 1600 nm (called the L-band), allowing more signals to be transmitted. At the other end, scientists have eliminated water molecules that greatly increase attenuation at 1400 nm, releasing this band (the S-band) for possible future use.
