Solution:

In the early days of optical fiber communication, fiber attenuation was best represented by the upper curve in Figure.

Partly for historic reasons, there are considered to be three “windows” or bands in the transmission spectrum of optical fiber.

The wavelength band used by a system is an extremely important defining characteristic of that optical system.

Short Wavelength Band (First Window):

This is the band around 800-900 nm. This was the first band used for optical fiber communication in the 1970s and early 1980s. It was attractive because of a local dip in the attenuation profile (of fiber at the time) but also (mainly) because you can use low-cost optical sources and detectors in this band.

Medium Wavelength Band (Second Window):

This is the band around 1310 nm which came into use in the mid-1980s. This band is attractive today because there is zero fiber dispersion here (on single-mode fiber). While sources and detectors for this band are more costly than for the short wave band the fiber attenuation is only about 0.4 dB/km.

Long Wavelength Band (Third Window):

The band between about 1510 nm and 1600 nm has the lowest attenuation available on current optical fiber (about 0.26 dB/km). In addition, optical amplifiers are available which operate in this band. However, it is difficult (expensive) to make optical sources and detectors that operate here. Also, standard fiber disperses the signal in this band.