The optical transmission medium is the best in a sense that it has ultra wide bandwidth and very low attenuation.
The attenuation history is given in the following Figure.
Initially in early 1970s due to technology limitation, the optical fiber had a low loss window around 800nm. Also the semiconductor optical sources were made of GaAs which emitted light at 800nm. Due to compatibility of the medium properties and the sources, the optical communication started in 800nm band so called the ‘First window' .
As the glass purification technology improved, the true silica loss profile emerged in 1980s. The loss profile shows two low loss windows, one around 1300nm and other around 1550nm. In 1980s the optical communication shifted to 1300nm band , so called the ‘ Second Window' . This window is attractive as it can support the highest data rate due to lowest dispersion.
In 1990s the communication was shifted to 1550nm window, so called ‘Third Window' due to invention of the Erbium Doped Fiber Amplifier (EDFA). The EDFA can amplify light only in a narrow band around 1550nm. Also this window has intrinsically lowest loss of about 0.2 dB/Km . This band has higher dispersion, meaning lower bandwidth. However, this problem has been solved by use of so called ‘dispersion shifted fibers'.
Both 1300nm and 1550nm band have approximately 100nm bandwidth each.
The frequency bandwidth is related to the wavelength bandwidth as
Where is the velocity of light in vacuum, is the refractive index of the medium, is the central wavelength of the band, and is the wavelength bandwidth (also called spectral width ).
For 1550nm window, , and . For silica optical fibers . We therefore get
So we have Approximately.
So , as a rule of thumb we can take for optical communication,
is the velocity of light in vacuum, 
is the refractive index of the medium, 
is the central wavelength of the band, and 
is the wavelength bandwidth (also called spectral width ). 
, and 
. For silica optical fibers 
. We therefore get 
