Generally speaking, Silica based glass optical fibers can transmit from 250nm to 2000nm wavelengths. But long distance optical transmission is limited to specific wavelength ranges due to the absorptive and scattering losses. These wavelength ranges are known as Optical Transmission Windows or Optical Fiber Bandpasses.
You might have heard about the O-bands, E-bands, L-bands, etc. These optical bands are nothing but the transmission wavelength ranges of optical fibers. The below list of optical bandpasses or transmission windows with typical attenuation is shown below;
|S #||Transmission Band||Meaning||Wavelength Range||Typical Attenuation|
|1||O – band||Original band||1260nm – 1360nm||0.33dB/km|
|2||E – band||Extended band||1360nm – 1460nm||0.19dB/km|
|3||S – band||Short wavelength||1460nm – 1530nm||0.22dB/km|
|4||C – band||Conventional||1530nm – 1565nm||0.20dB/km|
|5||L – band||Long||1565nm – 1625nm||0.23dB/km|
|6||U – band||Ultra-long||1625nm – 1675nm||0.28dB/km|
Telecommunication grade fibers are made of silica glass. When light travels through silica glass fiber it gets attenuated due to material absorption, scattering, waveguide attenuation, and leaky modes. It is important for the glass to become pure for low loss transmission.
Shorter wavelengths such as 250nm get more attenuated due to the absorption and Rayleigh scattering. Higher wavelengths like those above 2000nm also get attenuated due to infrared resonant absorption bands within the material. Material absorption is caused by mechanical resonance in the crystalline structure of the silica glass and by absorption peaks resulting from metal and OH ions in the glass.
The attenuation of a common glass which we use for windows is in the order of 1000dB/km. It was in the 1960s scientists started to remove the impurities from the glass to make it useful for light transmission. Scientists were successful to develop low water peak fiber by eliminating OH ions from the silica material and also by treating it in a Deuterium environment to secure from the possible intrusion of OH ions in the future.
The OH absorption loss happens at the 1380 nm region. The increase in attenuation due to absorption beyond 1550nm is due to atomic resonances. Ultraviolet and Infrared fibers like ZBLAN are also available but not generally used for optical transmission in a telecommunication line.
The U band or Ultra long band is used for system monitoring and maintenance. 1550nm region is used for DWDM transmission. As fiber optic technology advances, new windows of transmission will emerge. There was a recent report that 1000nm wavelength will be the future transmission window. Wait and see how new wavelength ranges are being added to the optical transmission!