Figure below shows three optical windows which offer minimum signal attenuation and also relationship between attenuation and wavelength.
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In case of optical transmission the loss is wavelength dependent. So, there is a specific band of wavelength where the signal attenuation is minimum which is known as optical or operating window. The wavelength of operation from the optical window is selected as they offer minimum attenuation.
The first optical window is defined from 800-900nm, where the minimum signal loss is 4dB/km. In early 's this window was used for operation of optical sources and detectors.
By reducing the concentration of hydroxyl ions and metallic impurities in the fiber material, in 's manufacturers were able to fabricate optical fibers with very low loss in the -nm region. This spectral band is called long wavelength region.
The second optical window is centered at nm also called O-band, which offers 0.5dB/km.
The third optical window is centered at nm also called C-band, which gives the loss of 0.2dB/km.
Hence while designing optical systems for long distance applications the nm wavelength is preferred because loss offered at this wavelength is minimum than any other wavelength.
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Further research with optical fibers found that the fiber's absorption and scattering effects which cause fiber's attenuation were lower as wavelength increased. Another spectrum located around nm would have attenuation losses reduced to 1.5 dB/km using multimode fibers which resulted in immediate cost savings due to the elimination of costly regenerators/repeaters. The development of new high performance photo detectors and edge emitting LEDs along with the development of new solid-state laser diodes in the late s and early s provided the essential optical components required. It was at this time that the term "second window" was first used implying that 850 nm was the first window.
The second "window" of nm was used to define a spectral region past and was defined as nm +/- 50 nanometers ( nm ' nm). With the high cost of amplifiers in the late 's which would be required for single-mode oceanic spans starting with TAT-8. By using laser transmitters with a center wavelength of .1 nm the expensive costs and numbers of amplifiers could be reduced. Rounding this number up to nm was a result that even today we use to call out single-mode fiber systems at nm vs nm. The term nm would be used by those using multimode fibers. Yet, both / nm are both in the spectral range of the second window.
The third window announced by NTT in would operate with a center wavelength of nm and provide lower attenuation (> .5 dB/km). Combined with the development of the Distributed Feedback (DFB) Laser, and erbium doped fiber amplifier this allowed for lower optical dispersion and the development of high speed and Dense Wavelength Division Multiplexing (DWDM) systems.
The fourth window of nm had higher optical attenuation but expanded the usable optical spectrum available for FTTx and WDM systems. Today, this window is also specified for maintenance of live and dark fiber systems per the International Telecommunications Union (ITU).
In our next article, I'll address how the ITU defined the term "Bands" to identify specific wavelengths and how they are used in current and future fiber optic transmission systems.
Key point: Rounding up .1 nm up to nm defines single-mode transmission to this day.
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