One of the most common terms used in fiber optic communication systems is transmission windows, yet where did the term come from, why are "windows" important and will they continue to provide the roadmap for how we use fiber optics in the future?
To understand the term "window" we need to review the early years of fiber optic technology. The main reason for the use of the term "window" applied to how a fiber span would initially operate at a specific optical frequency (wavelength). The first goal was the integration of a light source, photo detector and optical fiber to create a fiber link with the lowest signal attenuation which was accomplished in 1975. This marriage of the 3 basic elements continues to this day as new technologies including optical multiplexing and fiber-to-the-home continue to evolve.
In the late 1970s the only light source and photo detectors available operated at 850 nm. As single-mode fibers were not available till 1983 these early systems all used multimode fiber using light emitting diodes (LEDs) which had a wide emission spectral width of 100 nanometers (nm) or greater. 850 nm was the center wavelength of the 800-900 nm spectrum that would allow fiber optic systems to operate with the fiber’s attenuation of 4 dB/km attenuation value.
The 3 common windows and the optical spectrum.
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 1300 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 1970s and early 1980s 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 1300 nm was used to define a spectral region past and was defined as 1300 nm +/- 50 nanometers (1250 nm – 1350 nm). With the high cost of amplifiers in the late 1980’s which would be required for single-mode oceanic spans starting with TAT-8. By using laser transmitters with a center wavelength of 1308.1 nm the expensive costs and numbers of amplifiers could be reduced. Rounding this number up to 1310 nm was a result that even today we use to call out single-mode fiber systems at 1310 nm vs 1300 nm. The term 1300 nm would be used by those using multimode fibers. Yet, both 1300/1310 nm are both in the spectral range of the second window.
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The third window announced by NTT in 1977 would operate with a center wavelength of 1550 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 1625 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 1308.1 nm up to 1310 nm defines single-mode transmission to this day.
