Optical window

For the optical element, see Window (optics)

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Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light.

The optical window is the portion of the optical spectrum that is not blocked by the Earth's atmosphere. The window runs from around 300 nanometers (ultraviolet-B) up into the range the human eye can detect, roughly 400&#;700 nm and continues up to approximately 2 μm.[1][2] Sunlight mostly reaches the ground through the optical atmospheric window;[3][4] the Sun is particularly active in most of this range (44% of the radiation emitted by the Sun falls within the visible spectrum and 49% falls within the infrared spectrum).[5]

Definition

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The Earth's atmosphere is not totally transparent and is in fact 100% opaque to many wavelengths (see plot of Earth's opacity); the wavelength ranges to which it is transparent are called atmospheric windows.[6]

Disambiguation of the term 'optical spectrum'

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Although the word optical, deriving from Ancient Greek &#;πτ&#;κός (optikós, "of or for sight"), generally refers to something visible or visual,[7] the term optical spectrum is used to describe the sum of the visible, the ultraviolet and the infrared spectra (at least in this context).[8][9]

Optical atmospheric window

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Solar irradiance spectrum above atmosphere and at surface. Extreme UV and X-rays are produced (at left of wavelength range shown) but comprise very small amounts of the Sun's total output power.

The optical atmospheric window is the optical portion of the electromagnetic spectrum that passes through the Earth's atmosphere, excluding its infrared part;[10] although, as mentioned before, the optical spectrum also includes the IR spectrum and thus the optical window could include the infrared window (8 &#; 14 μm), the latter is considered separate by convention, since the visible spectrum is not contained in it.[11]

Historical importance for observational astronomy

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Up until the s, astronomers could only use the visible and near infrared portions of the optical spectrum for their observations. The first great astronomical discoveries such as the ones made by the famous Italian polymath Galileo Galilei were made using optical telescopes that received light reaching the ground through the optical window.[12] After the s, the development of radio telescopes gave rise to the even more successful field of radio astronomy that utilized the radio window.[13]

See also

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References

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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.

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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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