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The attenuation of SEI’s Z-PLUS Fiber unveiled at OFC2002 was 0.151dB/km. You may think there is marginal difference between 0.19dB/km and 0.151dB/km, but the improvement adds 30% to transmission distance in the C band (1.55µm band). In other words, with Z-PLUS Fiber a repeater would be required every 130 km instead of every 100 km, producing a significant cost reduction effect. SEI has continued its research on Z-PLUS Fiber and improved attenuation further to 0.148dB/km. The company is currently far ahead of the competition in the development of low-attenuation optical fiber products.
What made the development of low-attenuation optical fiber possible? To answer this question, we first need to take a look at the structure of optical fiber.
Optical fiber generally has a two-layer structure, where a central core is surrounded with a cladding layer. Both layers are made from silica, but the core material is germanium-doped. The addition of germanium gives the core a higher refractive index than the cladding, and the optical signal passing through the core is reflected by the cladding and transmitted without leakage. The difference in the refractive index between the two layers is critical.
The addition of germanium reduces the transparency of the silica, which in turn increases the attenuation. Thus, attempts to minimize the attenuation of optical fiber produce conflicting demands.
As a solution to this problem, SEI’s research team came up with the idea of using a pure silica core and adding a different substance lowering the refractive index to the cladding layer, thereby maintaining the transparency of the core as well as a difference in refractive index between the two layers.
The question was which substance to add to the cladding layer. It was not simply a matter of lowering the cladding layer’s refractive index, since the high quality and processability of the optical fiber could not be compromised. The research team’s solution was fluorine-doped silica. |
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| A preform from which optical fiber is made. The string-like form extending from the heated tip of the preform is an optical fiber. |
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