History of Optical Fiber Attenuation

We know that fiber optic technology has started a revolution in the human communication process and optical fiber is the key component in fiber optics. Light, of course, no doubt is the carrier of our communication signals. We have been using light to transmit information for hundreds of years from now. However, the new way of transmitting light through optical fibers opened up new avenues of technological innovations that have started enhancing human life.

Similarly, the invention of LASER (Light Amplification by Stimulated Emission of Radiation) in the 1960s was one of the major leaps that made the transmission of light through optical fibers possible. The laser can send a larger amount of data than telephone, microwave, and copper telecommunication systems.

Scientists have tried many different ways to transmit Laser light. They tried through different types of glass fibers and watched which one gave out lower transmission losses. The experiments span from space to glass tubes to solid glass wires, and so on. Solid glass wires yielded promising results and thus became a preferred medium for researchers.

Copper wires were used for telecommunication and the initial results on solid glass wires were not enough to replace the copper wires. Transmission loss in solid glass wires, called Attenuation was high. Researchers have recorded 1,000 dB/km for the optical fibers in their studies in the 1960s. Such loss levels were not acceptable for transmission, and researchers continued their pursuit to purify the glass in order to achieve lower loss.

In 1969, scientists concluded that the attenuation in glass wires can be brought down by removing the impurities. They found that the impurities in glass wires cause a signal loss in optical fibers. They devised techniques to purify the glass by removing the impurities. It was not silica that was causing scattering and absorption losses in a glass fiber, but the metals, metal oxides, and hydroxyl ions cause high attenuation.

Dr. Kao in his laboratory experimenting on optical fibers

Scientists in Corning’s laboratory in the United States made a multimode optical fiber in 1970, that had a loss of 20 dB/km. That was an amazing achievement in the history of optical fibers and attenuation. Charles Kao, the father of fiber optics proposed ways to achieve attenuation as low as 4dB/km.

Corning made a multimode fiber in 1972 that had an attenuation of 4 dB/km at 850nm. This invention was a milestone achievement and encouraged researchers all over the world to work for lower loss levels for silica glass fibers. Multimode fibers were the first optical fibers to be used for telecommunication networks. At 1300 nm, Multimode fibers exhibit an attenuation of around 1.5 dB/km. Those were for the initial versions of multimode fibers having a core diameter of 62.5 micrometers.

Now, we know that a multimode fiber having a 50-micrometer core diameter has an attenuation of less than 2.5 dB/km at 850 nm and 0.5 dB/km at 1300nm.

Attenuation is a wavelength-dependent characteristic. Attenuation at lower wavelengths is higher and that at higher wavelengths is lower. Note that this statement is true for a wavelength range from 800 nm to 1550 nm. From 1550nm onwards, the attenuation will increase. Towards the end of the 1980s, researchers could succeed in developing single mode fibers that could support the transmission of higher wavelengths in order to achieve lower attenuation values.

Broadly there are two basic types of optical fibers, multimode fibers, and single mode fibers. Both types are used for telecom networks. Single mode optical fibers are used for long haul networks and multimode optical fibers are used for short haul networks.

Attenuation in early single mode fibers, those classified as ITU-T G.652A fibers, at 1310 nm was 0.5 dB/km and 0.40 dB/km at 1550 nm. The second version of single mode fiber, that is ITU-T G.652B showed improvements in attenuation at both wavelengths. ITU-T G.652B fibers exhibited 0.4 dB/km at 1310 nm and 0.35 dB/km at 1550 nm. Practically, these attenuation values are lesser than those specified by the ITU-T.

G.652B fibers offered by optical fiber manufacturers have attenuation values less than 0.35 dB/km at 1310nm and 0.20 dB/km at 1550nm. This is the case with ITU-T G.652C and ITU-T G.652D fibers, though the ITU-T Recommendations specify 0.4 dB/km at both wavelengths for both types.

The pursuit for lower losses for optical fibers has led to the invention of Low loss optical fibers. Some manufacturers have achieved a level of 0.17 dB/km at 1550 nm and 0.32 dB/km at 1310 nm for G.652 category fibers.

Some fiber manufacturers were able to achieve attenuation less than 0.16 dB/km for cut-off shifted fibers, which are used for transoceanic ultra long haul networks. The lowest attenuation reported so far is 0.1419 dB/km. This was reported by Sumitomo, as per the new link here: Sumitomo Succeeds Mass Production of Ultra Low Loss Optical Fiber with 0.14 dB/km

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