Development of Low Attenuation Optical Fibers

A major breakthrough in optical communication happened when Charles K Kao and George A. Hockam of Standard Telephones and Cables, a British company proposed and promoted the idea that the attenuation in silica optical glass fibers could be reduced below the 20 dB/km. This proposal gave encouragement to the researchers to achieve low attenuation in optical fiber that would make the optical fibers suitable for telecommunication.┬áThe team proposed and theorized the right methods to reduce attenuation in optical fibers. Apart from scattering loss, impurities in silica optical fibers penalize heavily on attenuation. Kao’s researches towards purifying the glass found success.

Corning Incorporated has the distinction to witness the development of first silica optical fiber with an attenuation of 17 dB/km when their researchers Robert d. Maurer, Donald Keck, Peter C. Schultz and Frank Zimar demonstrated their invention in 1970. The team was successful to make first optical fiber that showed lower than 20 dB/km attenuation. They used Titanium, a rare earth element for doping the silica glass to achieve 17 dB/km. A few years later they excelled their own invention by producing an optical fiber with 4 dB/km attenuation by doping Germanium dioxide.

The invention took place in Corning Glass Works, which is now known as Corning Incorporated, which is a U.S based global pioneer in fiber optic industry. Doping was used in the core glass to increase the refractive index. The difference in refractive index between core and cladding allowed the glass optical fiber to act as waveguide.

In 1973, Gerhard Bernsee of Schott Glass in Germany invented the first modern silica optical fiber that used glass material for both core and cladding. In 1981, General Electric produced fused quartz ingots, earlier forms of current optical preforms. Optical fibers could be drawn into thin rods from these ingots. General electric could draw optical strands up to 40 kilometers. Now a days, drawing of optical fibers extend to a few thousand kilometers. Attenuation in optical fibers is significantly lower than that in copper electrical wires.

Research directions in the optical industry had changed from conventional optical fibers that use total internal reflection towards photonic crystal fibers that guides lights by diffraction from a periodic structure. The developments started in 1991. Photonic crystal fibers can carry higher power than conventional fibers and their wavelength dependent properties can be manipulated to improve performance. Multi-core fiber structures were also proposed and researches in this field shows increase of data carrying capacity of multi core optical fibers in multiples compared to a single core optical fiber.

Developments in conventional silica based optical fibers for telecommunications indicates towards lower attenuation and reduced diameter together with bend in-sensitiveness. As the fiber deployment focus has been shifting from long-haul to subscriber premise, reduced diameter and tighter bend characteristics become inevitable. Long haul application demand low attenuation loss to reduce number of amplifiers and regenerators. Fiber manufacturers have been successful in introducing conventional optical fibers having attenuation lower than 0.32 dB/km at 1310 nm and 0.18 dB/km at 1550 nm.

200 micrometer diameter fibers are available now in the market. The reduction in diameter from 250 micrometers to 200 micrometers will lead to design of reduced diameter cables that will help utilization of existing congested duct facilities in metro cities.

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Author: Fiber

Chief Editor of Fiber Optic Mania Magazine

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