The developments in the optical fiber industry have revolutionized modern communication. It is interesting to look back at those historical events. The first scientifically recorded experiment to guide light through a medium other than space or air, probably, was done by John Tyndall. He guided the light in a jet of water flowing from a tank. John Tyndall’s experiment shows that the light bends and followed the path of the water jet.
Moving from there, Heinrich Lamm in 1930 showed that transmission of image is possible through a fiber bundle. Heinrich Lamm was a medical student when he developed the first flexible fiber optic bundle capable of transmitting images around curves. He was the first to make a fiber optic endoscope. Fiber optic endoscopes are widely used in the medical field. Lamm’s purpose was to check inaccessible parts inside the human body. Though his endoscope did not give clear pictures of internal body organs, the experiments that followed his attempt created endoscopes that can transmit clear images or real-time streaming videos of the internal human body.
After 30 years, Lawrence Curtiss succeeded in making the first clad optical fiber. The attenuation measured on this clad fiber was 1,000 dB/kilometer, which is almost 5000 times higher than today’s commercially available conventional optical fibers. Charles Kao was the first to propose the ability of silica fibers to achieve attenuation less than 20 dB/km. Subsequent research at Corning Glass Works resulted in an optical fiber that has 20 dB/km attenuation. The wavelength used to measure this fiber was 633 nm. Scientists at Corning could reduce the attenuation to 17 dB/km and then to 4 dB/km in 1972 by doping Germanium dioxide to the Silica. In 1975, Corning made the fiber optical fiber cable, which was deployed by GTE in Long Beach, California.
The first transoceanic optical fiber cable was installed in 1988 – 1989. This transatlantic optical fiber cable (TAT-8) was made with 3 pairs of fibers that operated at a capacity fo 280 Megabits per second at 1300 nm. The repeater spacing was 50 kilometers, which was five times longer repeater-spacing compared to that of copper counterparts. EDFAs were used to amplify the signal.
Multimode fibers were used in the earlier telecom networks. By the end of the 1970s, fiber manufacturers were able to produce single mode fibers having much lower attenuation than the Multimode fibers. Multimode fiber attenuation was in the range of 3.5 to 4 dB/km at 850nm and 1 to 1.5 dB/km at 1300nm. With the introduction of Single mode fibers, the transmission wavelength is shifted from 850nm and 1300nm to 1310 nm and 1550nm. Earlier Single mode fibers, complying with ITU-T G.652A fibers were used at 1310nm only due to the compatibility of transmitters and receivers.
For an ITU-T G.652A single mode fiber, the attenuation was in the range of 0.4 dB/km at 1310nm. Optical fiber manufacturers kept on investing on the research and development of enhanced characteristics for the single mode fiber and improved parameters of transmission. Attenuation was improved on single mode fibers and then ITU-T G.652B fibers were introduced. The practical applications of optical fibers started expanding and in order to meet the market demand, single mode fibers that can provide lower levels of attenuation over an entire wavelength range of 1260nm to 1625nm were introduced as ITU-T G.652C fibers. This is also known as Low water peak fiber. When the signal transmission rate increased over the fiber, installers noticed a new problem called Polarization mode dispersion that limits the signal transmission capacity. To overcome this issue, Low water peak fibers with low Polarization mode dispersion characteristics were developed.
Applications of multimode fibers are limited to short distances due to their inherent modal dispersion characteristics. Single mode fibers propagate hundreds of kilometers before they need amplification and regeneration. Hence, multimode fibers are mainly used for Local area networks, office wiring, and campus connectivity. Single mode fibers run thousands of kilometers across the continents. Single mode fibers used for transoceanic network demands longer span for amplifiers and repeaters.
ITU-T G.652 fibers have around 0.33 dB/km attenuation at 1310nm and 0.2 dB/km attenuation at 1550nm. But the Chromatic dispersion is lower at 1310nm and higher at 1550nm. Chromatic dispersion is becoming zero in the wavelength range of 1285nm to 1330nm for this type of fiber, but it increases to around 18 ps/nm.km at 1550nm. Lower attenuation at 1550nm is an attractive value, but dispersion is a negative factor at 1550nm.
Manufacturers thought of overcoming the high dispersion values at 1550nm by changing the fiber design. They created a new fiber called Dispersion shifted single mode fiber, which is categorized as ITU-T G.653 fiber. G.653 fiber was active in the market for some years towards the end of the 1990s. When the data transmission capacity over the fiber increased, installers experienced a new issue called non-linear effects and the reason was the zero-dispersion value at 1550nm.
In order to overcome the non-linear effect, optical fiber manufacturers slightly modified the fiber design and gave a minimal value for the dispersion at 1550nm. This new fiber is known as Non-Zero dispersion shifted single mode fiber categorized by ITU-T as G.655 fiber.
When G,652 fibers reached at the customer premise locations, the harsh and congested installation areas demanded a much bend optimized single mode fiber. Such demands led to the development of Bend optimized single mode fibers, categorized by ITU –T under G.657 fibers.
Research and development of optical fibers is a continuous process. The market demands are now pushing fiber manufacturers to the mass production of Multicore fibers.
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