FIBER OPTICAL WIRE COMMUNICATION

The discovery of the laser in 1960 has provided light sources suitable for the transmission and processing of information. Optical transmission has several advantages over radio transmission: speed (of light), high speed and a wider carrier frequency. It had one drawback: its price, but it becomes less and less true.


Operation of fiber: the preform (Bar glass 2 indices) is heated in a furnace, and stretched continuously by a cylinder which wraps around the fiber. Along the way, various systems adjust the speed of stretch and the oven temperature as a function of fiber diameter, measured by laser.



From 1960 to 1975, researchers, especially American, have worked to find a low attenuation. The single crystals, first approached, were abandoned in favor of glass, transparent material par excellence. The manufacturing techniques of fiber glass, and especially the research of high purity, were not without mentioning the manufacturing process of semiconductors. Therefore CNET (1) Lannion B, with his experience in semiconductors, decided to embark on the epic fiber.


20 Years of Progress

By applying the method to fibers called MCVD (Modified Chemical Vapor Deposition), obtained by CNET in 1975 silica fiber whose heart is enriched in germanium. The variation index between the glass heart and the sheath allows a reduced signal in the order of 5 decibels (2) per km for a laser wavelength of 0.85 m. Gradually improve the performance: in 80 years, the multimode beam, due to interference limitations, is replaced by a single. Currently, with a fiber laser and a 1.3 m or 1.5 m, the loss of signal strength is only 0.3 to 2 db / km, the wavelength chosen. This choice is based on the target application: for short distances at high speed information, use 1.3 m, where chromatic dispersion is the lowest, at the expense of a slightly more attenuation important. The other option is usually considered for international routes and especially the submarine links.


-> Booting the fiber.


A problem of connectivity

After some distance, the signal is very attenuated force: it has to be regenerated. So far, a device called a repeater transforming the optical signal into an electrical signal and amplify it back before the optical form. For the transatlantic TAT9, immerse it took a repeater every 50 km. A new system is currently under development: the optical amplifier. A laser is coupled to the fiber line, the signal passes through a fiber amplifier before reaching the fiber line. This system, returning a signal more faithfully than does the repeater, will link more than 500 km without connections.


The challenge of CNET

Optical fibers are material costs: about 1.20 francs per meter compared to 30 cents for copper. This hampers the development of optical transmission, despite its high speed performance and speed. To stand up to major Japanese and American, CNET strives to reduce production costs. Target: the fiber optic 40 cents per meter. For that, he worked closely with the company of Cables de Lyon. Stay tuned!



NOTES:

(1) CNET: Center National d'Etudes des Telecommunications.
(2) db = decibel is the optical equivalent decibel acoustics. It represents the signal attenuation, proportional to the ratio of the logarithmic power on the power recovered.