A fiberoptic is a Fil in plastic Verre or very fine which with the property to lead the Lumière and is used in the terrestrial and oceanic transmissions as data. It offers a flow of Information S definitely higher than that of the coaxial cables and supports a network “broad band” by which can forward the Télévision as well, the telephone, the Visioconférence or computer data.

The principle of fiberoptic was developed in the Années 1970 in the laboratories of the American company Corning Glass Works.

Surrounded by a protective sheath, the fiberoptic can be used to lead Lumière between two distant places several hundreds, even thousands, de  kilometers. The light signal coded by a variation of intensity is able to transmit a great quantity of information. By allowing the communications very long distance and flows hitherto impossible, the fiberoptics constituted one of the elements key of the revolution of the Télécommunication S optics. Its properties are also exploited in the field of the Capteur S (Température, Pression, etc) and in the Imagerie.

A new type of fiberoptics, fibers with photonic crystals, also was developed these last years, allowing significant profits of performances in the field of the optical treatment of information by nonlinear techniques, in the optical Amplification or even in the generation of Supercontinuum S usable for example in the medical diagnosis.

In the data-processing networks of the type Ethernet, to connect it to other equipment, one can use a transmitter-receiver.

History

Precursors

At the time of the old Greek , the phenomenon of the transport of the light in cylinders of glass was already known. It was, seems it, made profitable by the craftsmen of glass to create parts Later decorative S., the techniques of manufacture used by the craftsmen Venetian S of the Renaissance to manufacture the “ millefiori ” would resemble much the current techniques of manufacture of fiberoptic. The use of glass in conjunction with the light is thus not recent.

The scientific first Démonstration of the principle of the total Réflexion intern was made by the Physicien Irish John Tyndall in front of the British Royal Société in 1854. With l´époque, the idea to curve the trajectory of the light, in some way that it is, was revolutionist since the scientists considered that the light travelled only in Straight line. Its demonstration consisted in guiding the light in a Jet of water poured of one hole at the base of a tank. While injecting light in this jet, this one followed the curve of the water jet well, showing thus that it could be deviated of its rectilinear trajectory. It could in this manner of showing the principle which is at the base of fiberoptic. Thereafter, of many invention S using the principle of the total reflection interns transfer the day; like the luminous fountains or devices allowing to transport the light in cavities of the human body.

One owes the first optical attempt at communication with Alexander Graham Bell, known for the invention of the Téléphone. Indeed, it developed, during the Années 1880, the photophone. This apparatus made it possible to transmit the light on a distance of 200 meters. The Voice, amplified by a Microphonous , made vibrate a Miroir which reflected the sunlight. Some 200 meters further, a second mirror collected this light to activate a Cristal Sélénium and to reproduce the desired sound. The receiver of this apparatus was almost identical to that of the first telephone. Although operational in discovered ground, this method proved little used. The Pluie, the Neige and the obstacles which prevented the transmission of the signal condemned this invention, although he considered itself that the photophone was its greater invention, since it allowed a communication without wire.

The advent

The possibility of transporting light along fines glass fibers was exploited during first half of the 20th century. In 1927, Baird and Hansell tried to develop a device images of Télévision using fibers. Hansell could make Brevet er its invention, but it never was really used. A few years later, in 1930, Heinrich Lamm succeed in transmitting the image of a filament of Flashlight thanks to a rudimentary fiber assembly of quartz. However, it was still difficult at that time to conceive that these glass fibers can find an application.

The first profitable application of fiberoptic took place at the beginning of the Années 1950, when the flexible Fibroscope was invented by Van Heel and Hopkins. This apparatus allowed the transmission of an image along glass fibers. It was particularly used in Endoscopie, to observe the interior of the Human body, and to inspect welding S in engines of Avion. Unfortunately, the transmission could not be made at a long distance being given the poor quality of fibers used. In 1957, the Fibroscope (medical Endoscope flexible) is invented by Basil Hirschowitz with the the United States.

The Télécommunication S by fiberoptic remained impossible until the invention of the Laser in 1960. The laser indeed offered the occasion to transmit a signal with enough power at a long distance. In its publication of 1964, Charles Kao, Standard Laboratories Telecommunications, described a communication system with long distance and weak loss by making profitable the joint use of the laser and fiberoptic. A little later is in 1966, it showed in experiments, with the collaboration of Georges Hockman, which it was possible to transport of information at a long distance in the form of Lumière thanks to fiberoptic. This experiment is often regarded as the first data transmission by fiberoptic.

However, the losses in a fiberoptic were such as the signal disappeared at the end of a few centimetres, not by loss of light, but because the various ways of reflection of the signal against the walls ended up making some lose the phase. That made it not very advantageous compared to the wire of traditional Cuivre. The losses of phase pulled by the use of a homogeneous glass fiber constituted the main obstacle with the current use of fiberoptic.

In 1970, three scientists of the company Corning Works Knell of New York, Robert Maurer, Peter Schultz and Donald Keck, produced first fiberoptic with sufficiently weak losses of phase to be used in the telecommunication networks (20 decibels per kilometer; today the conventional fiber posts losses of less than 0,25 decibel per kilometer for the wavelength 1550 Nm. used in telecommunications). Their fiberoptic was able to transport: 65,000 times more information than a simple copper cable, which corresponded to the report/ratio of the wavelengths used.

The first optical telephone communication system was installed with the downtown area of Chicago in 1977. In France, DGT installed the first optical link in Paris between the telephone centres of Tileries and Philippe-Auguste. It is estimated that today more than 80% of the communications with long distance are transported along more than 25 million kilometers of cables with fiberoptics everywhere in the world.

The fiberoptic, in a first phase (1984 with 2000), was limited to the interconnection of the telephone centres, them-only requiring strong flows. However, with the fall of the costs pulled by his mass production and needs increasing for the private individuals in very high banc, one considers since 2005 his arrival even at the private individuals: FTTH ( Fiber To The Home ), FTTB ( Fiber To The Building ), FTTC ( Fiber To The Curb ), etc

Principle

The fiberoptic is a Guide of wave which exploits the refracting properties of the light. It usually consists of a heart surrounded by a sheath. The heart of fiber has a Index of refraction slightly higher (difference of some thousandth) that the sheath and can thus confine the light which is entirely considered multiple times at the interface between two materials (because of the phenomenon of total Réflexion interns). The unit is generally covered with a plastic sheath of protection.

When a luminous ray enters a fiberoptic to the one of its ends with an adequate angle, it undergoes multiple internal total reflections. This ray is propagated then until the other end of fiberoptic without loss, by borrowing a course in zigzag. The light propagation in fiber can be done with very few losses even when the fiber is curved.

A fiberoptic is often described according to two parameters:

• the difference of Index standardized, which gives a measurement of the jump of index between the heart and the sheath: $\backslash Delta=\backslash frac\{n\_c-n\_g\}\{n\_c\}$.
• the numerical Opening of the fiber ( numerical aperture ), which is concretely the sine of the maximum angle of entry of the light in fiber so that the light can be guided without loss. This angle is measured compared to the axis of fiber: $N.A.\; =\; \backslash \; sin\; \backslash \; theta\_\; \{max\}\; =\; \backslash \; sqrt\; \{n\_c^2-n\_g^2\}$.

There exist several types of fiberoptic. In step index fiber, the index of refraction changes brutally between the heart and the sheath. In fiber with Gradient of index, this change of index is much more progressive. In the fibers with photonic crystals, the variation of index between different the Matériau X (in general the Silica and air) is much more important. Under these conditions, the physical properties of guidance differ appreciably from and gradient step index fibers of index.

In the field of optical telecommunications, the privileged material is the very pure Silice because it presents very weak optical losses. When the Atténuation is not the principal selection criteria, one can also implement plastic fibers.

A fiberoptic cable contains several pairs of fibers in general, each fiber leading a signal in each direction. When a fiberoptic is not fed yet, one speaks about black Fiberoptic.

Manufacture of a silica fiberoptic

The first stage is the realization of a silica bar very pure, of a diameter of several centimetres. The composition in the middle of the bar is adapted in order to modify the index of refraction of glass. One uses in particular the Germanium to increase the index. There exist various processes to obtain this bar: deposit of layers in a quartz tube (CVD), external deposit around a Chuck (OVPO), axial deposit (VAD). Does everything call to reactions in vapor phase, which makes it possible to obtain a very pure material. The doping agents are injected in the form of chlorides (gas) in the tube, oxidized with the passage of the blowtorch, and soots settle downstream from the blowtorch. Another passage of the blowtorch, at higher temperature, vitrifies the deposit obtained. The tube is then softened by a stronger heating, while remaining in rotation, and slowly narrows. A last passage of the blowtorch, slower and carefully controlled to avoid the formation of bubbles, closes again the tube.

The bar undergoes then a drawing in a tower of fiber drawing, while placing the end in a furnace carried to a Température close to: 2,000  °C. It is then transformed into a fiber of several hundred kilometers, at a speed about the kilometer per minute. The fiber is then covered with a double-layer of protective resin (this layer can be deposited by the tower of fiber drawing, just after the stretching) before being rolled up on a reel. This layer is particularly important to avoid any moisture, because the fiber becomes breakable under the effect of water: hydrogen interacts with silica, and any weakness or microphone-notch is amplified.

Characteristics

The principal parameters which characterize fiberoptics used for the transmissions are the following:

Attenuation

The attenuation characterizes the weakening of the signal during the propagation.

Are $P\_0$ and $P\_L$ the powers at the entry and the exit of a fiber length L. the linear attenuation results then in a decrease Exponentielle of the power according to the length of fiber: $P\_L\; =\; P\_0\; e^\; \{-\; \backslash \; alpha\; L\}$ where $\backslash \; alpha$ is the linear attenuation coefficient. One often uses the coefficient $\backslash \; alpha\_\; \{dB\}$ expressed in dB/km and connected to $\backslash \; alpha$ by $\backslash \; alpha\_\; \{dB\}\; =\; 4.343\; \backslash \; alpha$:

The principal asset of fiberoptics is an extremely weak attenuation. The attenuation will vary according to the Wavelength. The Diffusion Rayleigh limit thus performances in the field short lengths the wave (field of visible and the Infra-red close relation ). A peak of absorption, due to the presence of radicals - OH in silica, could also be observed around: 1385  Nm. The most recent progress in the techniques of manufacture makes it possible to reduce this peak.

The silica fibers know a minimum of attenuation towards: 1,550  Nm. This wavelength of the infra-red close relation will thus be privileged for the optical communications. Nowadays, the control of the manufactoring processes makes it possible to usually reach an attenuation as weak as: 0.2  dB/km with: 1,550  Nm: after 100 km of propagation, there will thus remain still 1% of the power initially injected into fiber, which can be sufficient for a detection. If one wishes to transmit information on thousands of kilometers, it will be necessary to have recourse to a periodic reamplification of the signal, most generally via Amplificateurs optics S which combine simplicity and reliability.

It should be noted that the signal will undergo additional losses with each connection between fibers, that it is by cross-pieces or by welding, this last technique reducing very strongly these losses.

Chromatic dispersion

The chromatic Dispersion is expressed in PS (Nm·km) and characterizes the spreading out of the signal related to its spectral width (two different wavelengths are not propagated exactly at the same speed). This dispersion depends on the wavelength considered and results from the sum of two effects: dispersion specific to material, and the dispersion of the guide, related to the form of the profile of index. It is thus possible to minimize it by adapting the profile. For a silica fiber, the minimum of dispersion is towards 1300-1310 Nm.

Non-linearity

See also: Optical non-linear

A transmission channel is known as nonlinear when its Transfer function transfer depends on the entry signal. The Effect Kerr, the Raman Diffusion and the Brillouin effect are the independent sources of nonlinearity in fiberoptics.

Modal dispersion of polarization (PMD)

The modal dispersion of polarization (PMD) is expressed in ps/km ½ and characterizes the spreading out of the signal. This phenomenon is due to defects in the geometry of the fiberoptics which involve a difference of speed of group between the modes being propagated on various axes of polarization of fiber.

Wavelength of cut and standardized frequency

The Wavelength of cut is the wavelength $\backslash \; lambda\_c$ in lower part of which the fiber is not monomode any more. This parameter is connected to the standardized frequency, noted V, which depends on the wavelength $\backslash \; lambda$, of the ray of heart $a$ of fiber and the indices of the heart $n\_c$ and the sheath $n\_g$ (see image “Principle of a fiberoptic with jump of index” for the notations). The standardized frequency is expressed by:

$V\; =\; (2\; \backslash \; pi\; has\; \backslash \; sqrt\; \{n\_c^2\; -\; n\_g^2\})/\backslash \; lambda$

A fiber is monomode for a standardized frequency V lower than 2,405. Abacuses provide the constant of standardized propagation, noted B, according to the frequency standardized for the first modes.

Monomode and multimode fibers

The fiberoptics can be classified in two categories according to their diameter and the wavelength used:

• the multimode fibers, were the first on the market. They have as characteristics to transport several modes (luminous ways) simultaneously. Because of modal dispersion, one notes a temporal spreading out of the signal. Consequently, they are used only for low flows and short distances. Modal dispersion can however be minimized (with a given wavelength) by carrying out a gradient of index in the heart of fiber. They are characterized by a diameter of heart of several tens to several hundreds of micrometers (the hearts into multimode are of 50 or: 62.5 µm for the low flow).
• For moreover long distances and/or moreover high bancs, one prefers to use monomode fibers (known as SMF, for Single Mode Fiber ), which are technologically more advanced because finer. A monomode fiber does not have intermodal dispersion. On the other hand, there exists another type of dispersion: dispersion intramodale. Its origin is the finished width wavelength of emission. All the wavelengths are not propagated at the same speed in the guide what induces a widening of the impulse in fiberoptic. It is called also chromatic dispersion (cf higher " Dispersion chromatique"). These monomode fibers are characterized by a diameter of heart of only some micrometers (the monomode heart is of 9 µm for high banc).

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