Unit for the amplification of light signals in transmission lines being provided with optical fibers

The invention relates to a unit for amplifying light signals in optical fiber transmission lines, comprising a fiber optic amplifier adapted to be capable of being connected at least to an optical fiber input line through which the light signals are transmitted to the amplifier, and at least to an optical fiber output line The optical fiber amplifier is fed into the optical fiber amplifier.

State of the art

As is known, in fiber optic telecommunication lines, the signal is gradually attenuated along the fiber optic due to the inevitable loss of light that occurs within the fiber optic. For this purpose, in the case of signal transmission over long distances, one or more amplification units must be used which are inserted into the optical fiber path at intervals of a predetermined length.

The type of amplification unit which is currently extended consists essentially in the use of a fiber optic amplifier which is operatively connected to the optical fibers by defining along their path an input line along which signals are transmitted to the amplifier itself as well as an output line, by which the amplified light signals are transmitted towards the optical receiver.

In the current state of the art, the use of these fiber optic amplifiers is a source of certain drawbacks, mainly due to the lack of mainly and exclusively useful signals to be amplified into the amplifier, but also to various noise signals which are consequently amplified and re-output knowledge.

It has been found that some of these noise signals originate from the output line and are caused by the diffusion phenomenon of the "s; sw" la, which inevitably occurs within the optical fibers. More specifically, some of the amplified signals are lost due to the diffuse effect occurring inside the optical fibers. Part of the back diffused light is returned to the amplifiers and is therefore re-amplified and fed to the output line. In addition, it should be noted that the amplifier, due to its intrinsic nature, emits a certain amount of noise signals that are input either to the supply line or to the output line. Due to the above-mentioned diffusion phenomenon, these noise signals come in part back to the amplifier and mix with the useful signals that are actually desirable to transmit.

From the above, it is apparent that the input of noise signals into the amplifier and their subsequent amplification causes interference and a beat phenomenon, so that the noyer values ​​are sufficiently high.

> 15 dB amplifier gain produces interferometric noise with an amplitude greater than the known noise produced by the amplifier.

This leads to an undesirable reduction in the ratio between the useful signal and noise on the output line in the amplifier itself. This reduction in the signal to noise ratio tends to increase by increasing the gain of the fiber optic amplifier as well as by placing a plurality of amplifiers along the fiber line.

In this situation, there is a great difficulty in obtaining a useful signal sufficiently clear and clear when it reaches a receiver located far away from the source of the signal itself.

From Japanese patents 52-155901 and 63-219186 Sz / Electronics Letters, Vol. 24, No 1 of 7.1.1988, p.

to 38 it is known that in a laser or optical semiconductor amplifier there is a risk of instability and oscillation due to reflections at the ends of the amplifier. In order to avoid these reflections, it is generally proposed in the aforementioned patents and articles to attach an optical insulator to the semiconductor laser, which prevents light reflected by the connecting surfaces between the conductor fibers themselves and these devices from reaching the lasers themselves.

In an active fiber amplifier there are no interfaces between the conduit fibers and the amplifier, since the fibers are directly welded to the active fibers of the amplifier. For this reason, a reflection effect is usually not expected. However, it has been found that in an active-fiber amplifier in the absence of a reflection-reducing agent towards the active fiber, it is impossible to achieve high amplification gain due to the occurrence of interferometric-type noise as a result of jitters between direct signal and reflected signals. to active fibers. The presence of interferometric noise is of little importance in a semiconductor amplifier, which has small gains and small design dimensions, while becoming particularly important in an active fiber amplifier capable of achieving relatively large gains and having active fibers of considerable length generally in the range of several tens of meters. is the coherence distance of the signal driving laser.

The optical fiber amplifier therefore has the problem of protecting the fiber against such a noise source and keeping the reflection towards the active fiber itself below the critical values ​​so that the transmission quality is not sacrificed and the amplification gain values ​​are still kept high.

essence of the invention

The problem is solved by the invention of a unit for amplifying light signals in optical fiber transmission lines, comprising a fiber amplifier adapted to be capable of being connected at least to an optical fiber input line through which the light signals are transmitted to the amplifier and at least to an optical fiber output line. fibers into which light signals amplified by said optical fiber amplifier are introduced, having at least one first optical isolator performing unidirectional selection to prevent transmission of optical signals between the optical fiber amplifier and the optical fiber output line

-and-.::-.

noise from said output line to the optical fiber amplifier /% cut optical fiber amplifier and the fiber optic input line is provided with at least one second optical isolator for unidirectional selection to prevent transmission of optical noise signals from said output line to the optical fiber amplifier, wherein the first optical insulator and the second optical insulator both have a reflectance lower by at least 10 dB than the Rayleigh scattering of the optical fibers forming the input and output lines.

This solution makes it possible to solve the problems of the prior art by providing an amplification unit that substantially prevents noise signals from entering the fiber optic amplifier. This leads to an increase in the useful gain of the amplifier, as well as an improved transmission of optical signals from the transmitter to the receiver located a long distance from the transmitter.

Overview of figures in the drawing

BRIEF DESCRIPTION OF THE DRAWINGS The invention is explained in more detail in the following description, by way of non-limiting example, with reference to the accompanying drawing, showing a block diagram of an amplification unit according to the invention adapted to operate in optical fiber transmission lines.

Examples

In the drawing, the entire amplification unit for amplifying the light signals in the optical fiber transmission line according to the invention is designated as an amplification unit of the invention.

The amplifier unit 2 typically comprises fiber optic amplifiers 2 adapted to be connected in use to at least one fiber optic input line 3 to transmit light signals emitted, for example, by an optical transmitter 6 or alternatively coming from the amplifier unit as shown. and is located in the duct in front of it.

The amplifier 2 is also connected to the fiber optic output line 5 to which an amplified light signal is to be supplied, which will be output to the optical receiver 6, or to another amplification unit such as that shown.

According to the invention, the amplifier unit 1 also comprises a first unidirectional optical isolator 7 interposed between the optical fiber amplifier 2 and the output line 5, with optical fibers to prevent transmission of noise optical signals from the output line to the amplifier. Furthermore, a second optical selection insulator is inserted between the optical fiber amplifier 24 and the input line 2 to prevent transmission of noise signals from the amplifier to the input line.

Optical insulators 1 and 8 for unidirectional selection may be one-piece, as shown in the drawing. However, it is within the scope of the invention that the insulators are used in multiple embodiments. Optical insulators having low reflectivity are suitable for this purpose. It is necessary that the reflectance of these optical insulators 7, 8 is lower by at least 10 dB than the reflectance corresponding to the Rayleigh scattering of the optical fibers forming the inlet conduit 3 and the outlet conduit 15.

The operation of the amplification unit according to the invention is as follows. The amplifier 2 receives, in a known manner, the light signals coming from the input line 13 and transmits the amplifiers. In addition to the above optical signals, the amplifier also transmits its noise signals in a known manner, which tend to be applied to both the input line and the output line. The presence of a second optical insulator i? immediately in front of the amplifier 42 does not allow these noise signals to be input to the input line 3.

In the absence of such an optical insulator, the input of the amplifier noise signals to the input line 3, due to the diffuse effect occurring inside the optical fibers, will generate additional noise signals, some of which re-enter the amplifier 2 and cause more interference. signals emitted by the optical transmitter 4.

Further, the presence of the first optical insulator T immediately after the amplifier 2 precludes that noise signals generated along the length of the output line 5 due to the light diffusion effect occurring inside the optical fibers can reach the amplifier. In the absence of the first optical insulator 7, these noise signals would be amplified and reintroduced into the output line 5 together with the amplified payload signals and would thus cause undesirable interference and / or a phenomenon.

Accordingly, the only signals that reach the output line 5 are amplified payload signals along with a small noise signal, negligible among other signals produced by the amplifier 2.

The invention achieves the desired purposes. As demonstrated above, due to the presence of optical isolators (immediately before and after amplification), the optical amplifier according to the invention makes it possible to significantly reduce the input of noise signals to the amplifier output line compared to the prior art.

This leads to an increase in the useful gain of the amplifier, as well as an improved transmission of the optical signals from the transmitter to the receiver also located at a great distance from one another.

Of course, the invention may include numerous variations and variations, provided that they do not abandon its inventive basic idea.