Unit for amplifying light signals in optical fiber transmission line

This invention relates to a device for amplifying light signals in transmission lines from optical fibers. The device comprises an amplifier made of optical fibers adapted to be connected in use to at least one of the optical fibers, an input line for transmitting light signals to the amplifier, and at least one output line of optical fibers fed to these light signals amplified by the aforementioned amplifier made of optical fibers.

In telecommunications composed of optical fibers, the signal is gradually weakened due to the inevitable loss of light inside the optical fibers.

Therefore, when transmitting a signal over a long distance, it is necessary to use one or more amplification devices which are inserted into the optical fiber line at pre-fixed length intervals.

The type of amplifier that is currently widely used is mainly an optical fiber amplifier, which is operatively linked to the optical fibers so as to set along an input line of the above-mentioned line through which amplified light signals are transmitted in the direction of the optical receiver.

In the state of the art, the use of fiber optic amplifiers presents some difficulties due to the fact that the amplifier amplifies not only the useful signal but also the various noise signals which are amplified and reintroduced into the output line.

It is known that a certain amount of these noise signals are generated in the output line due to the light scattering that inevitably occurs inside the fibers.

Part of the diffused light goes back to the amplifier and is amplified again and brought to the output line.

By its very nature, the amplifier is thought to generate a certain amount of noise signals that fall into the output or input lines.

As a result of the scattering phenomenon mentioned above, the noise signals return, intermingling with the useful signals that actually need to be transmitted.

As can be seen from the above, the input and amplification of noise signals to the amplifier results in interference and shock phenomena which, due to the amplifier's rather large amplification values ​​(> 15 dB), produce "interferometric noises" of known amplitude, amplifier-generated noises.

The above causes an undesired decrease in the payload-to-noise ratio along the amplifier line itself.

The ratio signal / noise reduction begins to weaken with an increase in the gain of the optical fiber amplifier, as well as an increase in the number of amplifiers in the fiber line.

In such a situation, it is difficult to obtain a sufficiently pure payload when it reaches a receiver at a considerable distance from the signal source.

From Japanese Patents 52-155901 and 63-219186 and from Electronics Letters 1988. month of January. Volume 24 no. 1, page 7 36-38 it is known that there is a risk of instability and oscillation in the laser or optical semiconductor amplifier due to reflections at the ends of the amplifier.

The patents and articles referred to generally describe how to reduce these reflections, a pair of optical insulators for a semiconductor laser that do not allow light reflected by the porous surfaces between the fiber lines and thereby prevent it from entering the laser itself.

In the amplifier, there are no intermediate surfaces between the active fibers between the fiber line and the amplifier, because the fiber lines are simply welded to the active amplifier fibers and as a result, reflection phenomena are generally not expected.

At the same time, it has been discovered that in the absence of active fiber reflectors in the amplifier, it is not possible to obtain a high gain factor due to interferometric noise caused by collisions between the direct signal and reflected signals in the fiber lines themselves and in all elements belonging to the active fiber. fiber.

Interferometric noises in a semiconductor amplifier are of little importance due to its low amplification coefficient and small construction dimensions. In the case of active fiber amplifiers, these noises are very important because these amplifiers give very high gain coefficients and consist of active fibers of quite large length of up to ten meters, far beyond the laser-generated signal coherence distance.

The problem with optical fibers in the amplifier is how to protect the amplifier fibers from such noise sources and reduce reflections in the direction of the active fiber to levels below the critical values ​​so as not to compromise transmission quality while setting high gain ratios.

The main object of the present invention is to provide a known solution to technical problems by providing an amplifying device manufactured in such a way as to significantly obstruct the passage of noise signals to an amplifier composed of optical fibers.

This and other objects of the present invention are substantially achieved by a device for amplifying light signals in fiber-optic transmission lines, characterized in that it consists of first unidirectional selection means disposed between said amplifier and optical fiber output line to prevent transmission of optical noise signals from said transmission. lines to the amplifier and second means of unidirectional selection, placed between the amplifier and the output line and preventing the transmission of noise signals from the amplifier to the aforementioned input line.

Other features and advantages will be more readily apparent from the detailed description of a more suitable but not exclusive description of a device for amplifying light signals in transmission lines from the optical fiber construction in accordance with the present invention. The foregoing description is given below with reference to the accompanying drawing, which is a non-limiting example only and in which the block diagram of an amplifying device which is the object of the present invention and intended to operate on optical fiber transmission lines is shown in a single drawing.

In the aforementioned drawing, reference 1 denotes an entire device for amplifying light signals in a line from optical fibers according to the invention.

The amplifying device 1 consists primarily of amplifiers 2 of optical fibers intended to be connected in use with at least one optical fiber input line 3 which transmits a light signal emitted, for example, by an optical transmitter 4 or, alternatively, exiting the amplifier device. to the one described and placed on the same line.

The amplifier 2 is also coupled to an output line 5 of optical fibers, which must receive an amplified light signal, which is output to the optical receiver 6 or, alternatively, to another amplifier similar to that described.

According to the present invention, the amplifying device 1 also comprises first unidirectional selection means 7, inserted between the amplifier 2 from the optical fibers and the output line 5 from the optical fibers to prevent transmission of optical noise signals from the output line to the amplifier. In addition, the second unidirectional selection means 8 is inserted between the amplifier 2 from the optical fibers and the input 3 to prevent transmission of noise signals from the amplifier to the input line.

Preferably, the unidirectional selection means 7 and 8 comprise in particular at least at least a first optical insulator and at least at least a second optical insulator and the insulators have a low reflectivity. It is assured that the reflectance of these optical insulators 7 and 8 is substantially at least 10 dB lower than the reflection caused by Reille scattering on the optical fibers forming the input 3 and output 5 lines.

The reinforcing device which is the object of the present invention operates as follows:

The amplifier 2 receives in a known manner light signals coming from the input line 3 and transmits the amplified signals in the direction of the output line 5.

In addition to the above-mentioned optical signals, the amplifier transmits its noise signals in a known way, which is transmitted to the input line 3 and to the output line 5.

A second optical insulator 8 located directly on line 2 of the amplifier prevents the passage of said noise signals to the input line 3.

Where no optical isolator is mentioned, the input of the amplified noise signals to the input line 3 must be due to the diffusion phenomenon occurring in the optical fibers, due to further noise signals, some of which must re-reach the amplifier 2, interfering shock with useful optical signals. transmitter 4.

Directly at the bottom of line 2 of the amplifier 2 is the first optical insulator 7, which also prevents noise from entering the amplifier along the output line 5 due to light diffusion phenomena inside the optical fibers. In the absence of the first optical isolator 7, these noise signals must be amplified and re-introduced to the output line 5 together with the amplified payload, thereby generating undesired interference and / or impact phenomena.

From the above, it is clear that the only signals that will reach the output line 5 are the amplified payloads, together with minor noise signals, which are completely small compared to other signals produced by the amplifier 2.

The present invention solves the challenges. Indeed, as shown above, with the optical insulator directly at the top and bottom of the amplifier line, the amplifying device which is the subject of the present invention makes it possible to significantly reduce the input of noise signals to the amplifier output line compared to the prior art.

This makes it possible to increase the gain of the amplifier, thereby improving the transmission of the optical signal from the transmitter to the receiver spaced far apart.

The invention may include various modifications and variations provided that they follow the same principle of the invention.