Unit for amplifying light signals in optical fiber transmission lines

This application is a continuation of application Ser. No. 07/552,918, filed Jul. 16, 1990 .Iadd.now abandoned.Iaddend..

BACKGROUND OF THE INVENTION

1. Field Of The Invention

The present invention generally concerns a unit for amplifying light signals in optical fiber transmission lines, and more specifically, to such a unit which reduces interference and noise in such lines and which is connected between a pair of optical fibers.

2. Description Of The Prior Art

As known, in the optical fiber telecommunications field, owing to the unavoidable losses of light occurring inside optical fibers, a gradual attenuation of the signal always takes place along the path of the optical fibers.

For this reason, when signals have to be transmitted long-distances, it is necessary to use one or more amplifying units which are interposed along the path of the optical fibers at intervals of pre-fixed length.

A type of amplifying unit that at present is in widespread use provides for the use of an optical fiber amplifier which in operation is connected to the optical fibers so as to define, along the path of the latter, an input line through which the signals of light are transmitted to the amplifier itself, as well as an output line through which the amplified signals of light are transmitted in the direction of an optical receiver.

At the present state of the technique, the use of these optical fiber amplifiers gives rise to some drawbacks, deriving mainly from the fact that the amplifier receives not only and exclusively the useful signal to be amplified, but also different noise signals which are consequently amplified and introduced again into the output line.

It is found that a certain number of these noise signals come from the output line and are caused by a phenhomena of diffusion of light unavoidably arising inside the optical fibers.

More precisely, a part of the light forming the amplified signals gets lost as a result of a phenomenon of diffusion arisinq inside the optical fibers.

A part of the back-diffused light returns to the amplifiers and, therefore, is again amplified and introduced into the output line.

Moreover, it is to be considered that the amplifier, owing to its intrinsic nature, emits a certain quantity of noise signals which are introduced either into the input line or into the output line.

Owing to the above phenomena of diffusion, these noise signals partly come back to the amplifier where they mix with the useful signals which it is desired be transmitted.

As it can be understood from the above, the entry of noise signals into the amplifier and their consequent amplification result in interferences and beat phenomena that, for sufficiently high values (>15 dB) of gain of the amplifier, originate an "interferometric noise" of an amplitude greater that the known noise produced by the amplifier.

The above problem results in an undesired reduction of the signal-to-noise ratio between the useful signal and the noise downstream of the amplifier itself. This reduction in the signal/noise ratio tends to increase by increases in the gain of the fiber optical amplifier, as well as by increasing the number of amplifiers arranged along the path of fibers.

In this situation it is extremely difficult to have the useful signal sufficiently clear when it reaches a receiver placed at a long distance from the source of the signal itself.

From Japanese patents 52-155901 and 63-219186 and from "ELECTRONICS LETTERS", vol. 24, no. 1, Jan. 7, 1988, pages 36-38, it is known that in a laser or in an optical semiconductor amplifier there is the risk of instability and generation of oscillations due to the reflections at the amplifier ends.

In the above patents and article, in order to eliminate these reflections, it is taught to couple an optical isolator to the semiconductor laser, which prevents the light reflected by the coupling surfaces between the line fibers and these devices from reaching the lasers themselves.

In an active-fiber amplifier no interface surfaces are present between the line fibers and the amplifier because the line fibers are directly welded to the amplifier's active fiber. Therefore, the reflection phenomena are not generally expected.

It has, however, been discovered that in an active-fiber amplifier, in the absence of means for limiting reflections towards the active fiber, it is impossible to reach high amplification gain due to the occurrence of noise of the interferometric type as a result of beats between the direct and reflected signals in the line fibers themselves and in any event directed towards the active fiber. The presence of interferometric noise is of little importance in a semiconductor amplifier which has low gains and small construction sizes, whereas it becomes particularly important in an active-fiber amplifier capable of reaching very high gain and having an active fiber of considerable length generally in the range of some tens of meters, much greater than the coherence distance of the signal generating laser.

In an optical fiber amplifier the problem arises, therefore, of protecting the amplifying fiber against such noise sources and keeping the reflections towards the active fiber itself below critical values so as not to jeopardize the transmission quality, while maintaining high values of amplification gain.

SUMMARY OF THE INVENTION

The main object of the present invention is to solve the problems of the known technique, by realising an amplifying unit formed in such a way as to considerably prevent the entry of noise signals into the fiber optical amplifier.

This aim and other ones that will be better apparent from the present description, are substantially achieved by means of a unit for amplifying light signals in optical fiber transmission lines, comprising first isolator means for unidirectional light transmission interposed between said amplifier and the output optical fiber line to prevent the transmission of optical noise signals from said output line to the amplifier and second isolator means for unidirectional light transmission between the amplifier and the input line to prevent the transmission of noise signals from the amplifier to said input line.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages will better appear from the detailed description of a preferred but not exclusive embodiment of a unit for amplifying signals of light in optical transmission lines, according to the present invention. Said description will be made hereinafter with reference to the attached sheet of drawings, supplied only by way of example, which in the single FIGURE shows a block diagram of an amplifying unit forming the object of the present invention arranged to operate along an optical fiber transmission line.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the FIGURE, reference numeral 1 generally indicates a unit for amplifying signals of light in optical fiber transmission lines according to the invention.

The amplifying unit 1 includes conventionally an optical fiber amplifier 2 arranged to be connected in use to at least an input optical fiber line 3 through which light signals emitted for instance by an optical transmitter 4 or, alternatively, coming from an amplifying unit like that shown and placed upstream of the same, are transmitted.

The amplifier 2 is also connected to an output optical fiber line 5 which convey the amplified light signal to an optical receiver 6 or, alternatively, to another amplifying unit like the shown one.

In accordance with the present invention, the amplifying unit 1 also includes first isolator means 7 for unidirectional light transmission interposed between the fiber optical amplifier 2 and the output optical fiber line 5 to prevent the transmission of optical noise signals from the output line to the amplifier. Moreover, second isolator means 8 for light transmission are interposed between the fiber optical amplifier 2 and the input 3 to prevent the transmission of noise signals from the amplifier 2 to the input line.

Preferably, the isolator means 7, 8 for unidirectional light transmission comprise at least a first optical isolator and at least a second optical isolator, respectively, both isolators having a low reflectivity. It is provided that the reflectivity of these optical isolators 7, 8 known per se, is lower by at least 10 dB with respect to the reflectivity due to Rayleigh scattering in the optical fibers forming the input 3 and output 5 lines.

The operation of the amplifying unit of the present invention will now be described.

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

Besides the aforementioned optical signals, the amplifier 2 also transmits, in a known way, its own noise signals, which tend to be introduced both into the input line 3 and into the output line 5.

Advantageously, the presence of the second optical isolator 8 immediately upstream of the amplifier 2 does not allow the entry of noise signals into the input line 3.

Absent such optical isolator 8, the entry of the amplifier noise signals into the input line 3 would originate, owing to the phenomena of diffusion arising inside the optical fibers, further noise signals, a part of which would again reach the amplifier 2 creating interferences of beats with the useful optical signals, i.e. those transmitted by the optical transmitter 4.

Advantageously, the presence of the first optical isolator 7 immediately downstream of the amplifier 2 additionally avoids noise signals reaching the amplifier 2 produced along the output line 5 as a consequence of the phenomena of diffusion of light arising inside the optical fibers. Absent the first optical isolator 7, these noise signals would be amplified and again introduced into the output line 5 together with the amplified useful signal, thus originating undesired interferences and/or beat phenomena.

From the above it is understood that the only signals that will reach the output line 5 are the amplified useful signals, together with the small noise signal, negligible among other things, produced by the amplifier 2.

The present invention achieves the objects of the invention. In fact, as demonstrated above, due to the presence of the optical isolators immediately upstream and downstream of the amplifier 2, the amplifying unit 1 forming the object of the present invention permits noticeable reduction, in comparison with known techniques, in the entry of noise signals into the output line 5 of the amplifier 2.

This invention increases the useful gain of the amplifier, as well as an improved transmission of the optical signals from a transmitter to a receiver remotely placed at a long distance one from the other.

While a particular embodiment of the present invention has been illustrated and described herein, changes and modification apparent to those skilled in the art may be made therein and thereto, but are included within the scope of the appended claims.