Apparatus for amplifing optical signals in light-pipe type telecommunication trunk lines

The present invention relates to a device for amplifying light signals in fiber optic transmission lines.

The attenuation of the electromagnetic wave in the optical fiber limits the permissible distance between the transmitter and the telecommunication track receiver. Longer sections of the track require the inclusion of at least one reinforcement device along the way.

In the fiber optic telecommunication lines used, at regular intervals, devices are placed to amplify the transmitted light signals, especially optical amplifiers, which amplify the signals without converting them into electrical signals.

One of the known and used types of optical amplifiers is a semiconductor laser. For example, Japanese Patent Nos. 52-155901 and 63-219186, and from Electronics Letters, vol. 24, No. 1, of January 7, 1988, pp. 36-38, an optical amplifier is known in semiconductor laser, in which, however, there is a risk of loss of stability and generation of oscillations as a result of interfacial reflections on the boundary surfaces of the amplifier.

In the solutions described in the aforementioned patents and article, to eliminate these reflections, an optical isolator has been coupled to the semiconductor laser, which prevents light radiation reflected from the boundary surfaces between the sections of the fiber optic track and these devices from reaching the laser itself.

Japanese Patent Nos. 52-155901 and 63-219186 describe optical amplifiers, which are devices that require the use of connecting elements to the input and output section of the fiber optic path, in particular lenses. In this situation, interphase reflections arising on the air-glass interface are unavoidable. In such conditions, there is a need to use technical measures to prevent this type of reflection. Preferably, such insulators are optical insulators.

In addition, U.S. Patent No. 4,947,134 describes a fiber optic system using semiconductor optical amplifiers. An isolator was used between adjacent amplifiers that protects the amplifier from instability, which could be the result of reflection of the amplified signal from contact connections or other discontinuities in the transmission path. It is also a safeguard against the formation of undesirable

167 672 3 positive feedback, which may be the result of the amplifier amplifying signals transmitted in both directions along the fiber optic line.

Fiber optic amplifiers in the form of doped optical fiber are also known. This type of amplifier can be welded directly into the fiber optic path, which eliminates the semiconductor - air and glass - air boundary surfaces. There are no interphase reflections here.

However, the use of this type of fiber optic amplifier has some disadvantages, mainly due to the fact that this amplifier receives not only signals to be amplified, but also various types of noise, which are then amplified and fed to the output section of the fiber optic path.

It has been found that some of these noise signals come from the output section of the fiber optic path and are the result of the phenomenon of light scattering that occurs in optical fibers, the so-called Rayleigh scattering. Thus, part of the light energy constituting the signal amplification is lost due to the scattering taking place inside the optical fibers. Part of the diffused light energy returns to the amplifier, is again amplified and introduced into the output section of the fiber optic path. In addition, it should be taken into account that the amplifier itself emits a certain amount of inherent noise that is fed into the input or output section of the fiber optic path.

As a result of the aforementioned Rayleigh scattering phenomenon, the noise signals return partly to the amplifier, where they overlap the transmitted useful signals. The noise coming to the amplifier and then amplifying it, as a result of interference and rumbling phenomenon, at high amplifier gain values, especially above 15 dB, is the cause of interferometric noise at a level higher than the amplifier's own noise level.

The above problem causes an undesirable reduction of the useful signal distance from the noise level of the amplifier itself. The useful signal distance from the noise level decreases unfavorably when increasing the fiber amplifier gain, as well as when increasing the number of these amplifiers welded in the fiber optic path.

In this situation, it is extremely difficult to obtain a reasonably clean useful signal when it is sent to a receiver located at a considerable distance from the signal source.

So, although there are no boundary surfaces in the fiber optic line with the fiber amplifier, so there is also no interfacial reflection phenomenon, it is not possible to achieve high gain due to the occurrence of interferometric noise. This type of noise is a light wave superimposed on an information signal transmitted in a fiber optic line. The presence of interferometric noise is of little importance in the semiconductor laser amplifier, which has low gain and small dimensions. However, it is very important in the fiber optic amplifier, which achieves very high gain and contains active optical fiber of considerable length, on the order of several dozen meters, much greater than the distance of maintaining a coherent laser signal generating this signal.

In the fiber optic amplifier there is a problem of protecting the reinforcing fiber from interferometric noise sources and keeping reflections towards the active optical fiber below critical values ​​to maintain the appropriate transmission quality while maintaining high gain values.

The device according to the invention is intended to amplify light signals in fiber optic transmission lines and comprises an amplifier with active optical fiber. This device is placed between the transmitter and the optical receiver, and is connected to the input and output section of the long-range optical fiber path, in which part of the light signals is reflected by Rayleigh scattering. The device is characterized in that the amplifier with active optical fiber is its input connected to the input of the amplifying device connected to the input section of the optical fiber path through the first optical isolator. At the same time, an amplifier with its active optical fiber

167 672 the output is connected to the output of the amplifying device connected to the output section of the optical fiber path through a second optical isolator. The first and second optical isolators are insulators unidirectally transmitting light signals from the optical transmitter to the optical receiver, and the reflection coefficient of these optical insulators is less than the Rayleigh scattering factor of the long range fiber optic path, by at least 10 dB.

The solution according to the invention is explained in the embodiment in the drawing, which shows a block diagram of a device for amplifying light signals transmitted in a fiber optic line.

As shown in the drawing, the amplifying device 1 comprises an amplifier with an active optical fiber 2 connected to the input fiber optic path section 3, through which the signals emitted by the optical transmitter 4, or signals coming from the amplifying device 1, in the opposite direction of transmission are transmitted.

The active optical fiber amplifier 2 is also connected to the output section of the optical fiber path 5, which supplies the amplified light signal to the optical receiver 6, possibly to the next amplifying device.

The amplification device 1 further includes a welded first optical isolator 7 for unidirectional transmission of light radiation between the amplifier with active optical fiber 2 and the input fiber optic path 3, for protection against the transmission of noise signals from the amplifier with active optical fiber 2 to the optical fiber path input 3. In addition, a second optical isolator 8 is welded between the active optical fiber amplifier 2 and the output fiber optic path section 5 to protect against optical noise coming from the output optical fiber path 5.

Optical insulators 7, 8, for unidirectional light transmission, are preferably low reflective insulators. As a result of the research, it was found that the reflection coefficient of these optical insulators 7 and 8 should be at least 10 dB lower than the reflection coefficient resulting from Rayleigh scattering in the optical fibers forming the input segment 3 and the output receiver 5 of the optical fiber path.

The amplification device 1 works as follows:

The active optical fiber amplifier 2 receives the light signals coming from the input fiber optic path section 3 in a known manner and sends amplified signals towards the output optical fiber path section 5.

In addition to the mentioned light signals carrying information, the amplifier with active optical fiber 2 also transmits its own noise, which is fed to both the input section of the optical fiber track 3 and the output section of the optical fiber track 5.

The presence of the first optical isolator 7, connected before the amplifier with the active optical fiber 2, preferably prevents the introduction of noise into the input section of the optical fiber path 3.

Lack of the first optical isolator 7 will cause 3 noise resulting from the phenomenon of scattering inside the optical fibers to reach the fiber optic path section 3. In addition, the noise, some of which will get to the amplifier with the active optical fiber 2, will come in rumble interference with useful light signals that are transmitted by the optical transmitter 4.

The presence of a second optical isolator 8 downstream of the amplifier with active optical fiber 2, preferably prevents the introduction into this amplifier 2 of noise generated along the output section of the optical fiber path 5 as a result of the phenomenon of light scattering inside the optical fiber. In the absence of a second optical isolator 8, these noises will be amplified and brought back to the output section of the fiber optic path 5, together with the amplified useful signals, causing adverse interference and / or rumbling.

It follows from the above that when introducing optical insulators 7 and 8, the only signals coming to the output section of the fiber optic path 5 are amplified signals

167 672 5 useful with a small amount of noise, negligible in relation to the signals amplified in the amplifier with active optical fiber 2.

Due to the presence of optical insulators connected upstream and downstream of the active optical fiber amplifier 2, the amplification device 1 of the invention provides, compared to known solutions, a significant reduction in noise to the output section of the optical fiber tube 5.

This leads to an increase in the useful gain factor of the amplification device, as well as to the improvement of the transmission of light signals from the transmitter to the receiver, in the long-range fiber optic line.

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