Optical wave-quide telecommunication line and optical amplifier therefor

The subject of the invention is a fiber optic link with an active fiber optical amplifier.

As is known, optical fibers doped with certain doped substances, especially rare earth ions, have forced emission properties and are preferably used as optical amplifier fibers in fiber optic telecommunication lines.

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This type of amplifier is described in European Patent Application No. EP-A-0409012.

In fiber optic amplifiers, also called active fiber amplifiers, the transmitted light signal is amplified directly in the form of a light signal, without the need to transform the signal, as is the case in electronic amplifiers. In electronic amplifiers, the optical signal is converted into an electrical signal, amplified, and again transformed into an optical signal. The amplifying element of an optical amplifier consists of a specific length of doped core optical fiber, connected in series between two sections of optical fibers that deliver and remove signals. The optical amplifier is also equipped with a power supply device that provides an optical pumping signal.

Amplifiers of this type are particularly advantageous for use in fiber optic telecommunications connections because they provide high gain when used as line amplifiers. The required amount of gain of these enhancers is obtained by choosing the length of active fiber and / or the content of dopant in the fiber. When such an amplifier is used as a power amplifier, high energy efficiency is obtained.

When transmitting a light signal along a fiber optic link, the operation of optical amplifiers is affected by signal reflections occurring at the fiber ends.

From Japanese Patent Nos. 52-155 901 from 24-12-1977. and 63-219 186 from 12-091988. and from the magazine Electronics Letters, vol. 24, No. 1, of January 7, 1988, pp. 36-38, it is known that in a laser amplifier or semiconductor optical amplifier, there is a risk of unstable operation and the formation of oscillations as a result of interfacial reflections on surfaces limiting amplifier terminals.

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

In an optical amplifier with an active fiber, there are no boundary surfaces between the amplifier and the fibers of the transmission cables, because the fibers of the cables are directly welded to the active fiber of the amplifier. In the active fiber amplifier, there is basically no signal reflection.

It has been found, however, that in the absence of reflection control measures, it is not possible to achieve high gain in an active fiber amplifier due to interference noise occurring as a result of the beating of the useful signal and the signals reflected in the interior of the fiber and directed to the active fiber . The presence of interference noise is not very important with semiconductor amplifiers that work at low amplifications and have small geometric dimensions. However, this becomes a big problem in optical amplifiers with active fibers, which have very high gain, and the active fiber is of considerable length, usually on the order of tens of meters, which significantly exceeds the distance corresponding to the maintenance distance of the coherent laser signal generating the transmitted signal.

Therefore, the problem arises of protecting the active fiber in amplifiers of this type against such sources of noise, and maintaining each type of reflection in the direction of the active fiber below a certain critical value, in order not to reduce the quality of the transmitted signal, while maintaining high gain values.

In the aforementioned description of the European patent application No. EP-A-0 409 012 it is recommended to introduce into the amplifiers with optical fiber an optical isolator, which should have a reflection coefficient limited to a value not exceeding a specified critical value.

The fiber optic connection according to the invention comprises an active fiber optical amplifier. In this connection, the transmitter of the transmitted light signal is connected to the signal receiver by a long-range fiber optic line divided into sections. Each section is terminated with a linear optical amplifier with active fiber, wypo4

168 016 infected with a laser pumping source. This link contains at least one optical isolator unidirectional transmitting the light signal from the transmitter to the signal receiver. In addition, the signal transmitter includes an optical fiber power amplifier. This type of connection is characterized by the fact that the input fiber optic line section is attached to the input fiber end of the optical amplifier through a series connection of the input optical isolator and dichroic coupler, which is connected by its input fiber section to the laser pumping source. However, the active fiber output terminal is attached to the output section of the optical fiber line through the output optical isolator. The active fiber input end of the optical power amplifier is connected to the output signal isolator of the signal transmitter via a dichroic coupler.

In a preferred embodiment, according to the invention, the input optical isolator is connected between the output of the dichroic coupler and the input of the active fiber.

The input and output optical isolators are adjustable polarity optical insulators. The surface of the tip of the input section of the dichroic coupler fiber is provided with a multilayer anti-reflection coating and / or is a bevelled surface relative to the axis of the fiber. Preferably, the surface of the tip of the input segment of the dichroic coupler fiber forms an angle of 5 ° to 10 ° with a plane perpendicular to the axis of this fiber.

In a different embodiment, in a fiber optic connection according to the invention comprising an optical fiber amplifier, the transmitter of the transmitted light signal is connected to the signal receiver by a long-range fiber optic line divided into sections. Each of these sections is terminated with an active fiber optical amplifier, equipped with a laser pumping source, and the active fiber of the optical amplifier is connected in series in the long-range fiber optic line. The connection is characterized by the fact that at least one optical isolator unidirectional transmitting the light signal from the transmitter to the receiver, and having a reflection coefficient with an absolute value higher by 10 dB than the amplifier's amplifier, is connected between the active fiber of the optical amplifier and the long-range optical fiber line.

The subject of the invention is shown in the embodiment in the drawing, in which Fig. 1 is a diagram of a fiber optic telecommunication link equipped with a power amplifier and linear amplifiers, Fig. 2 - block diagram of a linear optical amplifier with active fiber, Fig. 3 - block diagram of a second linear example an active fiber optical amplifier, and Figure 4 is a block diagram of an active fiber optical power amplifier.

As shown in Figure 1, a telecommunications fiber optic link includes a signal transmitter 1, a signal receiver 2, which are located at a large distance from each other, for example hundreds of or even thousands of kilometers away. Between the transmitter and receiver there is a long-range fiber optic line 3 with appropriate signal transmission parameters. Useful light signals are transmitted through the line.

To send a signal over a long distance, it is first of all necessary to send a signal with the appropriate power, and then gradually compensate for the attenuation of the signal formed during the passage of this signal along the fiber. The signal transmitter 1 containing the laser 4 emitting the transmitted signal is connected to the power amplifier 5. At a certain distance from the transmitter, for example after several hundred kilometers, optical fiber line 3 is connected to optical line amplifier 6, restoring the useful signal to the appropriate level. Next are subsequent sections of fiber optic line 3 and subsequent line optical amplifiers 6a, 6b etc., which occur along the entire length of the line between the transmitter and the receiver.

Amplifiers 5 and 6 are preferably optical amplifiers. During amplification, the signal maintains its optical signal form and there is no need to convert it into an electrical signal, gain, and re-convert it into an optical signal, as in the case of electronic amplifiers.

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Performing signal transformations limits the bandwidth in practice, mainly due to the decrease in mission speed tr ^^^^ of the mission, which is limited by the speed of electronic devices used.

However, in optical amplifiers, the signal remains in the form of a light signal at all times, and therefore the transmission speed is not limited and there are no other disadvantages. The use of optical amplifiers with an active core fiber is particularly preferred.

In practice, such amplifiers provide particularly favorable operating conditions, both in terms of gain and efficiency.

The structure of the optical fiber amplifier is shown schematically in Fig. 2. Optical signal 3 is transmitted to the useful signal, which should be amplified at K-wavelength. The segment of the optical fiber 3 is connected to the dichronic coupler 7, in which the signal is applied to the useful signal pump with wavelength λ ^. The resulting signal is fed through the input terminal 8 to the active fiber 10. The pumping signal is generated by the laser pumping source 9. The active fiber 10 is the amplifying member of the optical amplifier 6, from which the signal is fed to the next section of the fiber optic line 3 and directed to the destination.

The active fiber 10 is preferably a silicon fiber whose core is doped with a fluorescent substance. In the presence of light pumping energy with a wavelength λ ^ », it is capable of producing forced emission consistent with the useful signal of wavelength Xs. In this way, the output signal is significantly amplified compared to the input signal of the optical amplifier 6.

In each optical amplifier, the G gain is associated with the reflection coefficients Ri and R2, measured at the ends of the amplifier, according to the relationship:

G (dB) <- 1/2 [Ri (dB) + R₂(dB)], 1 / for which the reflection coefficients are defined as:

R (dB) = 10 1n (Pr / Pt), where Pt is the transmitted power and Pr is the reflected power.

The above dependencies show that the achievement of high gain in the amplifier is limited by reflection properties measured at the amplifier's terminals. Thus, to achieve high gain, it is necessary to obtain high reflection coefficients R1 and R2.

In practice, however, if part of the light signal present in the amplifier is reflected from the amplifier's end and returns to it, it is amplified, and partially re-reflected from the opposite end of the amplifier. Then it is returned to the amplifier again, and such a cycle can be repeated several times. If this type of reflection and amplifier gain reach a high value, it is possible for the amplifier to enter oscillation, which prevents its proper operation and causes the need to limit the maximum gain to avoid the occurrence of the described phenomenon.

In addition to the phenomenon of internal reflection of the transmitted signal from the various elements of this amplifier, which is present in the optical amplifier, the signals reflected in the fiber itself are also amplified by the amplifier and again reflected by the elements located in front of the amplifier. This causes the rumble between the transmitted signal and the reflected signals. As a result of this phenomenon, so-called interference noise is created.

Such interference noise is particularly important when using an active fiber optical amplifier in which the length of the reinforcing element, i.e. the active fiber, is greater than the distance corresponding to the distance of maintaining the coherent laser signal generating the useful signal. In such conditions, the coherence between the useful signal and the reflected signals disappears. The reflected signals are shifted in time relative to the useful signal and if their intensity is high enough, the phenomenon becomes detrimental to the quality of the transmitted signal.

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Reflections occurring in the optical amplifier are caused by the presence of interfacial boundary surfaces at its ends, as a result of the known phenomenon of refraction of the luminous flux. Reflections can also occur in the absence of such surfaces, as is the case with an active fiber amplifier, in which the active fiber 10 is directly welded to the dichroic coupler 7 and with the fibers of the cables. Reflections then occur inside the cable fibers themselves behind and in front of the amplifier. This phenomenon is known as Rayleigh scattering. It reflects light energy.

In practice, it has been found that Rayleigh scattering that occurs along the entire fiber causes a reflection coefficient of about -30 dB. Other types of reflection arise when transmitting high-power light signals, according to a phenomenon known as Brillouin scattering.

In the solution according to the invention, the limitation of the maximum gain in the linear optical amplifier, resulting from the described signal reflection phenomena, is eliminated by placing the optical isolators 11a, 11b before and after the active fiber 10, of which the input optical isolator 11a is located before the dichroic coupler 7 , just after the fiber optic section 3, and the output optical isolator 11b is located behind the active fiber 10, and before the next section of fiber optic line 3.

Optical isolators are devices that provide one-way flow of light energy from transmitter to receiver. In the solution according to the invention, preferably optical isolators are used, which are independent of signal polarization, have an isolation (light retention) degree at least 20 dB higher, and which have a low reflection coefficient, at least 10 dB lower than the reflection coefficient from Rayleigh scattering in an infinite length fiber, and preferably at least 15 dB lower than the above size.

It was found in research that the presence of optical insulators with the presented properties ensures that the doped active fiber of the optical amplifier works in more favorable conditions than in the case of noise arising as a result of reflections of different nature. The amplifier can then work with the amplifications usually obtained by active fiber amplifiers, i.e. about 30 dB, which basically corresponds to the absolute value of the reflection coefficient associated with Rayleigh scattering in an infinite length fiber.

To achieve higher gain, a correspondingly low reflection coefficient is needed, preferably with an absolute value at least 10 dB higher and even 15 dB higher than the amplifier's gain factor.

It results from the above that in order to obtain a gain of 40 dB, the reflection coefficient calculated in relation to the active fiber in each of the fibers leading to it should be less than -50 dB and even less than -55 dB at the wavelength of the transmitted signal λβ.

The reflected reflecting properties of optical insulators are obtained by known methods, preferably by using multilayer coatings, by using obliquely cut chamfered propagation direction of interfacial boundary surfaces, through which surfaces the transmitted signal and other signals, e.g. reflected, pass.

In addition, to avoid noise caused by reflections, the input fiber segment 12 of the dichroic coupler 7, which transfers light pumping energy from the laser pumping source 9 to the dichroic coupler 7, and further to the active fiber 10, should also have a reduced reflection coefficient compared to the active fiber. This necessity results from the fact that the part of the light energy of the transmitted signal wavelength, which propagates back to the dichroic coupler 7, is transmitted through the input fiber section 12, and the couplers usually used for these purposes in two connected flow branches do not have the ability to completely separate signals of different wavelength. Due to this incomplete separation, some of the energy at the wavelength of the transmitted signal, of a significant magnitude, e.g. a few percent, is coupled to the link branch carrying the pumping energy.

When this part of the light energy of the wavelength of the transmitted signal at the end of the input fiber section 12 optically connected to the laser pumping source 9

168 016 bounces, will travel back through the dichroic coupler 7 to the active fiber 10, which will cause interference noise.

Therefore, the input fiber segment 12 of the dichroic coupler 7 should have a reflection coefficient less by 10 dB and even 15 dB from the value corresponding to Rayleigh scattering in the infinite length fiber, minus the value of the attenuation caused by the passage of the transmitted signal through the pumping branch of the link .

Thus, it is necessary that at the end of the active fiber 10 attached to the input end 8 of the active fiber 10, the reflection coefficient of each fiber attached to it is 10 dB lower, and even 15 dB lower than the reflection coefficient corresponding to Rayleigh scattering in the fiber of infinite length, or, respectively, an absolute value by which it is higher than the gain of the optical amplifier 6. Similarly, the reflection coefficient on the output terminal 16 of the active fiber 10 must meet the same conditions.

The reflected reflection properties of the input fiber section 12 are achieved by using multi-layer or obliquely sheared coatings relative to the propagation direction of interfacial boundary surfaces. Especially the bevelled surface of the tip 13 of the fiber 12 of the dichroic coupler 7 attached to the laser pumping source 9, at an angle preferably in the range of 5 ° -10 °, provides a reflection coefficient lower than -15 dB. Taking into account the attenuation at the signal passing through the dichroic coupler 7, attenuation of -20 dB for each transition, gives an overall reflection coefficient in relation to the tip of the active fiber 10, with a size of -55 dB, i.e. 15 dB less than the reflection coefficient resulting from scatter Rayleigh, which is about 30 dB.

Reflection phenomena in the input fiber portion 12 of the dichroic coupler 7 can also be eliminated by inserting the input optical isolator 11a after the dichroic coupler 7 immediately before the active fiber 10 as shown in Figure 3. This solution, which allows the use of anti-reflection means at the end of the fiber inlet section 12 of the dichroic coupler 7, is used when the loss of pumping energy in such an insulator is not detrimental to the proper operation of the optical amplifier.

In the case of the optical power amplifier 5, switched on behind the laser 4 emitting the transmitted signal, which is fed by a high-power input signal higher than the saturation level, in addition to the phenomena discussed, there may also be a noise phenomenon arising as a result of Brillouin scattering. As a result of this scattering, light energy introduced into the optical fiber section coming out of the optical amplifier, induces the vibrations of the fiber atoms, which in turn causes the formation of a reflected signal, with a wavelength slightly shorter than the wavelength of the transmitted signal. The signal output from the optical power amplifier 5 depends only on the pumping energy.

The reflected signal can generate beats with a useful signal, and thus the formation of noise distorting the transmitted signal, if they overlap with the noise resulting from the previously described phenomena.

In the fiber optic telecommunications link, as schematically illustrated in Fig. 1, the signal transmitter 1 comprises a series connection of a laser 4 emitting the transmitted signal and an output optical isolator 15. The signal transmitter 1 is connected to an optical power amplifier 5, forming together a signal emission unit 14. The output optical isolator 15 of the signal transmitter 1 protects the laser 4 from reflections that could cause damage to its components.

In the optical power amplifier 5, located in the emission unit 14, the input optical isolator 11 can be omitted at its input, as shown in Fig. 4, because the task of removing reflections towards the active fiber 10 of the optical power amplifier 5 is fulfilled in this case by the output optical isolator 15 of the signal transmitter 1.

The other components of the optical power amplifier 5 shown in Fig. 4 are the same as those of the linear optical amplifier 6 already shown, and therefore they bear the same reference numerals.

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For example, a fiber optic telecommunications connection was constructed according to the scheme in Fig. 1, in which a direct modulated laser, DFB type, with a wavelength of transmitted signal of 1535 nm was used as the emission laser. As the signal receiver 2, a receiver of the known pin / HEMT type was used, followed by broadband amplifiers, not shown.

Fiber optic line 3 was composed of low attenuation, shifted dispersion types, having zero dispersion at wavelengths close to the wavelength of the transmitted signal. The total link length was 300 km, which corresponded to 60 dB attenuation.

The link included two linear optical amplifiers 6 and an optical power amplifier 5. These were amplifiers with active silicon fiber 10, doped with germanium and erbium, pumped with a laser pump source 9, which was a miniaturized Nd-YAG type neodymium laser, with a doubled frequency and with a pump diode. The linear optical amplifiers 6 had the structure as shown in Fig. 2, and the optical power amplifier 5 as shown in Fig. 4. Each of the linear 6 optical amplifiers had a 20 dB gain factor. The optical power amplifier 5 showed saturation power at 9 dBm and input power of 0 dBm. The optical isolators 11 a and 11 b were insulators with adjustable polarity, were independent of the polarization of the useful signal and had an isolation factor greater than 35 dB, and a diffusion factor lower than -50 dB. These types of insulators are generally available and their construction has not been described. The tip 13 of the fiber inlet portion 12 of the dichroic coupler 7 connected to the laser pumping source 9 was bevelled at an angle of 5 ° to the surface perpendicular to the axis of the fiber. Transmission made on such a link showed received power of -20 dB and noise level corresponding to -40 dBm.

For comparison, the signal transmission was carried out through the presented link, which uses commercially available optical isolators with a reflection coefficient of -30 dB, which corresponds to the reflection coefficient resulting from Rayleigh scattering in the fiber allowing to avoid oscillation in the presence of a signal amplified to a level of 30 dB . Under such conditions, although no oscillations occurred, noise of -30 dBm was noted, which significantly disrupted the proper signal reception. These noises probably arose as a result of interference vibrations caused by Rayleigh scattering and Brillouin scattering, created in optical amplifiers with active fiber.

Optical amplifiers are schematically shown in the figure as welded amplifiers, the use of which is particularly convenient for the construction of active fiber optical amplifiers. However, it is also possible to use other types of optical connectors, for example connectors used in microoptics. For connectors other than welded joints, a reflection coefficient that is at least 10 dB lower than the reflection coefficient, corresponding to Rayleigh diffusion, or an absolute value at least 10 dB higher than the gain factor of a given optical amplifier is also required.

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