Telecommunication system with signal amplification for transmission of multiplexed signals
Abstract
The present invention relates to a telecommunication system including optical amplifiers connected in cascade, particularly adapted for a wavelength division multiplexing transmission, in which a combination of dopants in the fibre core enables a high signal/noise ratio to be achieved for all channels in a predetermined wavelength band, even in the presence of several simultaneously fed signals. <IMAGE>

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10 claims: 2 independent, 8 dependent
- 1Zastrzeżenia patentowe 1. Sposób telekomunikacji światłowodowej, obejmujący etapy, w których wytwarza się przynajmniej dwa sygnały świetlne o określonych długościach fal leżących w określonym zakresie zawierającym pasmo około 1535-1540 nm, z których przynajmniej jeden leży w paśmie około 1535-1540 nm, dostarcza się wspomniane sygnały do włókna optycznego w światłowodowej linii telekomunikacyjnej, wzmacnia się przynajmniej jednokrotnie wspomniane sygnały świetlne w przynajmniej jednym wzmacniaczu optycznym zawierającym włókno aktywne oraz odbiera się sygnał w stacji odbiorczej, przy czym przynajmniej jeden wzmacniacz optyczny zawiera aktywne włókno optyczne o bazie krzemowej, którego rdzeń jest domieszkowany erbem jako główną domieszką fluorescencyjną i przynajmniej jedną domieszką dodatkową, znamienny tym, że dobiera się domieszki względem siebie w takiej funkcyjnej zależności, że stosunek sygnału do szumu sygnału świetlnego w odbiorniku, mierzony przy szerokości pasma filtra 0,5 nm, nie jest mniejszy niż 15 dB dla wspomnianych sygnałów.
- 2Sposób według zastrz. 1, znamienny tym, że wzmacnia się sygnał świetlny przynajmniej dwukrotnie za pomocą odpowiednich wzmacniaczy optycznych zawierających włókno aktywne, umieszczonych szeregowo wzdłuż włókna światłowodowego.
- 3Sposób według zastrz. 1, znamienny tym, że określony zakres długości fal jest zawarty między 1530 a 1560 nm.
- 4Sposób według zastrz. 3, znamienny tym, że określony zakres długości fal jest zawarty między 1525 a 1560 nm.
- 5Sposób według zastrz. 1, znamienny tym, że dobór domieszki w aktywnym włóknie wzmacniacza optycznego obejmuje użycie przynajmniej dwóch dodatkowych domieszek w rdzeniu włókna aktywnego.
- 6Sposób według zastrz. 5, znamienny tym, że dobór domieszki w aktywnym włóknie obejmuje użycie germanu, aluminium i lantanu w postaci odpowiednich tlenków, jako domieszek dodatkowych w rdzeniu włókna aktywnego.
- 7System telekomunikacyjny światłowodowy, zawierający stację nadawczą, wytwarzającąprzynajmniej dwa sygnały świetlne o różnych długościach fal leżących w określonym zakresie, zawierającym pasmo około 1535-1540 nm, z których przynajmniej jeden leży w paśmie około 1535-1540 nm, stację odbiorczą oraz łącze, światłowodowe między stacją nadawczą, a stacją odbiorczą, przynajmniej jeden wzmacniacz optyczny, zawierający włókno aktywne, połączony szeregowo w łączu dla przesyłania sygnałów ze stacji nadawczej do stacji odbiorczej, znamienny tym, że przynajmniej jeden wzmacniacz optyczny zawiera aktywne włókno optyczne wykonane na bazie krzemu, mające rdzeń domieszkowany erbem jako główną domieszką fluorescencyjną i przynajmniej jedną domieszką dodatkową, które są dobrane funkcjonalnie w taki sposób, że stosunek sygnału do szumu w stacji odbiorczej, mierzony przy szerokości pasma filtra wynoszącej 0,5 nm, jest większy lub równy 15 dB wspomnianych sygnałów.
- 8System według zastrz. 7, znamienny tym, że domieszkami dodatkowymi są aluminium, german i lantan w postaci odpowiednich tlenków.
- 9System według zastrz. 7, znamienny tym, że określone pasmo przesyłania jest zawarte między 1530 a 1560 nm.
- 10System według zastrz. 7, znamienny tym, że przynajmniej dwa wzmacniacze są połączone szeregowo wzdłuż linii. * * * 177 262
Independent claims10
188 paragraphs in 7 sections, as filed
The present invention relates to a method of fiber optic telecommunications and a fiber optic telecommunications system with amplification for transmitting a wavelength multiplexed multiplex signal, in particular comprising optical amplifiers. The method of transmitting the multiplexed wavelength signal is briefly referred to as WDM (Wavelength-Division Multiplexing) transmission.
In the case of WDM transmission, it is required to send multiple channels, or many independent signals, on the same fiber optic line with the use of visible frequency multiplexing. Transmission channels can apply to both digital and analog signals, and differ from each other because each of them is associated with a specific frequency.
In this type of transmission, all channels must be equivalent, i.e. none of them can be more or less privileged than the other in terms of signal level or quality.
In the case of amplifiers, in particular optical amplifiers, their identical dynamic properties are required for all channels. In addition, to ensure the transmission of a large number of channels, their bandwidth must be wide.
Optical amplifiers are based on the properties of dopants, characterized by fluorescence spectra, in particular on the properties of erbium, introduced as an admixture into the fiber core. It is erbium excited energetically by means of light to the pumping band that shows strong emission in the wavelength range corresponding to the minimum attenuation of silicon-based optical fibers.
When a light signal with the wavelength corresponding to said strong emission flows through the erbium doped fiber and in which the erb is kept excited, it causes the transition of the excited erbium atoms to a lower energy level and the emission of light with the wavelength of this light signal light signal.
Starting from the excited state, the energy transition of erbium atoms also occurs spontaneously, which results in random emission, producing background noise coinciding with the stimulated emission corresponding to the amplified signal.
The emission of light produced from the incoming light energy corresponding to the pumping band of the active or doped fiber can occur at many wavelengths characteristic of doping substances. So you can talk about the fluorescence spectrum of the fiber.
In order to obtain the fiber of the above-mentioned type of the highest gain and high signal-to-noise ratio, favorable from the point of view of signal reception, in fiber optic telecommunications, laser emitters with a wavelength lying in the band corresponding to the maximum on the characteristics of the fluorescent spectrum of the fiber containing doped or wavelength substances only
On the other hand, erbium doped fibers have an emission spectrum containing a limited width apex, and its parameters will change depending on the fiber material being subjected to erbium doping. Within the intriguing wavelength range, these fibers are characterized by a strong spectrum in the area adjacent to the emission peak. Thanks to this, it becomes possible to use optical amplifiers to amplify signals in a wide band.
However, known erbium doped fibers do not have stable emission spectrum characteristics. This uneven characteristic curve conditions the possibility of obtaining the same gain in the whole selected band.
In order to obtain a really "flat" gain curve, which means that the gain for different wavelengths is as constant as possible, while eliminating sources of noise resulting from spontaneous emission, filter elements such as those described in Patent EP 0426222, EP0441211, EP0417441.
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However, these patents do not describe the behavior of amplifiers under multiplexing wavelength signals, moreover, the analysis of their behavior did not take into account the cascading of multiple amplifiers together.
The characteristics of the emission spectrum significantly depend on the admixtures introduced into the fiber core to increase the refractive index. For example, this is disclosed in US Pat. No. 5,282,799, which shows that the fluorescence spectrum of the doped aluminum and erbium fiber has a less pronounced peak than the spectrum of the doped germanium and erbium fiber, and is shifted towards lower wavelengths (maximum occurs at a wavelength of about 1532 nm). This fiber has a numerical aperture - NA (optical opacity) 0.15.
ECOC, 93, ThC 12.1, pages 1-4, describes a fiber for an optical amplifier doped with aluminum (Al) and lanthanum (La) that has a very poor response to hydrogen. The Al-doped fiber described therein has a numerical aperture of 0.16, while the Al-La doped fiber has a numerical aperture of 0.30.
ECOC, 93, Tu4, pages 181-184, describes an optical amplifier containing erbium doped fiber. Experiments conducted with fibers whose cores were doped with aluminum, aluminum / germanium and lanthanum / aluminum were discussed. The best results were achieved with simultaneously doped aluminiuin / lanthanum fibers.
In the issue 12 Electronics Letters of June 6, 1991, vol. 27, on pages 1065-1067, it is pointed out that in optical amplifiers containing erbium-doped fiber and at the same time doped aluminum it is possible to achieve higher and flatter characteristics. The article also describes amplifiers containing aluminum, germanium and erbium doped fiber and compared with amplifiers containing lanthanum, germanium and erbium doped fiber. It was found that the largest flattening of the gain characteristic is obtained in the first case.
In the ECOC publication of 91, TuPS1-3, on pages 285-288, Al2O fiber is described<sub>3</sub>and O<sub>2</sub> doped with erbium and lanthanum to obtain a higher refractive index and to reduce the formation of clusters containing erbium ions. It is found that the fluorescence and absorption spectra of Er / La doped fibers are very similar to those of Al fibers<sub>2</sub>ABOUT<sub>3</sub>-SiO2 doped with erbium. A numerical aperture (NA) of 0.31 and an erbium concentration of 23-10 was obtained<sup>! S</sup> cm '<sup>3</sup>.
In Post-Deadline Papers (EC) from 89 years, PDA-8. on pages 33-36, issue 10-14 September 1989, describes an experiment with twelve cascaded optical optical amplifiers containing erbium doped fiber. A single signal with a wavelength of 1536 nm was used. It was found that for stable operation it is required to maintain a constant wavelength with an accuracy of 0.01 nm, due to the strong dependence of the transmission error rate (BER) on changes in wavelength.
US 5117303 describes a fiber optic transmission system containing blocked optical amplifiers which, based on the presented calculations, working under saturation conditions, give a high signal-to-noise ratio. The amplifiers described contain Al fibers<sub>2</sub>ABOUT<sub>3</sub>-SiO2 doped with erbium. They also provide for the use of filters. The calculated quality is achieved at one wavelength. The same quality cannot be achieved when supplying a system with a wide wavelength signal.
A method of fiber optic telecommunications, comprising the steps of producing at least two light signals of specified wavelengths lying in a specified range comprising a band of about 1535-1540 nm, of which at least one lies in a band of about 1535-1540 nm, providing said signals to the fiber optical in a fiber optic telecommunications line, at least once the said light signals are amplified in at least one optical amplifier containing the active fiber and the signal is received at the receiving station, at least one optical amplifier containing the active optical fiber with silicon base, whose core is doped with erbium as the main fluorescent admixture and at least one admixture additional, according to invented 262 zku, it is distinguished by that the admixtures are selected in relation to each other in such a functional relationship that the signal-to-noise ratio of the light signal in the receiver, measured at 0.5 nm filter bandwidth, is not less than 15 dB for said signals.
The light signal is preferably amplified at least twice by means of suitable optical amplifiers containing active fiber arranged in series along the optical fiber.
The specific wavelength range is preferably between 1530 and 1560 nm, more preferably between 1525 and 1560 nm. The choice of dopant in the active fiber of the optical amplifier preferably includes the use of at least two additional dopants in the active fiber core, preferably germanium, aluminum and lanthanum in the form of suitable oxides of the active fiber core.
Fiber optic telecommunications system, including a transmitting station, generating at least two light signals of different wavelengths lying in a specified range, containing a band of about 1535-1540 nm, of which at least one lies in the band of about 1535-1540 nm, a receiving station and a fiber link between the station transmitting and receiving station, at least one optical amplifier containing active fiber, connected in series on the link for transmitting signals from the transmitting station to the receiving station, according to the invention, it is distinguished by the fact that at least one optical amplifier comprises an active optical fiber made of silicon, having an erbium doped core as the main fluorescent admixture and at least one additional admixture, which are functionally selected in such a way that the signal to noise ratio at the receiving station, measured at a filter bandwidth of 0.5 nm, is greater than or equal to 15 dB of said signals.
The additional admixtures are preferably aluminum, germanium and lanthanum in the form of suitable oxides.
The specific bandwidth is preferably between 1530 and 1560 nm.
In a preferred embodiment of the system, at least two amplifiers are connected in series along the line.
According to the present invention, it has been found that a particular combination of dopant substances contained in the 'active fiber core makes it possible to make a fiber having a high numerical aperture and emission spectrum whose characteristics allow the implementation of optical amplifiers. These amplifiers used in systems employing wavelength division multiplexing give the same response at different wavelengths in the predicted waveband. This applies to both the case of a single amplifier and the case of multiple cascaded amplifiers.
The active fiber is characterized by an emission curve showing a high emission zone in the wavelength range containing a predetermined wavelength band within which there is a reduction in emissions relative to adjacent zones. The improvement of fiber properties is expressed by eliminating or reducing the reduction of the emission curve by choosing the type and amount of dopants in the active fiber.
The choice of dopants for fibers includes the use of a main fluorescent admixture and at least one additional admixture interacting with the dopant, the main in the glass substrate of the active fiber. The goal is to reduce the discussed reduction in the characteristic to less than 1dB, compared to the emission value in at least one of the zones adjacent to the band in question.
The solution, according to the invention, in the examples is reproduced in the drawing, in which Fig. 1 shows the diagram of the amplifier, Fig. 2 - diagram of the amplifier with window filter, Fig. 3 - diagram of the experimental system for determining emission spectra diagrams for various types of optical fibers, Fig. .4 - emission spectra plots for various types of active fibers, determined using the experimental setup shown in Fig. 3, Fig. 5 - the amplifier gain characteristics shown in Fig. 1 for signals with different wavelengths and two different input power levels, determined for the fiber according to the invention, fig 6 - the amplifier gain characteristics shown in fig. 2 for syg6
177 262 lead with different wavelengths and three different input power levels, determined for the fiber according to the invention, Fig. 7 - amplifier gain characteristics shown in Fig. 2 for signals with different wavelengths and three different input power levels, determined for the known fiber , fig. 8 - schematic diagram of the experimental transmission system in which there were multiple cascaded amplifiers and two signals of different wavelengths multiplexed on the same wavelength split, Fig. 9 - BER (bit error rate) graph determined during the experiment carried out according to the scheme shown in Fig. .8 using various amplifiers, Fig. 10 - schematic diagram of the experimental transmission system in which there were multiple cascaded amplifiers and four signals of different wavelengths multiplexed on the same wavelength split, Fig. 11 - signal power levels at the input of the first gain stage during the experiment carried out in Fig. 10 , using the amplifiers of the invention, Fig. 12 - signal power levels at the input of the second amplification stage during the experiment carried out according to Fig. 10, Fig. 13 - levels of signal power at the input of the third amplification stage during the experiment carried out according to Fig. 10, Fig. 14 - signal power levels at the input of the fourth stage gain during the experiment carried out according to Fig. 10, Fig. 15 - signal power levels at the preamplifier input during the experiment carried out according to Fig. 10, and Fig. 16 shows the signal power levels at the preamplifier input during the experiment carried out according to Fig. 10, using amplifiers of known type.
In Figure 1, the amplifier constituting the link amplifier comprises one active fiber 1 doped with erbium and a special pumping laser 2 attached to it by means of a dichroic connector 3. One optical insulator 4 is directed towards the fiber 1, in the direction of the amplified signal, and the other optical isolator 5 is directed to further sections of this active fiber. It is advantageous, although it is not necessary, that the dichroic connector 3 is positioned (as shown) with the current of the active fiber 1 so that it supplies pumping energy in counter-current to the signal. The further amplifier contains an erbium-doped second active fiber 6, associated with a special pumping laser 7 connected via a dichroic connector 8, connected in the example also countercurrently. For this reason, the further optical isolator 9 is oriented with the fiber current 6.
The pumping lasers 2 and 7 are of the Quantum Well type and have the following features:
- emitted wavelength λ<sub>ρ</sub>= 960 nm;
- maximum light signal output at the P output<sub>at</sub>= 89 mW.
Lasers of this type are known. Dichroic connectors 3 and 8 are fused from single-mode optical fibers for 980 nm and in the 1530-1560 nm band, with fluctuations in light output, depending on polarization, <0.2 dB. Dichroic joints of the above type are known and commercially available.
Optical insulators 4, 5, 9 are insulators in which the polarization control is independent of the polarization of the transmitted signal, they have insulation greater than 35 dB and reflection coefficient less than -50 dB.
The diagram shown in Fig. 2 is another embodiment of the amplifier. The corresponding elements in this diagram have been given the same reference numbers as in Figure 1. In this amplifier, whose elements have the same properties as the elements described above, there is a window filter 10, consisting of a part of an optical fiber having two optically coupled cores, one of which at a predetermined wavelength is coaxial with the attached optical fiber, the other is misaligned and cut off at the ends. This filter is sized to couple the wave or waveband corresponding to the emission spectrum of the amplifier to the non-coaxial core. The non-coaxial core, cut off at the ends, causes the wave with a length introduced into it to be dispersed in the fiber sheath so that it cannot be reintroduced into the main core.
177 262
In the presented example, the dual-core filter 10 has the following features: bandwidth coupled with the second core BW (band-3 dB) 8-10 nm filter length 35 nm
The filter had such dimensions that the greatest tuning possibilities were at the top of the emission curve of the active fiber used.
In the conducted tests, filters with the following parameters' attenuation at λ were used interchangeably <sub>s</sub> 1530 nm 5 dB or attenuation at λ s 1532 nm 11 dB.
This filter aims to reduce the signal strength in the highlighted band, in particular the peak of the fiber emission curve. This serves to obtain a maximum flat amplifier gain characteristic in the event of changing wavelengths. This requirement is particularly important when transmitting Wavelength Division (WDM) signals where it is desired to maintain equal gain conditions for all channels as accurately as possible.
Parameters of different types of active fibers doped with erbium used in the amplifiers described above and their optical properties are given in Table 1.
Table 1
<td rowspan="2">Fiber</td><td colspan="2">AbOj</td><td colspan="2">GeO,</td><td colspan="2">La<sub>2</sub>ABOUT<sub>3</sub></td><td colspan="2">er<sub>2</sub>ABOUT<sub>3</sub></td><td>ON</td><td></td>
<td>wt%</td><td>(moth%)</td><td>wt%</td><td>(moth%)</td><td>wt%</td><td>(moth%)</td><td>wt%</td><td>(moth%)</td><td></td><td>nm</td>
<td>AND</td><td> 4</td><td> (2,6)</td><td> 18</td><td> (11,4)</td><td> 1</td><td> (0,2)</td><td> 0,2</td><td> (0,03)</td><td> 0,219</td><td> 911</td>
<td>B</td><td> 1,65</td><td> (1,1)</td><td> 22,5</td><td> (14,3)</td><td> 0</td><td> (0)</td><td> 0,2</td><td> (0,03)</td><td> 0,19</td><td> 900</td>
<td>C</td><td> 4</td><td> (2,6)</td><td> 18</td><td> (11,4)</td><td> 0</td><td> (0)</td><td> 0,2</td><td> (0,03)</td><td> 0,20</td><td> 1025</td>
<td>D</td><td> 4</td><td> (2,6)</td><td> 0</td><td> (0)</td><td> 3,5</td><td> (0,7)</td><td> 0,2</td><td> (0,03)</td><td> 0,19</td><td> 900 __1</td>
where:
wt% = (average) percentage of oxide in the core in relation to mass, mol% = (average) percentage of oxide in the core in relation to molar mass, NA = numerical aperture (n1 - n2), λς = cut-off point (LP11) .
Composition analysis was performed on a preform (before fiber extraction) using a micro sample and scanning electron microscope (SeM Hitachi). The analyzes were conducted at 1300 magnifications, at discrete points distributed along the diameter and spaced apart at 200 pm. The fibers considered were made by vacuum deposition technique inside a quartz glass tube. Introducing germanium admixtures into the SiO network<sub>2</sub> in the core of the fiber concerned is carried out in the synthesis process. The introduction of erbium, aluminum and lanthanum into the core took place in solution doping technology in which the aqueous chloride solution of the dopant comes into contact with the material forming the fiber core, while it is in a special condition before the preform cures.
More details regarding solution technology can be found, for example, in US Pat. No. 5,282,079.
The higher value of the numerical aperture (NA) for fiber A in relation to the compared fibers was caused by the fact that during the fiber core the change of the flow of the reagent previously selected for fiber C (Al / Ge / Er) was not carried out, in particular it was forgotten to close germanium supply. The resulting introduction of lanthanum and aluminum during the application of the technologist of the solution led to a refractive index in the core higher than expected, in addition to the unexpected characteristics of the amplification and transmission conditions
177 262
The structure of the experimental system under consideration adapted to determine the emission spectrum of the fiber is schematically shown in Fig. 3, while the emission spectra plots determined for the active fibers A, B, C, D are quoted in Fig. 4. The pumping laser diode 11, corresponding to the wavelength 980 nm, was connected to the tested active fiber 13 via a dichroic junction 12. The emission of the fiber was tested using a light spectrum analyzer 14. Laser diode 11 gave a power of about 60 mW (in fiber 13). The active fiber 13 was of a length suitable for effective reinforcement for the assumed pumping power. For the tested fibers, each of which had the same erbium content, this length was about 11m. For fibers with different erbium content. the correct length is determined using known criteria. The light spectrum analyzer was the TQ8345 model. The measurements were carried out by pumping the fiber with 980 nm wavelength and testing the spontaneous emission spectrum of the fiber. The results obtained are shown in Fig. 4, in which curve 15 corresponds to fiber A, curve 16 - fiber B, curve 17 - fiber C, curve 18 corresponds to fiber D. As the graphs show, the emission spectrum for B, C, D fibers has a major peak with high intensity giving a maximum at about 1532.5 nm, and the resulting high emission zone for larger wavelengths extends to about 1560-1565 nm, including an additional a very broad peak. A comparison of curves 16 and 17 (determined for B and C fibers, respectively) shows that the higher aluminum content in the fiber increases the level of this high emission zone. Replacement of germanium with lanthanum (D fiber, curve 18) enables an even higher level to be reached in the 1535-1560 nm range.
At the same time, for all B, C, D fibers a decrease in spectrum characteristics can be seen in the d zone (located approximately between 153 and 1540 nm), contained between the main peak of the emission characteristics with adjacent, and the additional peak. In this reduction, the emission value is at least 2 dB lower compared to the emission value in adjacent zones, i.e. in relation to both the main and the additional peak. In the graph, this has been marked with the value only for curve 16, but it can also be clearly read for curves 17 and 18.
Conversely, curve 15 shows that under the given experimental conditions fiber A does not show a clear reduction in the spectrum d of the spectrum characteristics or, when this reduction can be determined, it is always lower than about 0.5 dB. Curve 15 also shows that the peak of maximum emission for fiber A, being located at about 1530 nm, occurs at a lower wavelength than for fibers B, C, D and that the fiber has a high emission level almost from 1520 nm.
The amplifiers whose structures were shown in Fig. 11 Fig. 2 were made of fibers A. The first active fiber 1 was about 8 m long, and the second active fiber 6 was in the cases given in Fig. 1 and Fig. 2 respectively about 15 and 13 m.
The curves in Fig. 5 give the gain for different wavelengths at two different levels of input power in the amplifier system shown in Fig. 1. The curves in Fig. 6 give the gain for different wavelengths for three different levels of input power in the amplifier system shown in fig. 2.
In particular, in the case of the amplifier shown in Figure 1, curve 19 in Figure 5 refers to an input power of -20 dBm, while curve 20 refers to an input power of -25 dBm.
In turn, for the amplifier shown in Figure 2, curve 21 in Figure 6 refers to the input signal power -20 dBm, curve 22 refers to the input signal power -25 dBm, and curve 23 refers to the input signal power -30 dBm.
As can be read from the charts, in particular by comparing curves 19 and 21 corresponding to a power level - 20 dBm, which is particularly interesting in telecommunications and determined in systems without a filter and with a filter, the use of fiber containing an aluminum-doped core, germanium and lanthanum as erbium supplements, enables flat gain characteristics, particularly in the area between 1536 and 1540 nm. The same result can also be achieved in a system without a filter.
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In particular, without a filter, at -20 dBm, the gain difference for signals with different wavelengths was less than 1.6 dB, while with a filter, at -20 dBm, the gain difference for signals with different wavelengths was less than 0, 9 dB.
The curves shown in Fig. 7 provide gain at different wavelengths for three different levels of input power in an amplifier having the structure shown in Fig. 2 and made of fiber C (Al / Ge / Er).
In particular, curve 24 in Fig. 7 refers to an input signal power of -20 dBm, curve 25 refers to an output of -25 dBm. and curve 26 refers to an input signal power of -30 dBm. At -20 dBm, the gain difference for signals of different length was about 2.1 dB.
Comparing it can be concluded that also in an amplifier without a filter, the A (Al / Ge / La / Er) fiber provides a flatter gain characteristic than the C (Al / Ge / Er) fiber in an amplifier equipped with a filter.
Long distance transmission tests have been carried out with many amplifiers in a cascade, i.e. in series. The amplifiers made according to FIGS. 1 and 2 were used either with fiber A (Al / Ge / La / Er) or fiber C (Al / Ge / Er). One of the experimental setups is shown in Fig. 8. One signal 27 with a wavelength λ, = 1536 nm and the other signal 28 with a wavelength λ 2 = 1556 nm · were fed to fiber 29 via multiplexer 230.
One attenuator 31 has been positioned in accordance with the direction of wave transfer from the power amplifier 32a. The remaining, further suppressors 31, identical in terms of properties, were placed in the track along which four amplifiers 32, 32 ', 32, 32' connected with each other on the way to the receiver 33 Receiver 33 was preceded by an optical demultiplexer 34 consisting of an interference filter with a bandwidth of 1 nm at -3 dB. by means of which the selection of the wave subject to detection took place. Both 27.28 signals generated by the respective lasers had a power of 0 dBm. The total switched power in fiber 29 was 0 dBm (as a result of 3 dB coupling loss). Multiplexer 30 was a known 1x2 coupler. The 32a power amplifier was a commercially available optical amplifier with the following features:
input power from -5 to +2 dBm output power 13 dBm working wavelength 1530-1560 nm.
The power amplifier had no window filter. The amplifier uses erbium doped C-type active fiber (Al / Ge / Er). By power amplifier is meant an amplifier operating under saturation conditions and in which the output power depends on the pumping power. After the first attenuator 31, at the input of the amplifier 32, the total light signal power was about -18 dBm. The well-known Va5 muffler was used as the 31 suppressor. Each of them provided 30 dB attenuation corresponding to about 100 km of optical fiber. The 32. 32 ', 32' ', 32' '' amplifiers were identical and each gave a gain of about 30 dB for both wavelengths λ and λ<sub>2</sub>, at +12 dBm total output power.
Signal 27 with a wavelength λ, = 1536 nm was a signal directly modulated with a 2.5 Gbit / s signal, generated by the DFB laser included in the SLX-1/16 model SDH available on the market. This camera was used as a 33 receiver.
Signal 28 with a wavelength λ2 = 1556 nm was a continuous signal (CW), generated by the DFB laser model MG0948L3 with a power of 0 dBm. Model TB4500 interference filter was used.
Example 1. In the first example, amplifiers using A (Al / Ge / La / Er) fiber were used, without a window filter 10, having the structure shown in Fig. 1.
Example 2. In the second example, amplifiers using fiber A (Al / Ge / La / Er), with window filter 10, having the structure shown in Fig. 2 were used. For the signal with wavelength λ and (1536 nm) the error rate was measured transmission (BER) when the average power received by the receiver changes 33.
The results are presented in the form of a graph in Fig. 9, where curve 35 refers to example 1 and curve 36 to example 2. As can be seen from the graph in fig. 9, despite the fact that
177 262 gain characteristics of a single amplifier using the A (Al / Ge / La / Er) fiber and having a window filter was identical, or even flatter than for an amplifier without a 10 window filter but also using the A fiber, this signal was reflected on the 1536 nm signal negatively more, giving a clearly higher error rate with equal received power.
Example 3. The scheme of the second of the experimental schemes used is shown in Fig. 10. In this experiment, four signals 37, 38.39.40 with lengths λ, = 1536nm, λ were input into fiber 41 through fiber 41<sub>2</sub>= 1556nm, X<sub>3</sub>= 1550 nm and λ4 = 1544 nm.
The signal level at the input to the link was tuned by means of the initial equalizer 43. The signals after exiting the power amplifier 44 were sent to the link containing four 45, 45, 45, 45 '' amplifiers having corresponding attenuators 46 placed between the amplifiers in order to simulation of optical fiber sections. The receiving station consisted of a preamplifier 47, a light signal demultiplexer 48 and a receiver 49. The corresponding signals were generated by a DFB laser with a wavelength of 1536 nm directly modulated by a 2.5 Gbit / s signal, which is part of the apparatus forming the receiver 49, by a DFB laser with a wavelength of 1556 nm, giving a continuous signal, by a DFB laser with a wavelength of 1550 nm, giving a continuous signal, by an ECL laser with variable but predetermined wavelength of 1544 nm, giving a continuous signal, model HP81678A.
The initial equalization system 43 consisted of four variable dampers 43a. Their setting was carried out depending on the power of the light signal in the appropriate channel. Multiplexer 42 was made using a 1x4 splitter. The 44 power amplifier was the TPA / E-13 model already described, commercially available. Each of the 45, 45 ', 45' ', 45' amplifiers was the same and gave a gain of about 30 dB with a total output power of +12 dBm. The amplifiers 45 have the structure shown in Fig. 1, and they use A fibers (Al / Ge / La / Er). Each attenuator 46 introduces a 30 dB attenuation corresponding to approximately 100 km of optical fiber length.
The light attenuators were VA5 models.
The 47 preamplifier, commercially available, has the following characteristics:
gain 22 dB noise factor <4.5 dB output power from -26 to -11 dBm operating band 1530-1569 nm
The commercially available RPA / EF amplifier used was C-type active fiber (Al / Ge / Er). A pre-amplifier should be considered an amplifier selected to receive a very weak signal (e.g. -50 dBm) and amplify it before sending it to the receiving device after the signal reaches the appropriate power for that device. The 48 light signal demultiplexer consists of a Fabry-Perot filter with a tunable wavelength of 0.8 nm (-3 dB). This filter is included in the pre-amplifier 47. To perform the experiment, the Fabry-Perot filter was tuned to wavelength λ = 1536 nm (found as critical) by the pilot signal sent by the transmitter 37. The receiver 49 consists of the SDH model SLX-1/16 , commercially available.
Figures 11 to 15 show the signal passing through successive stages, in particular at the input of the 45, 45 ', 45, 45' amplifier and at the input of the preamplifier 47, respectively. The pre-equalizer 43 is used to perform initial signal correction to approximately 7 dBm between different channels, as shown in Fig. 11. This is to compensate for saturation effects at lower wavelengths occurring in the cascade of amplifiers.
Pre-equalization was carried out in such a way that the signal-to-noise ratio (S / N) at the output of the pre-amplifier 47 could be corrected.
In the next levels of gain you can see a decrease in the gain characteristics in the region of smaller wavelengths caused by the saturation phenomenon described above,
177 262 while the S / N ratio in each channel remained high (S / N 15 dB for Δλ = 0.5 nm) up to the preamplifier output 47.
In the experiment carried out using the amplifiers according to the scheme of Fig. 2, containing the C-type active fiber, both a strong reduction in signal power at 1536 nm and 1544 nm as well as a large image of the signal-to-noise ratio between different channels was found, as can be seen from Fig. 16 showing the power in different channels at the preamplifier input. Even more negative effects can be expected for a channel operating in a 1540 nm wavelength environment.
In this case, pre-equalization could cause an imbalance between different channels (in some, especially those working between 1535 and 1540 nm, this would cause greater damage than in others) which should be limited. As a result of such pre-equalization, the acceptable signal-to-noise ratio could not be maintained in all cases for all signals in the band of interest. In fact, in order to make it possible to pre-equalize the channels, the most-favored channels (1550 and 1556 nm) should be strongly suppressed at the beginning, which could lead to very low values of S / N ratios (in the order of 8-10 dB), which would make impossible the correct reception of signals transmitted.
It can be assumed that the obtained better results compared to the case of using an amplifier equipped with a window filter and Al / Ce / Er fiber result from the fact that for fiber A the emission characteristics practically show no decreases or local minima with a clear value, and in particular has no minimum in the wavelength range adjacent to the peak emission characteristics in the zone 1535 - 1540 nm.
In fact, it can be assumed that when several signals of different wavelengths are introduced into the fiber, the existence of depressions or local minima on the emission characteristics (appearing in the spectra of the compared fibers) cause that the signals with the wavelengths corresponding to the said depressions are amplified less than the signals of wavelengths lying in adjacent ranges.
According to the above interpretation, greater signal gain for wavelengths from adjacent areas reduces pumping energy for a signal that is saturated to the lower level (which means that this level after amplification no longer depends on the input level but only on the pumping power available in the fiber) thereby increasing the level difference between the different signals.
In the presence of a cascade of amplifiers and WDM transmission, such a phenomenon deepens in every degree and it can be thought that it is responsible for the observed inequality of response, which cannot be compensated by initial equalization or otherwise similar to the discussed one.
It has been noticed that the above phenomenon occurs for signals corresponding to the decreases in the emission curve and depending on the result of the comparison of the signal gain for the wavelength in the vicinity of these depressions, and it does not (or at least not to the same extent) for signals with wavelengths on the useful edges bandwidth, although for these lengths the emission value may be equal to or lower than that in the discussed reductions.
According to the smaller invention, the incorporation of lanthanum into the Al / Ce / Er fiber unexpectedly made it possible to eliminate these local minima on the emission characteristics, although this could not be predicted on the basis of the available data for the Al / La / Er and Al / Ce / Er fibers.
In fact, the Al / La / Er and Al / Ce / Er fibers show a significant reduction in the emission characteristics in the 1535-1540 nm zone, and therefore, knowing the design of these popular fibers, various favorable behaviors of the Al / Ce / La / Er fiber could be excluded and that such fiber could enable amplified multiplexed wavelength sharing.
It was unexpectedly discovered that, according to another, even more important feature, when there is a peak in the high emission zone, the presence of this depression in the area adjacent to the peak, or in any case its unfavorable relation to the adjacent zones, was
177 262 responsible for the insufficient signal-to-noise ratio for the signals in the discussed lowering and that the active fiber that is capable of spontaneous elimination or reduction of this lowering allows to solve the problem thanks to the possibility of multiplexed wavelength transmission in a system with one or more amplifiers.
Hence, according to the present invention, it was found that active fiber whose admixtures give an emission curve with a relatively high value in the wavelength band, not showing local depressions in the zone within this band and being in functional dependence with the other zones in this band (such that could cause significant differences in gain for telecommunications signals for different wavelengths in their multiplexing band in the fiber), they enable the production of amplifiers particularly suitable for use in telecommunications links containing at least two optical amplifiers divided in series with wavelength division for multiplexed signals. This solution ensures high quality.
On the other hand, according to the smaller invention, it has been discovered that controlling the S / N signal to noise ratio in the transmission systems discussed here can be obtained not only by means of filters or by adjusting the transmission bandwidth of limited width (capable of bypassing uncomfortable wavelength zones), but by choosing the type and concentration of dopants in the core of the active amplifier fiber. This choice should be like this. that the emission characteristics that can be drawn in a sufficiently wide band (extending from 1525 to 1560 nm, or at least from 1530 to 1560 nm) do not result in an undesirable increase in signal gain in one or more selected zones of the emission curve, although the emission peak in the band in question occurs.
By functional dependence it should be understood, as it was explained above, that the occurrence of increased emission in the zones adjacent to the depressions, in particular the emission peak and the presence of signals in these adjacent zones will have negative effects on signal amplification at the wavelengths corresponding to the said depressions.
By the content of the concept of an emission curve (or spectrum) having a relatively large value in the wavelength band it should be understood that in a given wavelength band, most preferably between 1525 and 1560 nm, the pumped fiber has an emission exceeding the emission outside this band and it is adapted to signal amplification in the discussed band. As the indicator shows, this zone is determined as a zone between two extreme values at which the emission is 3 dB lower than that contained within the range or band (most preferably in relation to this zone in the band where the emission is constant). In fact, this band corresponds to the band in which the enhancement operation can be successfully carried out.
The peak of emission should be understood as the occurrence in a certain range of emission wavelengths much higher than in other spectral zones - outside this range. This causes different properties of the optical fiber with respect to the signals input into it for the wavelengths' inside and outside the compartment in question.
A significant difference in gain in the discussed band should be understood, for example, a difference higher than 2 dB between the gains in the most and least-favored wavelength range (with input power equal to or less than -20 dBm).
Local reduction of the emission curve should be understood as the wavelength range within the band at which the minimum emission occurs, i.e. less than on the edges of this range. The emission value should also be smaller by the previously determined value in relation to the values in the adjacent wavelength ranges (in particular the erbium for the main emission peak, below the lowering and the second peak, for higher wavelengths). For the purposes of the present invention, said lowering greater than 0.5 dB. and more practical - greater than 1 dB produces noticeable effects.
Attention to the above considerations will be able to determine the specific operating conditions and specific content of admixtures by selecting them for the intended use in order to achieve specific end results.
1ΊΊ262
Within the scope of the present invention, acting on fibers containing the main dopant, most preferably erbium, when it concerns applications in telecommunications, which exhibits fluorescent features in the interesting wavelength range, in combination with other dopants cooperating there on the basis of adding or mixing features, specific admixtures or combinations thereof and their respective compositions may be determined to obtain changes in fiber emission curves and corresponding versions of amplifiers and systems based thereon, both lasers, optical gyroscopes and the like, as well as transmission, telecommunications or measuring systems in which they are used , as well as to obtain the desired quality from the point of view of the signal to noise ratio within the interesting band.
In a special area of particular interest, research was limited to erbium as the main fluorescent admixture and Ge, Al, La introduced into the fiber in the form of oxides as additional dopants, since the results of these tests were sufficient to solve specific technical problems.
The guidelines given in the present invention can be used to solve their own problems by those familiar with this technique in an average way. These problems may be similar or different to those described here. It is also possible to test various admixtures and their dosing in the same way as it was tested and described, to apply the results in practice or to use the same functional relations between the results and the means used. These persons are advised not to abandon trials with individual admixtures or sets until their completion, even if the partial results considered separately are unsatisfactory from the point of view of the signal-to-noise ratio. The combination of admixtures can provide better results, as is seen in the description of the examples according to the present invention m 262
<img file="PL177262B1_D0001.tif" />
<img file="PL177262B1_D0002.tif" />
1ΊΊ262
<img file="PL177262B1_D0003.tif" />
-λ- (nm)
Fig. 6
<img file="PL177262B1_D0004.tif" />
Fig. 5
1560 λ (ηη)
177 262
<img file="PL177262B1_D0005.tif" />
A (nm)
Fig. 7
<img file="PL177262B1_D0006.tif" />
Fig. 9
177 262
<img file="PL177262B1_D0007.tif" />
177 262
MS9701B
AKKR A: 1.5405jin> frl.SńSjmn BA - '? T.5nm
LMKR C: -26.62SdBm D ~ 3t625dftn C-D4i) B
<img file="PL177262B1_D0008.tif" />
Figure 11
SPECTRUM
ΓΠ 262
MS9701B
<td>AMKR</td><td>AND<sup>:</sup>t5405um</td><td>8-A <24.Sujm</td><td> —</td>
<td>IMKR</td><td>C<sup>;</sup>-26.625dBm</td><td>& -3t.625dBm CD'8dB</td><td></td>
-25dBm
5dB / d
40dBm
-55dBm
<td>TM</td><td>(R</td><td></td><td></td><td>AND</td><td></td><td></td><td></td><td></td><td></td>
<td rowspan="2">1 EREF</td><td rowspan="2">.5555 -24dB</td><td>um</td><td></td><td></td><td>t And 9</td><td></td><td></td><td></td><td></td>
<td>ΓΠ</td><td></td><td></td><td>r- AND</td><td></td><td></td><td></td><td></td>
<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td> : —··'</td><td></td><td></td>
<td>D</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
<td></td><td></td><td></td><td></td><td></td><td>N</td><td></td><td>L</td><td></td><td></td>
<td></td><td></td><td></td><td></td><td>l</td><td></td><td></td><td></td><td></td><td> \</td>
<td></td><td></td><td></td><td></td><td></td><td>1 L</td><td></td><td></td><td></td><td></td>
1515; the
R<sup>;</sup>0.5nm
1.54jjm lvc • 1
INT <sup>;</sup>0s
SM • OFF
1565) IU
5nm / d
Flg.12
ΥΠ 262
WIDMO HS9701B
<img file="PL177262B1_D0009.tif" />
Fig. 13
177 262
WIDMO MS9701B
<img file="PL177262B1_D0010.tif" />
Fig.1L
ΥΠ 262
WIDMO MS9701B
<img file="PL177262B1_D0011.tif" />
Figure 15
177 262
RL -5.27 dBm MKR -1WVL 1535.88 nm
<td colspan="2">SENS.-21 .5.88, dB / 0</td><td>dBm V</td><td></td><td></td><td></td><td></td><td colspan="2"></td><td> •22.75</td><td>dBm -1</td>
<td colspan="2">MARKER</td><td></td><td></td><td></td><td></td><td></td><td colspan="2"></td><td></td><td></td>
<td> 1535 -22.</td><td colspan="2">jQ8nm i8dB m</td><td></td><td></td><td></td><td></td><td colspan="2"></td><td></td><td></td>
<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
<td rowspan="2"></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>
<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td colspan="2"></td><td rowspan="2"></td><td rowspan="2"></td>
<td></td><td></td><td></td><td></td><td></td><td></td><td></td><td colspan="2"></td>
START 15 26.88 nm STOP 1578.68nm
RB8.5nm VB 388 kHz Sfc58asec
Fig. 16
177 262
<img file="PL177262B1_D0012.tif" />
UP Department of Publications. Circulation of 70 copies Price PLN 4.00
Contents7
23 sheets
Sheet 1 Sheet 2 Sheet 3 Sheet 4 Sheet 5 Sheet 6 Sheet 7 Sheet 8 Sheet 9 Sheet 10 Sheet 11 Sheet 12 Sheet 13 Sheet 14 Sheet 15 Sheet 16 Sheet 17 Sheet 18 Sheet 19 Sheet 20 Sheet 21 Sheet 22 Sheet 23
39 members in 22 offices
Priority claims1
| Document | Office | Kind | Date |
|---|---|---|---|
| MI940712 | Italy | A |
Members39
| Document | Office | Kind | |
|---|---|---|---|
| ITMI940712D0 | Italy | D0 | |
| NO951446D0 | Norway | D0 | |
| HU9501071D0 | Hungary | D0 | |
| ITMI940712A1 | Italy | A1 | |
| CA2147035A1 | Canada | A1 | |
| FI951813A | Finland | A | |
| FI951813A7 | Finland | A7 | |
| FI951813L | Finland | L | |
| NO951446L | Norway | L | |
| PL308104A1 | Poland | A1 | |
| EP0677902A1 | European Patent Office (EPO) | A1 | |
| AU1620095A | Australia | A | |
| SK49795A3 | Slovakia | A3 | |
| BR9501362A | Brazil | A | |
| CN1113622A | China | A | |
| KR950035162A | Republic of Korea | A | |
| JPH0846592A | Japan | A | |
| CZ93995A3 | Czechia | A3 | |
| PE26096A1 | Peru | A1 | |
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| RU95105451A | Russian Federation | A | |
| TW301087B | Taiwan Province of China | B | |
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| US5748364A | United States of America | A | |
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| HU216228B | Hungary | B | |
| PL177262B1This record | Poland | B1 | |
| PL177566B1 | Poland | B1 | |
| RU2146853C1 | Russian Federation | C1 | |
| CN1084098C | China | C | |
| EP0677902B1 | European Patent Office (EPO) | B1 | |
| AT220482T | Austria | T | |
| ATE220482T1 | Austria | T1 | |
| DE69527325D1 | Germany | D1 | |
| DE69527325T2 | Germany | T2 | |
| CA2147035C | Canada | C | |
| MY130572A | Malaysia | A |
Numbers
- Application
- 30810495
Titles2
- English
- TELECOMMUNICATION SYSTEM WITH SIGNAL AMPLIFICATION FOR TRANSMISSION OF MULTIPLEXED SIGNALS
- Polish
- Sposób telekomunikacji światłowodowej i system telekomunikacyjny światłowodowy
Classification
- CPC, 12
- H04B10/2912
- H04B10/2581
- H01S3/06754
- H01S3/06787
- H01S3/1608
- H01S3/1691
- H01S3/1696
- H01S2301/04
- H04B10/291
- H04B10/2935
- H04B10/294
- H04B2210/003
- IPC, 7
- H04J14 00
- G02B6 26
- H01S3 067
- H01S3 16
- H04B10 12
- H04B10 17
- H04J14 02