WO2008001020A1

WO2008001020A1 - Long distance passive optical access network

Info

Publication number: WO2008001020A1
Application number: PCT/FR2007/051560
Authority: WO
Inventors: Naveena Genay; Erwan Pincemin
Original assignee: France Telecom
Priority date: 2006-06-29
Filing date: 2007-06-29
Publication date: 2008-01-03

Abstract

The invention relates to a passive optical network featuring a central optical unit connected by at least optical fibre forming a branch from said network to at least one first optical component, wherein said central optical unit comprises: - transmission/reception means for at least one signal comprising at least one optical data component associated to a specific wavelength, - first amplification means of the optical power of said optical data component, - wherein said branch permits bidirectional transmission of the signal. The network according to the invention comprises in at least one of its branches, at least one first passive amplification medium, wherein said first amplification medium is excited by the first amplification means so as to amplify said at least one optical data component of said signal bidirectionally.

Description

Long distance passive optical access network

The invention relates to the field of optical access networks and more particularly, the field of passive optical access networks or PON (Passive Optical Networks).

Access networks are expensive networks for telecommunication operators because they are often tree networks serving many subscribers. Such networks are equipped with many components sometimes consuming electrical energy. In order to limit these operating costs while improving the quality of services offered to subscribers, operators have developed passive optical access networks. All the components in the network between the optical center and the users' equipment are passive, that is, they do not need to be electrically powered to operate. Such optical networks offer subscribers a high connection rate of the order of 2.5 Gbit / s (Gigabit per second). Such speeds make it possible to offer services such as high-definition television, the Internet or video telephony, thus meeting a demand from subscribers.

A passive optical network is, for example, a tree network of the point-to-multipoint type. Such a network is shown in FIG. 1. The network comprises at a first end an optical central station OC at the output of which is connected a first end of an optical fiber 12. A second end of the optical fiber 12 is connected to the input of at least one optical coupler 13 an input to N outputs, N representing the number of branches that owns the network. A first end of an optical fiber

14 _j , I {1, 2, ..., N}, is connected to one of the N outputs of the optical coupler

13. A second end of the optical fiber ^\ _4i is connected to a line termination device OLT _1, ie {1, 2, ..., N} which are connected one or more subscribers. The optical central OC comprises a laser 10 emitting an optical signal for conveying data to the different subscribers connected to the network as well as receiving means R receiving the signals emitted by the line termination devices. The passive optical network described above uses the principle of Time Division Multiplexing (TDM). In such a network, the optical signal emitted by the laser 10 is cut into a plurality of time intervals of the same duration. Each time slot is then associated with one of the OLT line termination devices ₁ according to their needs.

There are also passive optical networks using wavelength division multiplexing or Wavelength-Division Muliplexing. In such a network, the optical center comprises a plurality of lasers each emitting a data optical component associated with a wavelength of its own. An optical multiplexer placed at the output of the optical center and to which is connected a first end of the main fiber of the network makes it possible to inject a wavelength multiplexed signal into the latter. In such an optical network, each line termination device is associated with an optical component from the optical center and therefore at a particular wavelength.

Passive optical access networks, whether they use time division multiplexing or wavelength division multiplexing, have a conventional range of the order of 20 km (kilometers). This limited range of the network is due to the fact that in passive optical networks, the various optical components that are, for example, optical couplers, optical multiplexers or optical fibers, cause losses of optical power of the signals passing through the network and that the transmitted signals can not be amplified without constraints to compensate for such losses. Indeed, in a passive optical network, the downward optical signals, that is to say the optical signals transmitted by the exchange to the subscribers, and the signals optical upstream, that is to say the optical signals transmitted by the subscriber equipment to the optical center, are carried by a single optical fiber. This reduces the cost of the network. However, the use of a single optical fiber to convey the downward and upward optical signals introduces constraints on the transmission powers of these optical signals.

In particular, it is necessary, on the one hand, that the transmission power of the data signals is sufficient to compensate for the losses associated with traversing the network and thus to allow correct reception. On the other hand, it is necessary that the power is not so high as to generate backscattered signals which may dazzle the reception means R used to detect the signals propagating in the subscriber-central direction. The result of this compromise on the value of the optical power of transmission of the signals in a passive optical network results in a limited range of the network. If the range of passive optical access networks is sufficient in urban areas where subscribers are located at relatively short distances from optical exchanges, of the order of 5 to 10 km, this is not the case for subscribers located in rural areas. In these areas, the subscribers are often geographically dispersed and are therefore most often located at a distance from the optical exchanges higher than the conventional range of a passive optical network. These subscribers can not therefore benefit from the broadband transmission offered by the passive optical networks and consequently the services offered which require a broadband connection.

There is therefore a need to increase the range of passive optical networks to serve subscribers in rural areas.

However, because of the previously mentioned constraints, it is difficult to increase the optical transmission power of the optical signals in order to increase the range of the network without affecting the quality of reception of the data. A known solution is to introduce amplification means such as EDFAs into the different branches of the network. A disadvantage of this solution is that it is an operation whose cost is high both in terms of equipment and in terms of energy consumption of the network because such a network is no longer passive.

The invention responds to the need to increase the range of a passive optical network without introducing active elements into the network. Such a solution does not require increasing the optical transmission powers of the transmitted optical signals.

Indeed, the present invention relates to a passive optical network comprising an optical central connected by at least one optical fiber forming a branch of said network to at least a first optical component, said optical central comprising:

means for transmitting / receiving at least one signal comprising at least one optical component of data associated with a particular wavelength; first means for amplifying the optical power of said optical data component

- said branch for the bidirectional transmission of the signal.

The network according to the invention is particular in that it comprises, in at least one of its branches, at least a first passive amplification medium, said first amplification medium being excited by the first amplification means of the amplifier. so as to amplify said at least one optical component of data of said signal bidirectionally.

The network according to the invention by proposing passive amplification means arranged between the optical central and the subscriber equipment remains a passive network.

In the network according to the invention, the optical signal from the amplification means, known as a pump and in the form of laser diodes, excites the amplification medium at a specific wavelength. which corresponds to the wavelength associated with a particular optical data component of said transmitted signal. Thus, when the optical data component passes through the amplification medium, its optical power is amplified. Thus the losses incurred by the optical component are compensated. The optical component of data having regained optical power, it then becomes possible to increase the range of the network without increasing the optical transmission power. The optical power emitted at the central office being substantially the same as that emitted in a conventional optical access network, the effects of backscattering remain minimal and do not disturb the reception of signals.

It should be noted that the signals emitted by the subscriber terminal equipment destined for the optical central unit also have their amplified optical power in accordance with the principle of the invention.

Thus, for the same optical budget as a conventional passive optical network whose range is 20 km, the passive optical network according to the invention makes it possible to reach a range of the order of 100 km.

Alternatively, the present invention responds to another need felt in urban areas. By increasing the range of the optical access network, it is possible to bring the optical center closer to the core network. This makes it possible to group the optical exchanges at the same point, which has the consequence of reducing the costs of the infrastructure. This is particularly advantageous in urban areas where population density is such that many amenities are needed to cover all subscribers.

According to a second characteristic of the network according to the invention, the amplification medium is constituted by at least one erbium-doped optical fiber section inserted in said branch of the network.

In this embodiment of the system according to the invention, an erbium doped optical fiber section or EDF for Erbium-Doped Fiber, is inserted into a branch of the network. By injecting a wavelength excitation in the erbium doped fiber section, it is possible to amplify the optical component of data passing through this branch of the network. Such an amplification technique is called remote optical amplification technique or ROPA, for Remote Optical Pumped Amplification. When the amplification medium is at least one erbium-doped optical fiber section inserted in said branch of the network, the first amplification means comprise a laser amplification diode.

Indeed, a single laser diode is necessary to excite the erbium atoms present in the amplification medium, returning to their equilibrium state, the erbium atoms release photons at a plurality of distinct wavelengths but whose values are a range. Thus, if an optical component of data whose associated wavelength is in this range of value, it sees its optical power amplified. When the network uses wavelength division multiplexing, the optical data components transmitted by the optical central office are associated with wavelengths within a given value range corresponding to the range of wavelength values that the section of erbium doped fiber is able to amplify.

According to a third characteristic of the network according to the invention, the first amplification medium consists of the optical fiber constituting the branch of said system.

In this embodiment of the system according to the invention, the optical fiber constituting a branch of the system is the amplification medium of the optical data component. In this embodiment, it is said that the amplification is distributed, that is to say that the optical component sees its optical power amplified over the entire length of the optical fiber. This amplification technique uses the RAMAN scattering effect. By exciting the constituent atoms of the optical fiber, by means of an appropriate wavelength from the pump located in the optical center of the grating, it is possible to amplify the optical power of the optical component of the optical fiber. data circulating in the network. The wavelength used to amplify the optical component of data must be appropriate in that it must be able to excite the constituent atoms of the optical fiber in such a way that they will amplify the optical component of data. If this is not the case, the data component will not see its amplified optical power.

Finally, this embodiment has the advantage of using as an amplification medium an optical fiber already present during the establishment of the network. It is not necessary to introduce new elements in the network which would lead to heavy maintenance operations in terms of costs and duration.

In this embodiment, when the network uses the time division multiplexing technique, the first amplification means comprise a laser amplification diode.

In the case of a TDM network, the amplification means consist of a laser diode that amplifies the optical power of a single optical component of data.

In this embodiment, when the network uses the wavelength division multiplexing technique, the first amplification means comprise at least two amplifying laser diodes. In the case of a WDM network, the amplification means are in the form of a plurality of laser diodes. The presence of these laser diodes makes it possible to excite the amplification medium, here the optical fiber, so that it is able to amplify the optical power of a plurality of optical components comprised in a wavelength band. whose width is of the order of 30 nanometers.

According to a fourth characteristic of the network according to the invention, the system comprises a first amplification medium consisting of an optical fiber forming a branch of said system, and a second medium amplifier comprising at least one erbium-doped optical fiber section inserted in said branch of the system.

Such an embodiment makes it possible to further amplify the optical power of an optical signal transiting on a branch of the network if it is decided to amplify in the two amplification media the same wavelength. In the opposite case, this embodiment makes it possible to dedicate a first amplification medium to a particular wavelength or to a direction of communication.

According to a fifth characteristic of the network according to the invention, said first optical component is an input distribution element to N outputs, where N represents a number of secondary branches connecting said distribution element to N second optical components of said network.

The network according to the invention then comprises a main branch connecting the output of the optical central to the input of the distribution element. Secondary branches then connect the different outputs of the distribution element to the equipment of the subscribers.

The distribution element may be an optical coupler 1 to N in the case of a TDM optical network or an optical multiplexer in the case of a WDM network. By locating the amplification medium in the main branch of the network, it is possible to simultaneously amplify the various data optical components for subscribers.

According to another characteristic of the network according to the invention, second amplification means are arranged close to said first optical component.

By having a third laser diode near the distribution element, it is possible to reduce the costs of the network by allowing the pooling of the pump between the different terminal equipments. According to an alternative embodiment of the network according to the invention, the second optical component is a line termination device comprising means for remodulating the optical component of data. This makes it possible to suppress the emission laser in the line termination device. This reduces the cost of infrastructure.

According to a first embodiment, the remodulation means comprise a first semiconductor amplifier at the output of which is connected a remodulation device itself connected to the input of a second semiconductor amplifier.

Such a solution of modulation, called external as opposed to a modulation solution of a signal from a so-called internal modulation laser, is more efficient in terms of scope of the remodulated optical component because this remodulated optical component has a more chromatic dispersion. weak than in the case of the direct modulation of an optical component derived from a laser.

According to an alternative embodiment, the previously defined remodulation means can be integrated at the same optical component. Such a solution makes it possible to reduce the costs of these remodulation means and to facilitate their introduction into the line termination devices.

According to a second embodiment, the remodulation means are a semiconductor amplifier in reflection. Other features and advantages will be apparent from the reading of preferred embodiments described with reference to the drawings in which:

FIG. 1 represents a passive optical access network according to the prior art, FIG. 2 represents a long-range TDM passive optical network using both the optical fiber and an erbium-doped optical fiber section as amplification media;

FIG. 3 represents a long-range TDM passive optical network using the optical fiber as amplification medium,

FIG. 4 represents a long-range TDM passive optical network using the optical fiber as an amplification medium and in which the optical signal emitted by the optical central unit is remodulated by the line termination devices; FIG. 5 represents an optical network; long-range TDM passive signal using the optical fiber as an amplification medium, in which the optical signal emitted by the optical central unit is remodulated by the line termination devices and comprising mutualization amplification means between the different line termination devices, FIG. 6 represents a long-distance WDM passive optical network using the optical fiber as an amplification medium,

FIG. 7 represents a long-distance WDM passive optical network using both the optical fiber and an erbium-doped optical fiber section as amplification media. FIG. 2 represents a point-to-multipoint TDM long-range passive optical network according to a first embodiment of the invention. An optical central office OC constitutes a first end of the network. A first end of an optical fiber 24 is connected to the output of the optical central office OC. A second end of the optical fiber 24 is connected to the input of at least one optical coupler 25 an input to N outputs, N representing the number of branches that owns the network. The optical fiber 24 is called the main branch of the network. A first end of an optical fiber 26 ,, I {1, 2, ..., N}, is connected to one of the N outputs S, of the optical coupler 25. A second end of the optical fiber 26, is connected to a line termination device 27 ,, ie {1, 2, ..., N} to which one or more subscribers are connected. The optical fibers 26 1 to 26 _N are called secondary branches of the network.

The optical central OC comprises a first laser 20 emitting an optical signal composed of an optical component associated with a particular wavelength. This optical component conveys data into the network for the different subscribers connected according to the principle of time division multiplexing. The optical center also comprises a second laser diode 210 emitting an optical signal composed of an amplifying optical component associated with a particular wavelength distinct from the wavelength associated with the optical data component.

The outputs of the laser 20 and the laser diode 210 are each connected to an input of an optical multiplexer 22 two inputs to an output (2: 1). The optical central office OC also comprises a reception module R of the signals emitted by the line termination devices 27 ' ₁ to 27 _N. The output of the optical multiplexer 22 and the input of the reception module R are each connected to an input of an optical circulator 23 having three ports. A first end of the optical fiber 24 is connected to the third port of the optical circulator 23 allowing on the one hand the signals emitted by the laser 20 and the laser diode 210 to pass through the network towards the line termination devices 27-, at 27 _N , and on the other hand to the signals emitted by the line termination devices to pass through the network towards the optical central office OC and the reception means R.

Each of the optical fibers 24 and 26i to 26 _N constituting the network allows a bidirectional transit of the optical signals in the network, that is to say that the upward signals, from a line termination device to the optical central, and the downstream signals, from the central to the line termination devices, flow in the same optical fiber. This reduces costs when setting up the network and facilitating its maintenance. In this embodiment, an erbium-doped optical fiber section 28 is inserted into the main optical fiber 24. Such an optical fiber section 28 serves as a passive amplification medium.

The amplification optical component derived from the first laser diode 210 excites the erbium atoms present in the optical fiber section 28. When the erbium atoms return to their non-excited state, they release photons under the principle of the stimulated emission whose wavelength corresponds to the wavelength of an optical component of data passing through the network. By way of example, an amplification optical component whose emission wavelength is 1480 nanometers makes it possible to amplify the transmission power of an optical data component whose emission wavelength is 1550 nanometers. Such a technique is called remote amplification technique since the amplification medium 28 is in the network while the amplification means, here the laser 210, are located in the optical central OC. Such an amplification medium makes it possible to amplify the signals transmitted both in the upward direction, from the line termination devices towards the central station, and in the downstream direction, from the optical central station to the line termination devices. In an alternative embodiment of the present invention not shown in the figures, an erbium doped optical fiber section 28 is inserted in one or more of the secondary branches 26i to 26 _N of the network.

Such an amplification method makes it possible to extend the range of the passive optical network from 20 kilometers to a hundred kilometers while maintaining the passive nature of the network without increasing the signal transmission power. Thus, backscattering effects are minimal and the quality of data reception is preserved.

FIG. 3 represents a long-distance TDM passive optical network according to a second embodiment of the present invention. The elements Network components common to the network described with reference to Figure 2 bear the same references and will not be described.

In this embodiment, the optical fiber is the amplification medium of the optical signals. This embodiment makes it possible to amplify the upstream signals and the downstream signals in the same amplification medium.

Such a technique uses the RAMAN effect and makes it possible to have amplification distributed over the entire length of the optical fiber.

The optical amplification component emitted by a laser diode 21 disposed in the optical center makes it possible to excite the atoms of the optical fibers 24 and 26i to 26 _N constituting the network. Returning to their non-excited state, the atoms of the optical fiber release energy by stimulated emission in the form of photons having a particular wavelength different from that of the optical amplification component. This wavelength corresponds to the wavelength of the optical component of data passing through the network. Such an optical component of data may be derived from the optical central or from one of the line termination devices 27,. Thus, by crossing the optical fiber whose atoms have been excited by the amplification optical component, the optical data component sees its amplified emission power. The wavelength of the optical amplification component is chosen so that the wavelength of the photons emitted during the return to the non-excited state of the atoms of the optical fiber corresponds to the wavelength of the signal at amplify. By way of example, to amplify an optical component of wavelength 1550 nanometer data, the amplification optical component is emitted at a wavelength of 1455 nanometers.

In an embodiment illustrated in FIG. 2, an erbium-doped optical fiber section 28 is inserted into the optical fiber 24. Such an optical fiber section 28 serves as a second amplification medium. Thus, the optical central OC is equipped with two laser diodes 21 and 210 each emitting an amplifying optical component whose wavelengths are distinct from one another and whose function is to excite the first medium respectively. amplifier constituted by the optical fiber 24 and the second amplification medium constituted by the section of optical fiber 28.

The following two tables indicate by way of example the optical power budgets in the downward direction and in the upward direction corresponding to a network comprising two amplification media 24 and 28.

In these tables, we see that the optical budgets corresponding to the downward and upward transmission directions are balanced. In the upward direction, it can be seen that the value of the optical power received by the optical central office OC is greater than the power of the optical signal of Rayleigh backscatter. Thus, the reception means R placed in the optical central office OC are not dazzled by the backscattered signal and the reception of the upstream data optical components is not disturbed.

Fig. 4 shows a third embodiment of the present invention. This embodiment relates to a long-distance TDM passive optical network using the technique of distributed amplification or RAMAN.

The components of the network common to the network described with reference to Figure 2 bear the same references and will not be described. This embodiment can also be implemented in combination with the embodiments described above.

In such an embodiment, it is proposed to delete the optical data transmission lasers of data located in the line termination devices. The line termination devices 27bis to 27biSN according to the invention are able to remodulate the optical data component from the optical central, or downstream. Once the downward optical component of the data is remodulated, it is then sent back into the network for the optical central office OC, it is then called upstream optical data component.

A line termination device 27bis according to the invention comprises an optical coupler 30 with an input to two outputs (1: 2). Such a coupler 30 is a 50/50 coupler. This makes it possible to distribute the optical power received identically between the two outputs of the coupler 30. A first output of the coupler 30 is connected to a first port of an optical circulator 31 whose operating principle has been described above. A second port of the optical circulator 31 is connected at the input of a first semiconductor optical amplifier 32 or SOA (Semiconductor Optical Amplifier) which amplifies the optical power of the received signal.

The output of this first semiconductor optical amplifier 32 is connected to the input of a remodulation device of an optical signal 33, such as for example an MEA (Electro-Absorption Modulator for Modulator to Electro-Absorption). Such an optical component by its remodulation function of an incoming optical signal introduces significant optical power losses. This is why it is necessary to connect the output of the remodulation device 33 to the input of a second semiconductor optical amplifier 34 so as to amplify the optical power of the remodulated signal. Finally, the output of the second semiconductor optical amplifier 34 is connected to the third port of the optical circulator 31.

The assembly consisting of the two semiconductor amplifiers 32, 34 and the remodulation device 33 may be replaced by a single optical component such as a reflective semiconductor optical amplifier or Reflective Semi-Conductor Amplifier (RSOA). Such an assembly can also be integrated at the level of the same optical component that is inserted in the line termination device.

The second output of the coupler 30 is connected to receiving means R 'of the downstream optical data component.

In a passive optical network according to the invention, an optical data component emitted by the laser 20 disposed in the optical central office OC, or downward data optical component, is phase modulated according to the DPSK (Differential Phase Shift Keying) modulation technique. , or in French Differential Phase Modulated Coding). In the context of this modulation technique, the intensity of the descending data optical component remains constant because the data is carried by two phases 0 and π. When the downlink optical component arrives at the input of a line termination device 27bis, a portion of the associated optical power is sent, by means of the coupler 30, to the reception means R 'and a second part is sent to the optical circulator 31.

The first part of the downlink optical data component sent to the reception means R 'which comprise means for such as a DPSK detector consisting of a DPSK demodulator and a DPSK photoreceptor with single or double photodiode.

The second part of the downlink optical data component is inputted to the first semiconductor amplifier 32. Once the optical power associated with this portion of the amplified downlink optical component is made, the downlink optical component is then remodulated. in intensity by the remodulation device 33. The intensity modulation performed by the remodulation device 33 is a remodulation called NRZ (Non Retum to Zero). The optical data component thus remodulated again sees its optical power amplified by the second semiconductor amplifier 34 before being sent in the network to the OC central optical.

The following two tables indicate by way of example the optical power budgets in the downward direction and in the upward direction corresponding to a network according to the embodiment described above.

In these tables, we see that the optical budgets corresponding to the downward and upward transmission directions are balanced. In the upstream direction, it can be seen that the value of the optical power received by the optical central office OC is greater than the power of the optical Rayleigh backscattering signal. Thus, the reception means R placed in the optical central office OC are not dazzled by the backscattered signal and the reception of the upstream data optical components is not disturbed.

FIG. 5 represents another embodiment of a long-distance TDM passive optical network. The constituent elements of the network common to the embodiments described in the preceding figures bear the same references and will not be described.

In this embodiment, a third laser diode 29 emitting an amplifying optical component is connected by means of a multiplexer 31 at the input of the optical coupler 25. In order to prevent the laser diode 29 from being dazzled by the optical component amplification from the laser diode 21, there is an isolator between the multiplexer 31 and the laser diode 29. The amplification optical component from this third laser diode 29 is injected into the optical coupler 25 so as to amplify the components data optics going up and down. Such a laser diode 29 emits, for example, an amplification optical component associated with a wavelength of 1455 nanometers.

Such an embodiment operates with conventional line termination devices 27-, 27 _N or line termination devices as described with reference to FIG. 4. The wavelength associated with the optical amplification component from the third laser diode 29 is used to excite the amplification medium which may be the optical fiber 24 or a section of optical fiber doped with erbium 28. There must also be two amplification media in the network, namely the optical fiber 24 and an erbium-doped optical fiber section 28. In such a case, a fourth laser diode 30 must be connected to the optical coupler 25. This additional laser diode 30 emits an amplifying optical component for exciting the second amplification medium.

The pooling of the third laser diode 29 makes it possible to reduce the costs of the network. Indeed, in the case where such a laser is not used, each line termination device 27 ,, 27bis must be equipped with amplification means. FIG. 6 represents a WDM long-range passive optical network of point-to-multipoint type. In this figure, an optical central OC constitutes a first end of the network. A first end of an optical fiber 24 is connected to the output of the optical central office OC. A second end of the optical fiber 24 is connected to the input of at least one optical demultiplexer 251 an input to N outputs, N representing the number of branches that owns the network. The optical fiber 24 is called the main branch of the network. A first end of an optical fiber 26 _j5 j <≡ {1, 2, ..., N}, is connected to one of the N outputs S _j of the optical demultiplexer 251. A second end of the optical fiber 26 _j is connected to a line termination device 27 ,, 27bis, ie {1, 2, ..., N} to which one or more subscribers are connected. The optical fibers 26i to 26 _Nj are called secondary branches of the network.

The optical central OC comprises a first laser 20 emitting an optical signal composed of several optical components each associated with a particular wavelength. There are as many transmitted optical components as there are line termination devices connected to the network. These different optical components carry data in the network for the different subscribers connected to the network. The optical central unit also comprises a second laser diode 21 1, a third laser diode 212 and a fourth laser diode 213, each emitting a signal optical system comprising an amplification optical component associated with a particular wavelength distinct from the wavelengths associated with the optical data components.

The outputs of the laser 20, the laser diode 21 1, the laser diode 212 and the laser diode 213 are each connected to an input of an optical multiplexer 220 four inputs to an output (4: 1). The optical central office OC also comprises a reception module R of the signals emitted by the line termination devices 27-, at 27 _N , 27bis-ι to 27bis _N. The output of the optical multiplexer 220 and the input of the reception module R are each connected to a port of an optical circulator 23 having three ports. A first end of the optical fiber 24 is connected to the third port of the optical circulator 23 allowing on the one hand the signals emitted by the laser diodes 20, 211, 212 and 213 to pass through the network towards the line termination devices 27 ,, 27bis ,, and on the other hand to the signals emitted by the line termination devices to pass through the network towards the optical central office OC and the reception means R.

Each of the optical fibers 24 and 26-ι to 26 _N constituting the network allows a bidirectional transit of the optical signals in the network, that is to say that the upstream signals and the downstream signals circulate in the same optical fiber.

The optical amplification components emitted by the second 21 1, third 212 and fourth laser diodes, 213 are used to excite the amplification medium so that it is able to amplify the optical power of optical data components whose different wavelengths associated are in a band of a few tens of nanometers wide. By way of example, such a strip has a width of the order of 30 nanometers. In this embodiment, the optical fiber 24 is the amplification medium of the optical signals.

FIG. 7 illustrates another embodiment of a long-distance WDM passive optical network according to the invention. The constituent elements of the network common to the network described with reference to Figure 6 have the same references and will not be described.

In this embodiment, a first amplification medium is constituted by the main optical fiber 24 and a second amplification medium consists of an erbium doped optical fiber section 28. This section of optical fiber 28 is inserted into the main fiber 24.

In order to be able to excite the second amplification medium 28, the optical center is equipped with a fourth laser diode 214. This laser emits an optical amplification component whose function is to excite the erbium atoms present in the section. 28. Although a single optical amplification component is necessary to excite the amplification medium 28, it is capable of amplifying the optical power of several optical data components whose respective wavelengths. are contained in a band of a few tens of nanometers wide. Thus, such an amplification medium 28 can be used in the context of a passive optical network WDM.

In another embodiment of a long-distance WDM passive optical network, it comprises only one amplification medium. This amplification medium consists of an erbium-doped optical fiber section inserted in the main fiber 24 or in at least one of the secondary fibers 26 _j . In such an embodiment, the optical center comprises a single amplification laser. This emits an amplifying optical component for exciting the amplification medium 28. The embodiments described above can be combined with another embodiment of a long-distance passive optical network WDM in which the devices of FIG. line termination are modified so as to remodulate the optical data component from the optical central OC associated with each of them. Such line termination devices 27bis-ι to 27a _N have been previously described with reference to FIG. 4.

The following two tables indicate by way of example the optical power budgets in the downward and upward direction corresponding to a network using the optical fiber 24 as the medium. amplification and wherein the line termination devices are modified so as to remodulate the optical data component from the optical central OC associated with each of them.

In an alternative embodiment, one or more amplification lasers are connected to the input of the optical multiplexer 251. These lasers have the function of each emitting an optical amplification component whose function is to excite the medium or the amplification 24, 28. When the amplification medium consists of the optical fiber 24, three amplification lasers are connected to the multiplexer 251. This makes it possible to amplify several components optical data associated with wavelengths included in a band of a few tens of nanometers wide.

When the network comprises two amplification media 24, 28, four amplification lasers are connected to the multiplexer 251. The first three lasers serve to excite the first amplification medium consisting of the optical fiber 24, the fourth laser, as to it serves to excite the second amplification medium consisting of the erbium-doped optical fiber section 28. Thus, it is possible to amplify the power of the upstream data optical components. Finally, when the only amplification medium consists of the erbium-doped fiber section 28, only one amplification laser is connected to the multiplexer 251.

The present invention in all its embodiments can be implemented in point-to-point optical networks. In such an embodiment, it is necessary to provide either an amplification laser for each branch of the network or a single laser and means for pooling it over all the branches of the network.

Claims

1) passive optical network comprising an optical central (OC) connected by at least one optical fiber (24, 26 _j ) forming a branch of said network to at least a first optical component (25, 251, 27, 27bis,), said central optical device comprising:

means for transmitting / receiving (20, R) at least one signal comprising at least one optical component of data associated with a particular wavelength,

first amplification means (21, 210, 211, 212, 213, 214) of the optical power of said optical data component,

- said branch (24, 26 _j) for the bidirectional signal transmission, characterized in that it comprises in at least one of its branches at least a first passive amplification medium (24, 26 _j5 28) said first amplification medium being excited by the first amplifying means so as to amplify said at least one optical data component of said signal bidirectionally.

2) The network of claim 1 characterized in that the first amplification medium is constituted by at least one erbium-doped optical fiber section (28) inserted into said branch of the network.

3) A network according to claim 2 characterized in that the first amplification means comprise an amplifying laser diode (210).

4) The network of claim 1 characterized in that the first amplification medium is constituted by the optical fiber (24, 26 _j ) constituting branch of said system. 5) The network of claim 4 characterized in that, when the network uses the time division multiplexing technique, the first amplification means comprise an amplification laser diode (21).

6) The network of claim 4 characterized in that, when the network uses the wavelength division multiplexing technique, the first amplification means comprise at least two amplification laser diodes (21 1, 212, 213).

7) A network according to any one of claims 2 to 6 characterized in that the network comprises a second amplification medium consisting of at least one erbium doped optical fiber section (28) inserted in said branch of the network ( 24, 26,).

8) A network according to any one of the preceding claims characterized in that said first optical component is a distribution element an input to N outputs (25, 251), where N represents a number of secondary branches (26,) connecting said element of N-second distribution of optical components of said network.

9) A network according to claim 8 characterized in that second amplification means (29, 30) are arranged near said first optical component (25, 251). 10) A network according to any one of claims 8 to 9, characterized in that the second optical component is a line terminating device (27a,) having remodulation means (32, 33, 34) of the optical component of data.

1 1) A network according to claim 10 characterized in that the remodulation means comprise a first semiconductor amplifier (32) output of which is connected a remodulation device (33) itself connected to the input of a second amplifier semiconductor (34).

12) Network according to claim 10 characterized in that the remodulation means are integrated at the same optical component. 13) The network of claim 10 characterized in that the remodulation means are a semiconductor amplifier in reflection.