FR2687030A1 - Installation for bi-directional information transmission using optical fibre(s) - Google Patents

Abstract

Bi-directional optical transmission installation including at least one optical amplifier (3) on the line (2). This amplifier (3) is decoupled with the aid of at least one optical circulator (5, 6). Another redundancy amplifier (7) can be connected between the terminals (54, 62) which are left free of the two circulators used (5, 6), in the case of a two-fibre link (F1, F'1 and F2, F'2) per line (2).

Description

TWO-WAY TRANSMISSION INSTALLATION
OF INFORMATION BY FIBER (S) OPTICAL (S)
The present invention relates to an installation for bi-directional transmission of information by optical fiber (s), this installation comprising at least one optical amplifier placed on the line.

 It is now common to use, in particular for underwater telecommunications or for terrestrial networks, semiconductor or doped fiber optical amplifiers, whose net fiber to fiber gain is positive, at the wavelength of the signal.

 In the case of doped fiber amplifiers, the optical signal passes through a purely optical amplifying medium, where it picks up by stimulated emission the photons necessary for obstinacy of sufficient amplitude.

 Generally, an optical amplifier of this kind is placed on a line optical fiber and it comprises a means of coupling a pump light energy, which reverses the population in an amplifying medium, constituted for example by an optical fiber doped with Erbium or by a fluorinated fiber doped with Praseodymium. There is thus creation of a single-pass laser medium. The pump is produced using semiconductor components, capable of coupling, for example, around twenty milliwatts.

 The patent application in France filed on April 22, 1991 under number 91.04930 deals for example with a telecommunications system with optical amplifiers of fiber doped with Erbium for the long-distance transmission of signals of wavelength 1550 nanometers, finding in particular an application in underwater telecommunications. This document also gives the references of some articles published on this same subject.

 This known art concerns uni-directional connections.

Bi-directional links, whether with optical amplifiers of the aforementioned type or with more conventional semiconductor optical amplifiers, are currently produced as follows For bi-directional links which have two distinct fibers per link, one of which fiber for the "rising" signal, that is to say propagating in a first direction, and another fiber for the "falling" signal, that is to say propagating in the other direction, the most satisfactory method in terms of performance consists in providing, for each amplification station, a separate optical amplifier on each of the two fibers. The cost of such a solution is however high, and it is sometimes considered prohibitive. It is then planned to use a single optical amplifier, both for the signal going in one direction, or "amount", as for that going in the other direction, or "down", which is possible because the optical amplifiers are often reversible. We then use, between each access, respectively upstream and downstream, of this amplifier, and the pair of fibers associated with it, a 3dB coupler that must necessarily be connected to each fiber using an isolator: this then results in total insertion losses which are greater than 6 dB, which is quite penalizing.

 In the other case where it is a bi-directional link with a single optical fiber per link, this fiber ensuring both the path of the "uplink" optical signal and that of the "downlink" signal, the technically the solution simpler consists in providing a single optical amplifier per bi-directional link, for each amplification station, which is made possible by the aforementioned reversible nature of the optical amplifiers. This solution is not always acceptable, however, since such a single amplifier cannot be adapted to each of s two directions of the connection, since the two channels, m () ntir1te and descendant, can be different in terms of wavelength and / or power of the signal (s). In a fiber optic guidance system, for example, communication on the forward channel is at high speed and 1.5 microns in wavelength, while the opposite direction channel is at low bit rate and has a wavelength of 1.3 microns: the use of a single optical amplifier, for example with fluoride fiber doped with Praseodymium, is then not possible for these two channels, since an amplifier of this type is very absorbent in the spectral window located around 1.5 microns. We then use two separate amplifiers, one for each channel, rising and falling respectively, which we couple from part and d other on the line using coupling devices ors 3dB and insulators, as mentioned previously this again leads to detrimental insertion losses in these coupling systems.

 Furthermore, in particular in the case of submarine links where the successive amplification stations are submerged, it is preferable to provide a redundancy of these optical amplifiers, in order to be able, in the event of failure of these active components, to replace them by command remotely without having to physically intervene at their level. In the case of bidirectional links with two fibers per link, it is then commonly provided, in each submerged case corresponding to an amplification station, not only two amplifiers per pair of fibers instead a single, but also mechanical switches controlled remotely and intended to achieve, by switching in the housing, the replacement of one amplifier by the other. The need to provide these mechanical switching flip-flops in the submerged case greatly penalizes the size, the cost price, and the reliability of the total optical amplification device used.

 The invention aims to remedy these drawbacks. To this end, it relates to an installation for the bidirectional transmission of information by optical fiber (s), with either one or two optical fibers per bi-directional optical link line, this installation comprising at least one optical amplification inserted on the bi-directional line and characterized by the fact that this amplification station comprises at least one optical amplifier which is coupled to said bidirectional line by means of at least one optical circulator capable of ensuring the decoupling of the two bi-directional link, "up" and "down" channels, respectively.

 According to one embodiment, it is an optical link comprising one fiber per direction of the signal, "up" and "down", or in all two fibers, and said amplification station comprises a single optical amplifier per link. each access of which, "upstream" or "downstream", is connected to the corresponding pair of fibers by a respective optical circulator, each of these two circulators having four successive doors clockwise, with 1 upstream access to this single amplifier connected to one of the four doors of the upstream circulator and the two adjacent doors, preceding and following, of this circulator respectively connected to the “upward” signal fiber and to the “downward” signal fiber, and with downstream access from this amplifier connected to one of the four doors of the downstream circulator and the two adjacent doors, preceding and following, of this circulator respectively connected to the “down” signal fiber and to the fib re signal "amount".

 Very advantageously, this same embodiment is produced in redundant form, and for this purpose, another optical amplifier, of redundancy, is then connected between the two fourth doors, remained free, of these two circulators, upstream and downstream, the replacement of the aforementioned amplifier by this redundancy amplifier then being carried out simply by inversion, at the head of the line, of the connections of each of the two fibers of the bi-directional link.

 According to another embodiment, it is, on the contrary, an optical link comprising a single fiber conveying both the uplink signal and the downlink signal, and there are then provided two optical amplifiers, one for each channel, downlink and uplink respectively, which are decoupled accordingly by at least one four-port optical circulator.

In any case, the invention will be well understood, and its advantages and other characteristics will emerge during the following description of some nonlimiting exemplary embodiments, with reference to the appended schematic drawing in which
Figure 1 is a block diagram of an optical amplification station placed on a bi-directional optical link comprising
two line optical fibers, one for the "uplink" path and one for the "downlink"path;
Figure 2 shows the optical amplification station according to Figure 1, but equipped with an additional optical amplifier capable of operating in redundancy;
Figure 3 similarly shows an optical amplification station placed on a bi-directional optical link comprising a single line optical fiber;
Figure 4 is another embodiment relating to a bi-directional optical link to a single line optical fiber; and
Figure 5 shows an input / output device usable on a bi-directional optical transmission line with a single line optical fiber.

 Referring first to Figure 1, the reference 1 designates an optical amplification station which is interposed on a bi-directional optical line 2 forming part of an installation for bi-directional transmission of information by optical fibers . It is for example a submarine telecommunications installation at long distances, and this amplification station 1 is one of the submerged amplification stations which are placed in cascade, every 50 kilometers for example, on the line 2. The optical signals are generally of wavelength around 1500 nanometers.

In the illustrated case, this line 2 is a link to two optical fibers, a first fiber F1, F ′ 1 which carries the signals S1 going in a first direction, here from left to right, which will be called "rising"signals; and
a second fiber F2, F ′ Z which carries the signals S2 going in the opposite direction, here from right to left, which should be called "downlink" signals.

 The optical amplification station 2 comprises a single optical amplifier 3, for example a fiber optic amplifier doped with Erbium or a semiconductor optical amplifier, which operates in a direction S1 as in the other S2.

Such a configuration can be used without problems in the case where the signals Si and S2 have substantially the same characteristics: for example, they are both of wavelength equal to 1532 nanometers and of the same flow rate. This configuration can also be used if these two signals are of different wavelengths and similar bit rates, provided that 1 amplifier used has sufficient gain at these two wavelengths.

 This optical amplifier 3 has an input port 4 and an output port 16 for the "uplink" signals Si, and of course these two ports 4 and 16 are, conversely, respectively the output and input ports for the "downlink" signals S2.

According to the invention, the grouping of the two fibers
F1, F'2, located to the left of the amplification station 1 in the drawing, in a single bi-directional input / output fiber 4, is here produced not using a coupler and two separators, but simply using an optical circulator 5 with four ports or doors 51, 52, 53, 54 succeeding each other at regular intervals clockwise.

Optical circulators are known per se in the art, and their use with optical fibers is for example described in the article by Y. FUJI: "High-isolation polarization-independent optical circulator coupled with single-mode fibers", published in the American magazine IEEE: "Journal of
Lightwave Technology ", vol. 9, 1991, p. 456 to 460, as well as in the publication" Proceedings of the 16th European Conference on Optical Communication ", September 16-20, 1990, Amsterdam, p.99 to 102, article by S. NISHI, K. AIDA and K. NAKAGAWA "Highly efficient configuration of erbium-doped fiber amplifier".

 As shown, the fiber F1, which carries the rising signal S1, is connected to the first door 51 of the circulator 5, while the fiber F2, which carries the falling signal S2, is connected to the third door 53 and that the fiber 4 is connected to the second door 52.

 In this way, the signals Fi are transferred from the fiber Si to the common fiber 4, and therefore from there to the amplifier 3, while on the contrary the downlink signals which are present, at the output of the amplifier 3, on the fiber 4 are transferred, via the circulator 5, to the fiber F '2. In fact, a circulator is a passive component which transfers any signal which is applied to it on one of its doors, in this example Si for the rising signal and 52 for the downlink signal, on the next door clockwise, that is to say respectively here on the gate 52 for the uplink signal and on the gate 53 for the downlink signal.

 Note that the fourth door 54 is here unused.

 Symmetrically with respect to the amplifier 3, the two fibers F '1 and F2 are joined on the single fiber 16 by means of a circulator 6, identical to the circulator 5 and therefore with four doors 61 to 64, the door 62 then being unused as shown.

According to an experimental realization carried out by the
Applicant according to the diagram according to Figure 1, 1 optical amplifier 3 is an optical fiber amplifier doped with Erbium whose gain between points 4 and 16, by simple passage of a rising signal of wavelength 1532 nanometers, is 26 dB for an incident signal of one microwatt power. The optical circulators used 5 and 6 have respective insertion losses which, depending on the doors used and to the nearest 0.2 dB, of the order of 1.2 to 1.4 dB, their isolation rate between channels being greater than 30 dB and possibly even reaching 36 dB.These characteristics are sufficient, especially since special care has been taken to avoid forming an oscillating cavity on either side of the amplifying medium 3.

Under these conditions, 1532 nanometers have been obtained for rising and falling signals of the same wavelength. tln total gain, in one direction as in the other, of 23.4 dB for t amplification station 1, which represents an addition of more than 5 dB compared to the amplification device with couplers and isolators of the prior art.

Figure 2 is an embodiment identical to that according to
Figure 1, but with an additional optical amplifier 7 operating in redundancy with the amplifier 3.

 In such a case, according to a particularly advantageous characteristic of the invention, this redundancy amplifier 7, which is of course identical to the amplifier 3, is quite simply permanently connected between the "fourth" doors previously left free, 54 and 62, of the two circulators 5 and 6.

 In normal operation, the operation takes place according to Figure 1, and no signal passes through 1 redundancy amplifier 7 since the gates 54 and 62 are never supplied by the signals Si or S2.

 In the event of failure of the "normal" amplifier 3, the operators invert, at each end terminal, the connections of the two line fibers.

 The rising signal S'1 is then applied to the fiber F'2 (and no longer to the fiber Fil), and it then passes to the fiber F2 (and no longer to the fiber F'1) via the circulator 5 (gate 53 to door 54), the redundancy amplifier 7, and the circulator 6 (door 62 to door 63).

 Similarly, the downward signal S'2 is this time applied to the fiber F '1 (and no longer to the fiber F2), and it then passes to the fiber F'2 via the circulator 6 (door 61 towards door 62 ), The redundancy amplifier 7, and the circulator 5 (door 54 towards door 53).

Ultimately, it can be seen that, by a simple reversal of the connections to the two terminal terminals at the line ends, the faulty amplifier 3 is no longer in circuit and is replaced by the redundancy amplifier 7, which is indeed the aim. sought, this result being obtained without the use, as in the prior art, of mechanical switching flip-flops controlled remotely and immersed like the amplifier station 1
The realization which has just been described with reference to
Figures 1 and 2 apply in the case of a bi-directional link with two optical fibers per line, for which it is possible, taking into account the identical or similar characteristics of the up and down signals, to use one and the same bi-directional optical amplifier for each amplification station.

 In the case of a bi-directional link with a single optical fiber per league, these conditions may not be met, so that one and the same optical amplifier may not be suitable for both the uplink and the downlink signal.

 In this regard, Figure 3 shows an embodiment, in accordance with the invention, of an amplification station i placed on a bi-directional optical link with a single optical fiber FF t, with signals, respectively up Si and down S2, with characteristics (wavelength, bit rate, etc.) so different that each requires an appropriate optical amplifier, which can be mono-directional, respectively 8 and 9.

 In this case, the signals Si incident on the fiber F are routed, via a circulator 5 identical to that of FIGS. I and 2 and from the gate 54 towards the gate 51 thereof, towards the input of the amplifier 8 adapted to the rising signal. They are then routed, leaving this amplifier 8, on the fiber F 'via a circulator 6 identical to that of Figures 1 and 2 and from door 61 to door 62 thereof.

 Similarly, the downlink signals S2 are routed from gate 62 to gate 63 of circulator 6 at the input of 1 suitable amplifier 9, and they are then routed, after amplification, from this amplifier 9 to fiber F via the circulator 5, from door 53 to door 54 of the latter.

 Decoupling is thus carried out with losses much lower than those achieved with the devices of the prior art.

 Note that the risk of self-oscillation of this amplifier i is quite low. If we want to get rid of it completely, it is possible to provide isolators at the output of amplifiers 8 and 9 while still benefiting from a significant gain in efficiency compared to the devices of the prior art.

 The embodiment shown diagrammatically in FIG. 4 is similar to that according to FIG. 3 in that it also uses two separate optical amplifiers 8 and 9 for the rising signal S1 and for the falling signal S2. However, only one optical circulator 10 is used here.

 Line fiber F, F 'is connected as shown on the diametrically opposite gates 104 ("upstream" fiber F) and 102 ("downstream" fiber F'), while amplifiers 8 and 9 are respectively connected to the other two doors 101 and 103 of this circulator 10.

 According to a configuration known per se, the outputs of each amplifier 8 and 9 are connected to a reflecting mirror, respectively il and 12.

 As indicated by the arrows, the rising signal S1 passes a first time through the circulator 10 (from the door 104 to the door 101), then a first time through the optical amplifier 8. It is then reflected on the mirror 11 , again passes through the amplifier 8, a second time through the circulator 10 (from door 101 to door 102), finally ending up on the fiber F '.

 Conversely, the falling signal S2 passes a first time through the circulator 10 (door 102 towards door 103), a first time through the amplifier 9, a second time through the latter after reflection on the mirror 12, then a second times through the circulator 10 (door 103 towards door 104) to finally reach the fiber F.

 Note that this arrangement has the advantage, compared to that according to Figure 3, of using only one circulator. However, it requires two mirrors il and 12, and it presents, in the case of a plurality of such amplification stations 1, greater risks of self-oscillation in many cases, the assembly according to Figure 3 will therefore preferential.

 FIG. 5 represents a mode of connection of an input / output terminal which is applicable in the case where, for example according to FIGS. 3 and 4, the bi-directional link is carried out on a single and same line fiber F.

 As shown, the rising signal S1, which is applied to the input terminal E of the link, passes through an optical amplifier 13 and it is then directed to the line fiber F via an optical circulator 14 (from gate 141 towards door 142 of the latter).

Likewise, the falling signal S2 arriving on the line
F is directed, via circulator 14 (from door 142 to door 143 of the latter), to another optical amplifier 15 whose output is connected to the output terminal S of the link.

 In the aforementioned case where the signals Si and S2 are of different characteristics, it is thus possible to obtain an amplification, at the start and / or at the end of the line respectively, using amplifiers 13 and 15 which are different and each adapted to the signals, Si and S2, which pass through them.

 It goes without saying that the invention is in no way limited to the few exemplary embodiments which have just been described.

On the contrary, it is capable of being implemented in other ways and by using various equivalent means. It applies equally to the use of optical amplifiers with doped fiber, as of optical amplifiers with semiconductors.

Claims (7)

F2, F'2) optical fibers per bi-directional optical link line (2), this installation comprising at least one optical amplification station (1) interposed on the bi-directional line (2), characterized in that this amplification station (i) comprises at least one optical amplifier (3) which is coupled to said bi-directional line (2) by means of at least one optical circulator (5,6) capable of ensuring the decoupling of the two channels , respectively 1, uplink "(S1) and" downlink "(82), of the bidirectional link.  1- Installation of bi-directional transmission of information by optical fiber (s) with either one (F, F ') or two (Fl, F'1 and 2- optical transmission installation according to claim 1, the optical link comprising a fiber in the direction of the signal, "up" (Si) and "down" (S2), or altogether two fibers (F1, F'1 and F2, F'2) per line (2), characterized in that said amplification station (1) comprises a single bidirectional optical amplifier (3) of which each access, "upstream" (4) or "downstream" (16) and vice -versa, is connected to the corresponding pair of fibers (F1, F'2 and F'1, F2) by a respective optical circulator (5,6) each having four doors (51 to 54 and 61 to 64) succeeding each other clockwise, with said "upstream" access (4) of this single amplifier connected on one (52) of the four doors of the "upstream" circulator (5), and the two adjacent doors, previous (51) and next (53), of this circulator (5) respectively connected to the upstream signal fiber (F1) and to the downstream signal fiber (F'2), and with symmetrical access "downstream" (16) of this amplifier (3) connects on one (64) of the four doors of the downstream circulator (6) and the two adjacent doors, precederlte (63) and following (61), of this downstream circulator (6) respe (.tiv-ely connected to the fiber (F2) of downlink signal and to the fiber (F'1) of uplink signal. 3- Installation according to claim 2, characterized in ctl that a bi-directional redundancy optical amplifier (7) tbst further branch between the two fourth doors (54. 62), previously remained free, of these two circulators, upstream ( 5) and downstream (6), the replacement of the aforementioned amplifier (3) by this redundancy amplifier (7) then being carried out simply by inversion, at the head of the lines, of the connections of each of the two fibers (Fî, F 'i and F2, F'2) of the bi-directional line (2). 4- Installation according to claim 1, the optical link comprising a single fiber (F, F ') conveying both the uplink signal (Si) and the downlink signal (S2), characterized in that said amplification station ( 1) comprises two optical amplifiers (8, 9) of different characteristics and each capable of respectively amplifying the uplink signal (S1) and the downlink signal (S2), these two separate optical amplifiers (8, 9) being each decoupled from this single fiber (F, F ') using at least one optical circulator (5, 6 or 10). 5- Installation according to claim 4, characterized in that there are provided two optical circulators (5, 6), one on either side of each optical amplifier (8, 9), including an "upstream" circulator (5 ) of which a door (54) is connected to the fiber (F ') of incident rising signal and of which the two adjacent doors, preceding (53) and following (51), are respectively connected to the output of the amplifier (9) signal downstream and at the input of the upstream signal amplifier (8), and a "downstream" circulator (6) on which the connections (62, 61, 63) are symmetrical with the previous ones. 6- Installation according to claim 4, characterized in that there is provided a single optical circulator (10) which is interposed on the line (2), with a door (104) connected to the fiber (F ') of falling signal , while each optical amplifier (8,9) is respectively connected to one (101,102) of the two other diametrically opposite doors with its output respectively connected to a reflecting mirror of its own (11,12). 7- Installation according to one of claims 1, 4, 5 or 6, the optical link comprising a single fiber (F) carrying both the uplink signal (S1) and the downlink signal (S2), characterized in that '' at least one of its end, input and / or output terminals, has its two input (E) and output (S) terminals of the signals (S1, S2) which are decoupled with respect to said fiber (F) through a common optical circulator (14) and a respective optical amplifier (13, 14).