CODIRECTIONAL ROPA SUPPLIED WITH POWER VIA A SEPARATE FIBER TRANSMITTING DATA IN OPPOSITE DIRECTION

Abstract

The invention discloses a method of amplifying an optical signal, in particular a data signal, transmitted from a first location (A) to a second location (B) via a first transmission link (10a), wherein said optical signal is amplified by means of a transmitter side remote optically pumped amplifiers (ROPA) (18) comprising a gain medium (24), wherein the gain medium (24) of said transmitter side ROPA (18) is pumped by means of transmitter side pump power (20) provided from said first location (A), characterized in that at least a part of said transmitter side pump power (20) is provided by means of light supplied from said first location (A) to said transmitter side ROPA (18) via a portion of a second transmission link (10b) provided for transmitting optical signals from said second location (B) to said first location (A).

Claims

1. A method of amplifying an optical signal transmitted from a first location to a second location via a first transmission link, wherein said optical signal is amplified by means of a transmitter side remote optically pumped amplifier (ROPA) comprising a gain medium, wherein the gain medium of said transmitter side ROPA is pumped by means of transmitter side pump power provided from said first location, wherein at least a part of said transmitter side pump power is provided by means of light supplied from said first location to said transmitter side ROPA via a portion of a second transmission link provided for transmitting optical signals from said second location to said first location, wherein said at least part of said transmitter side pump power is transferred from said second transmission link to said first transmission link by means of a connection comprising a connection link which is connected at one end with the first transmission link and at the other end with the second transmission link, wherein said connection link is connected to said first and second transmission links by means of couplers, and wherein a splitter is provided in said connection link, said splitter allowing for splitting part of the light passing said connection link from said second transmission link to said first transmission link and feeding it back into the second transmission link, wherein said method further comprises a step of amplifying an optical signal, in particular an optical data signal transmitted from the second location to the first location via said second transmission link, wherein said step of amplifying said optical signal comprises amplifying said optical signal by means of a receiver side ROPA provided in said second transmission link, wherein said receiver side ROPA comprises a gain medium, wherein the gain medium of said receiver side ROPA is pumped by means of receiver side pump power provided from said first location, wherein at least part of the receiver side pump power is power split off from the light passing said connection link from said second transmission link to said first transmission link and fed back into the second transmission link.

2. The method of claim 1, wherein at least 30% of the transmitter side pump power is provided by means of said light supplied from said first location to said transmitter side ROPA via a portion of a second transmission link.

3. The method of claim 1, wherein said light providing said pump power for pumping the gain medium of said transmitter side ROPA is provided by one or more of a transmitter side pump signal for pumping said gain medium of said transmitter side ROPA, a pump signal for pumping a further amplifier, in combination with a seed signal for generating a transmitter side pump signal for pumping said gain medium of said transmitter side ROPA and/or a shorter wavelength signal which upon one or more stimulated Raman scattering processes in combination with one or more seed signals produces a transmitter side pump signal for pumping said gain medium of said transmitter side ROPA, the wavelength of said shorter wavelength signal being shorter than the wavelength of said transmitter side pump signal.

4. The method of claim 1, wherein additional pump power for pumping said gain medium of said transmitter side ROPA is supplied from the first location to said transmitter side ROPA via said first transmission link.

5. The method of claim 4, wherein the amount of said additional pump power supplied from the first location to said transmitter side ROPA via said first transmission link is smaller than the amount of said transmitter side pump power provided by means of light via said portion of said second transmission link.

6. The method of claim 1, wherein the transmitter side ROPA is located 20 to 70 km away from said first location.

7. The method of claim 1, wherein said method further comprises amplifying said optical signal transmitted from said first location to said second location via said first transmission link by means of a receiver side ROPA provided in said first transmission link and comprising a gain medium, wherein said receiver side ROPA is pumped by means of pump power provided from said second location.

8. The method of claim 7, wherein said receiver side ROPA provided in said first transmission link is located 60 to 150 km from said second location.

9. The method of claim 7, wherein said receiver side ROPA and said transmitter side ROPA are located within said first transmission link at a distance of at least 10 km from each other.

10. The method of claim 1, wherein said light providing said pump power for pumping the gain medium of said receiver side ROPA in said second transmission link is provided by one or more of a receiver side pump signal for pumping said gain medium of said receiver side ROPA, a pump signal for pumping a further amplifier, in combination with a seed signal for generating a receiver side pump signal for pumping said gain medium of said receiver side ROPA and/or a shorter wavelength signal which upon one or more stimulated Raman scattering processes in combination with one or more seed signals produces a receiver side pump signal for pumping said gain medium of said receiver side ROPA, the wavelength of said shorter wavelength signal being shorter than the wavelength of said receiver side pump signal.

11. The method of claim 1, wherein said optical signal transmitted from the second location to the first location via said second transmission link is further amplified by means of a transmitter side ROPA comprising a gain medium, wherein the gain medium of said transmitter side ROPA is pumped by means of transmitter side pump power provided from said second location, wherein at least a part of said transmitter side pump power is provided by means of light supplied from said second location to said transmitter side ROPA via a portion of said first transmission link.

12. A bidirectional optical link comprising first and second transmission links extending between a first location and a second location, said first transmission link for transmitting optical signals from the first location to the second location, and said second transmission link for transmitting optical signals from the second location the first location, wherein in said first transmission link, a transmitter side remote optically pumped amplifier (ROPA) comprising a gain medium is provided, wherein the gain medium of said transmitter side ROPA is configured to be pumped by means of transmitter side pump power provided from said first location, wherein said transmitter side ROPA is arranged and configured such that at least a part of said transmitter side pump power can be provided by means of light supplied from said first location to said transmitter side ROPA via a portion of said second transmission link, further comprising a connection for transferring said at least part of said transmitter side pump power from said second transmission link to said first transmission link, wherein said connection comprises a connection link which is connected at one end with the first transmission link and at the other and with the second transmission link, wherein said connection link is connected to said first and second transmission links by means of couplers, and wherein a splitter is provided in said connection link, said splitter allowing for splitting part of the light passing said connection link from said second transmission link to said first transmission link and feeding it back into the second transmission link, wherein said bidirectional optical link further comprises a receiver side ROPA provided in said second transmission link, wherein said receiver side ROPA comprises a gain medium, wherein the gain medium of said receiver side ROPA is configured to be pumped by means of receiver side pump power provided from said first location, wherein at least part of the receiver side pump power is power split off from the light passing said connection link from said second transmission link to said first transmission link and fed back into the second transmission link.

13. The bidirectional optical link of claim 12, further configured such that at least 30% of the transmitter side pump power is provided by means of said light supplied from said first location to said transmitter side ROPA via a portion of a second transmission link.

14. The bidirectional optical link of claim 12, wherein said light providing said pump power for pumping the gain medium of said transmitter side ROPA is provided by one or more of a light source for providing a transmitter side pump signal for pumping said gain medium of said transmitter side ROPA, a light source providing a pump signal for pumping a further amplifier, in combination with a seed signal for generating a transmitter side pump signal for pumping said gain medium of said transmitter side ROPA and/or a light source for providing a shorter wavelength signal which upon one or more stimulated Raman scattering processes in combination with one or more seed signals produces a transmitter side pump signal for pumping said gain medium of said transmitter side ROPA, the wavelength of said shorter wavelength signal being shorter than the wavelength of said transmitter side pump signal.

15. The bidirectional optical link of claim 12, further comprising a pump light source configured to supply additional pump power for pumping said gain medium of said transmitter side ROPA from the first location to said transmitter side ROPA via said first transmission link.

16. The bidirectional optical link of claim 15, further configured such that the amount of said additional pump power supplied from the first location to said transmitter side ROPA via said first transmission link is smaller than the amount of said transmitter side pump power provided by means of light via said portion of said second transmission link.

17. The bidirectional optical link of claim 12, wherein the transmitter side ROPA is located 20 to 70 km away from said first location.

18. The bidirectional optical link of claim 12, further comprising a receiver side ROPA provided in said first transmission link and comprising a gain medium, wherein said receiver side ROPA is configured to be pumped by means of pump power provided from said second location.

19. The bidirectional optical link of claim 18, wherein said receiver side ROPA provided in said first transmission link is located 60 to 150 km away from said second location.

20. The bidirectional optical link of claim 18, wherein said receiver side ROPA and said transmitter side ROPA are located within said first transmission link at a distance of at least 10 km from each other.

21. The bidirectional optical link of claim 12, wherein said light providing said pump power for pumping the gain medium of said receiver side ROPA in said second transmission link is provided by one or more of a light source for providing a receiver side pump signal for pumping said gain medium of said receiver side ROPA, a light source for providing a pump signal for pumping a further amplifier, in combination with a seed signal for generating a receiver side pump signal for pumping said gain medium of said receiver side ROPA and/or a light source providing a shorter wavelength signal which upon one or more stimulated Raman scattering processes in combination with one or more seed signals produces a receiver side pump signal for pumping said gain medium of said receiver side ROPA, the wavelength of said shorter wavelength signal being shorter than the wavelength of said receiver side pump signal.

22. The bidirectional optical link of claim 12, wherein said second transmission link further comprises a transmitter side ROPA comprising a gain medium, wherein the gain medium of said transmitter side ROPA is configured to be pumped by means of transmitter side pump power provided from said second location, wherein at least a part of said transmitter side pump power is provided by means of light supplied from said second location to said transmitter side ROPA via a portion of said first transmission link.

23. A remote optically pumped amplifier (ROPA) comprising a gain medium, said ROPA for installation in a first transmission link for amplifying optical signals, in particular optical data signals transmitted therein, wherein said ROPA further comprises a connection for transferring pump power from a second transmission link to said first transmission link, wherein said connection comprises a connection link for connecting at one end with the first transmission link and at the other and with a second transmission link, wherein said connection link comprises couplers, in particular WDM couplers for connecting said connection link to said first and second transmission links and wherein a splitter is provided in said connection link, said splitter allowing for splitting part of the light passing said connection link from said second transmission link to said first transmission link and feeding it back into the second transmission link.

24. The ROPA of claim 23, further configured to act as a transmitter side ROPA claim 12.

25. A remote optically pumped amplifier (ROPA), which is formed in a double clad fiber, said double clad fiber having a core suitable for carrying data signals, an inner cladding having a first index of refraction and suitable for carrying pump signals, and an outer cladding having a second index of refraction, wherein said double clad fiber has at least one first section, in which a boundary between the inner and outer claddings of the double clad fiber is at least nearly rotationally symmetric with respect to the axis of said double clad fiber, thereby promoting the formation of modes of light that have a comparatively little overlap with the core, and at least one second section, in which said boundary between the inner and outer claddings of the double clad fiber deviates from the rotationally symmetric shape of the first section and the propagation of said modes of light having little overlap with the core is impeded, wherein a portion of the core of said double clad fiber located in and/or adjacent to the at least one second section comprises a gain medium, in particular an erbium doping.

26. The ROPA of claim 25, wherein said first index of refraction is higher than said second index of refraction.

27. The ROPA of claim 25, wherein a plurality of first and second sections are alternatingly formed in said double clad fiber wherein with each of said second sections, a corresponding portion of the core of said double clad fiber comprising said gain medium is associated.

28. The ROPA of claim 25, wherein said double clad fiber has a first end, wherein at said first end, a transmitter is operatively connected with said double clad fiber such that data signals provided by said transmitter are coupled into the core, and wherein at said first end, a source of pump light is operatively connected with said double clad fiber such that pump light provided by said source of pump light is coupled into the inner cladding.

29. The ROPA of one of claim 25, wherein at least a part of said at least one gain medium comprising portion of the core of said double clad fiber is located downstream from a corresponding second section with regard to data signals and pump light injected to the double clad fiber at said first end.

Description

SHORT DESCRIPTION OF THE FIGURES

[0062] FIG. 1 is a diagram showing the reach improvement provided by different combinations of amplification technologies when using different modulation formats as compared with the base configuration operated with 10 Gbit/s.

[0063] FIG. 2 shows a setup showing providing pump power to a receiver side ROPA and a transmitter side ROPA via the corresponding transmission fiber.

[0064] FIG. 3 shows an alternative setup, where pump power is fed to a transmitter side ROPA via a dedicated separate fiber.

[0065] FIG. 4 shows an embodiment of the invention, in which pump power is supplied to the transmitter side ROPA via a fiber provided for data transmission in the opposite direction.

[0066] FIG. 5 shows a further embodiment similar to the setup of FIG. 4, in which additional pump power is supplied codirectionally with the data signals to the transmitter side ROPA.

[0067] FIG. 6 shows a related embodiment employing a thulium doped fiber and a seed signal for generating a pump signal.

[0068] FIG. 7 shows a further related embodiment employing a thulium doped fiber and a seed signal for generating a pump signal.

[0069] FIG. 8 shows a double clad fiber used for guiding pump power to the ROPA position.

[0070] FIG. 9 shows a further embodiment of a transmitter side ROPA using a double clad fiber as shown in FIG. 8.

[0071] FIG. 10 shows a plurality of transmitter side ROPAs formed in a double clad fiber, as well as the corresponding power distribution.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0072] For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the preferred embodiments illustrated in the drawings and specific language will be used to describe the same. It will, nevertheless, be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates.

[0073] FIG. 4 shows an embodiment of the present invention. FIG. 4 shows a first transmission link 10a extending between first and second locations A and B, and a second transmission link 10b likewise extending between the first and second locations A and B for enabling data transmission in opposite direction. In this regard, the embodiment of the invention makes use of the fact that in fiber communication systems typically pairs of fibers (i.e. links 10a and 10b) are employed that transmit signals in opposite directions. In the embodiment shown in FIG. 4, the first link 10a is for transmitting data signals 12 from location A to location B, and the second link 10b is for transmitting data signals 12 from location B to location A. In the first link 10a, a transmitter side ROPA 18 is provided, which in the shown embodiment is located 30 km away from location A. In the second link 10b, a receiver side ROPA 14 is provided, which in the embodiment shown is 100 km away from location A, which for the second link 10b corresponds to the receiver side. While not shown in FIG. 4, which only shows the components close to location A, a similar transmitter side ROPA 18 would typically be provided in the second link 10b at approximately 30 km away from location B, and a similar receiver side ROPA 14 would be provided in the first link 10a approximately 100 km away from the second location B.

[0074] As is further shown in FIG. 4, the transmitter side ROPA 18 comprises an erbium doped fiber (EDF) 24 and an isolator 26 in the same way as the ROPA 18 shown in FIGS. 2 and 3. However, the transmitter side ROPA 18 further comprises a connection 30 allowing transmitter side pump signals 20 to be fed from location A to the transmitter side ROPA 18 via the second transmission link 10b. In the embodiment shown, the connection 30 comprises two WDM couplers 28 connected by a connection link 32. The WDM couplers 28 are devised to pass the wavelength of the data signal 12 but to couple the wavelength of the pump signal 20 out of the second transmission link 101) and into the first transmission link 10a. Within the connection link 32, a splitter 34 is provided which splits part of the pump signal 20 from the connection link 32 and feeds it back into the second transmission link 10b by means of a further WDM coupler 28. This split part of the pump signal 20 is then used to pump the receiver side ROPA 14 provided in the second transmission link 10b, and hence has the same function as the receiver side pump signal 16 shown in FIGS. 2 and 3.

[0075] FIG. 4 hence shows a solution for supplying the pump power (pump signal 20) for the transmitter side ROPA 18 via the second transmission link 10b used for data transmission in the opposite direction. Thus, the pump signal 20 is always travelling counterdirectionally to the data signals 12, and this way, signal distortions, such as phase distortions resulting from nonlinear fiber effects induced by the pump 20 signals are almost completely avoided. An appropriate location for coupling the pump signal 20 out of the second transmission link 10b and feeding it to the transmitter side ROPA 18 in the first transmission link 10a is at a distance between 30 km (as shown in the example) and 80 km. As shown in FIG. 4, an appropriate distance of the receiver side ROPA 14 in the second transmission link 10b is about 100 km away from location A, or in other words, from the receiver (not shown) to receive the data signals 12 carried on the second transmission link 10b.

[0076] Please note that the total pump power of the pump signal 20 that can be transmitted to the transmitter side ROPA 18 in this way is limited due to the Raman amplification induced thereby in the second transmission link 10b. However, it is possible to in addition transmit an additional pump signal 36 from location A to the transmitter side ROPA 18 via the first transmission link 10a codirectionally to the signals 12 provided that its power is small enough to avoid severe nonlinear distortions upon codirectional supply. This situation is shown in FIG. 5, where an additional pump signal with a wavelength of 1495 nm and moderate power is sent from location A to the transmitter side ROPA 18 via the first transmission link 10a. The wavelength of this additional pump signal 36 is chosen slightly different from the wavelength of the pump signal 20 (1480 nm) such as not to be affected by the WDM coupler 28 in the first transmission link 10a. The remainder of FIG. 5 is identical to FIG. 4.

[0077] Please note that all solutions presented so far are also compatible with higher-order pumping schemes. In fact, pump power can be transferred from a smallest wavelength to one or more intermediate low power seeds in the fiber segment from the receivers to the corresponding ROPA cassette. The wavelengths of the seeds are adapted to provide sufficient amplification in the EDF coils 24. Proper adjustment of the seed powers allows to optimize the ROPA gain of the receiver side ROPA 14 and the transmitter side ROPA 18 separately.

[0078] While the pump power launched into the ROPA cassette should be as high as possible, as indicated above, excessive Raman amplification of the signals 12 in the fiber section from the terminal where pump part light is injected (in the embodiment previously discussed, location A) to the ROPA cassette limits the maximum pump power. Using higher-order pumping, it is possible to provide the required pump power at smaller Raman gain. Thus, larger pump powers are acceptable.

[0079] FIG. 6 shows a variant in which Raman amplification by the ROPA pump is avoided completely. The setup is generally the same as that of FIG. 4, except that a thulium-doped fiber (TDF) 38 is provided in the second transmission link 10b close to the connection 30 between the first and second transmission links 10a, 10b. In the shown embodiment, the TDF 38 is in fact arranged in the ROPA cassette of the transmitter side ROPA 14 just before the coupler 28. A TDF is suitable to provide amplification in the wavelength range from 1440 nm to 1480 nm and can be pumped at 1050 nm or 1064 nm. In the embodiment shown, the TDF 38 is pumped with a corresponding pump signal 40 at 1050 nm. Further, a seed signal 42 at 1465 nm is injected to the second transmission link 10b together with the pump signal 40 at location A. Due to the large wavelength separation, no power is transferred from the pump signal 40 to the seed signal 42 via stimulated Raman scattering. Furthermore, the data signals 12 carried in the second transmission link 10b do not experience phase fluctuations since the high power pump signal 40 is propagating counterdirectionally to the signals 12. In the first ROPA cassette of the transmitter side ROPA 14, the seed signal 42 is amplified in the TDF 38 before being provided to the two EDF coils 24 provided in the ROPAs 14, 18 for amplification of the signals 12 in the corresponding transmission links 10a, 10b. Please note that the TDF 38 is transparent for signals in the C-band (1530 nm to 1565 nm). However, in case there is some signal attenuation associated with signals 12 passing through the TDF coil 38, a corresponding bypass (not shown) could be used. Furthermore, instead of arranging the TDF 38 in the cassette of the transmitter side ROPA 18, it could also be placed in the fiber segment of the second transmission link 10b anywhere between location A and the connection 30.

[0080] Please note that instead of the thulium doped fiber any other medium that is suitable for amplifying wavelengths that may be used for pumping the gain medium 24 of the ROPA and that can be provided with energy by lightwaves that almost do not interact with the signals via SRS in the transmission fiber can be used.

[0081] FIG. 7 shows a modification of the setup of FIG. 6. A high power pump at 1480 nm (transmitter side pump signal 20) is launched into the second transmission link 10b for providing pump power to the transmitter side ROPA 18 in the same manner as explained with reference to FIG. 4. Furthermore, a low power seed signal 42 at 1465 nm and a high power pump signal 40 at 1050 nm (or 1064 nm) are launched into the second transmission link 10b. In the TDF 38, which in this embodiment is arranged to the right of the connection 30 (in other words, further away from location A than the connection 30), the low power seed 42 is amplified to a level that is sufficient for amplification in the receiver side ROPA 14 provided in the second transmission link 10b. Please note that the high power 1480 nm pump will slightly deplete the 1465 nm seed by stimulated Raman emission. However, this effect is quite weak due to the small wavelength separation.

[0082] The presented technique can be applied to any kind of waveguides transmitting signals in opposite directions. In the examples above, the waveguides have been identified with different single core fibers. However, the same technique could also be applied to pairs of cores of a multi-core fiber.

[0083] In all of the embodiments of FIGS. 4 to 7, the pump power for the transmitter side ROPA 18 in the first transmission link 10a, which would have to be transmitted codirectionally with the data signals 12 if provided within the transmission link 10a as well, is transmitted to the transmitter side ROPA 18 via the second transmission link 10b, where it is provided counterdirectionally to the data signals 12 carried in this second transmission link 10b. This way, a codirectional co-propagation of pump signal and data signal, and the signal distortion associated therewith, in particular the occurrence of severe phase noise, can be avoided.

[0084] FIG. 8 shows an alternative solution to the same problem of supplying pump power to the transmitter side ROPA 18 without inducing noticeable signal distortions due to nonlinear fiber effects. FIG. 10 shows a longitudinal section and three cross-sections of a double-clad fiber 44 that can be used for providing large pump powers to a part doped with erbium without inducing significant nonlinear effects. The double-clad fiber 44 has a core 46, and inner cladding 48 and an outer cladding 50. In the embodiment shown, the inner cladding 48 has a higher refractive index than the outer cladding 50. Thus, light can be guided within the inner cladding 48 in the same way as in the core 46, but at different wavelength.

[0085] The left end of the fiber 44 shown in FIG. 8 corresponds to the transmitter location. To the right of the section B-B in FIG. 10, the core 46 is doped with erbium as shown under reference sign 52, such that the right portion of the double clad fiber 44 effectively represents a transmitter side ROPA. In the illustration of FIG. 10, pump power 20 is coupled into the inner cladding 48 and propagates within this part of the fiber, while the data signals 12 propagate in the smaller core 46 like they are doing in standard fibers. In the left portion of the fiber 44, a symmetrical refraction index profile is chosen, as is apparent from section A-A. This symmetrical refraction profile is maintained for at least the main part of the distance where the erbium doping 52 starts, which distance could, by comparison with the embodiments previously described be on the order of 30 km for example.

[0086] The circular shape of the border separating the inner cladding 48 from the outer cladding 50 leads to many modes of the light that hardly overlap with the core 46. In the language of geometrical optics, these modes can be viewed as helical rays that do not pass the core 46. As a consequence, the overlap between the modes of the pump light 20 and the modes of the data signal 12, is very small, which keeps nonlinear interactions of the co-propagating light low and hence avoids the problems usually encountered when the pump light and the signal light are codirectionally propagating in the same fiber core.

[0087] Close to or at the location of the erbium doping 52 (or in other words, the ROPA) the shape of the outer cladding 48 is modified such as to be noncircular. At this part of the fiber 44, thanks to the lower symmetry, helical rays are suppressed and the overlap with the core is significantly increased. Thus, pump power of the pump signal 20 is directed into modes that have a high overlap with the doped core resulting in efficient pumping of the erbium ions in the core.

[0088] FIG. 9 shows how this double clad ROPA can be used as a transmission side ROPA in a transmission link. On the left of FIG. 9, a plurality of transmitters 64 are shown, which are connected to a multiplexer 68. A booster amplifier 60 is provided between the multiplexer 68 and the left end of the double clad fiber 44. Optical data signals are traveling in the double clad fiber 44, and more precisely in the core 46 of the double clad fiber 44, from left to right in FIG. 9. Further, a multimode light source 72 for providing pump light is coupled by means of a suitable coupling apparatus 74 with the inner cladding 48 of the double clad fiber 44 such as to travel codirectionally with the optical data signals from left to right, but at least predominantly in different geometrical regions of the double clad fiber 44.

[0089] Toward the right end of the double clad fiber 44, an erbium doped region 52 within the core 46 of the double clad fiber 44 is formed, which resembles a transmitter side ROPA 18 as disclosed herein. While not shown in detail in FIG. 9, close to or at the location of the erbium doping 52, the shape of the outer cladding 48 is modified such as to be noncircular, thereby causing an increased overlap of the pump light traveling within the inner cladding 48 with the erbium doped core 46 such as to pump the same and to cause an amplification of the data signals passing their through. The ROPA 18 further comprises an optical isolator 26. In addition, a receiver side ROPA 14 is provided, which is pumped counterdirectionally by pump light provided by a further multimode light source 72. At the right end of the transmission link, a preamplifier 62, a demultiplexer 70 and a number of receivers 66 are provided. The distance from the transmit side ROPA (after the isolator 26) to the receiver side is preferably bridged by single mode transmission fibers due to cost reasons.

[0090] In the presented example, the transmitters 64 are connected via the multiplexer 68 directly to the considered span. But of course, the optical signals might be provided to the span under consideration also via additional spans. Analogously, the signals transmitted over the considered span might be forwarded to the respective receivers via some additional fiber spans.

[0091] Instead of employing a single double clad fiber-based transmitter side ROPA 18, it is easily possible to provide for a plurality of transmitter side ROPAs 18, as is shown in FIG. 10. As is seen therein, the double clad fiber 44 in this case comprises five separate regions 52 with erbium doping, and a corresponding change in the shape of the outer cladding 48, each resembling effectively a transmitter side ROPA. Further shown in FIG. 10 is the power distribution of the optical signal as a function of propagation distance. The solid line shows the power distribution if only a single transmitter side ROPA was provided at the end of the double clad fiber 44. The dashed line shows the power distribution in case of the plural ROPAs, which can be kept nearly constant along the propagation distance.

[0092] The examples described above and the drawings merely serve to illustrate the invention and its advantages over the prior art, and should not be understood as a limitation in any sense. The scope of the invention is solely determined by the appended set of claims.

REFERENCE SIGNS

[0093] 10 transmission link [0094] 12 data signal [0095] 14 receiver side ROPA [0096] 16 receiver side pump signal [0097] 18 transmitter side ROPA [0098] 20 transmitter side pump signal [0099] 22 separate fiber [0100] 24 EDF coil [0101] 26 optical isolator [0102] 28 WDM coupler [0103] 30 connection [0104] 32 connection link [0105] 34 splitter [0106] 36 additional pump signal [0107] 37 further connection [0108] 38 thulium doped fiber [0109] 40 pump signal for thulium doped fiber [0110] 42 seed signal [0111] 44 double clad fiber [0112] 46 core of double clad fiber 44 [0113] 48 inner cladding of double clad fiber 44 [0114] 50 outer cladding of double clad fiber 44 [0115] 52 erbium doping within core of double clad fiber 44 [0116] 60 booster amplifier [0117] 62 preamplifier [0118] 64 transmitter [0119] 66 receiver [0120] 68 multiplexer [0121] 70 demultiplexer [0122] 72 multimode light source [0123] 74 coupling apparatus [0124] 80 power distribution with one TX ROPA [0125] 82 power distribution with several TX ROPAs