TECHNICAL FIELD

The present disclosure relates to optical networks and multi-band optical transmission systems. The disclosure presents an amplification device for a multi-band optical transmission system, the multi-band optical transmission system, and a method for operating the multi-band optical transmission system.

BACKGROUND

Exemplary optical transmission systems can multiplex several channels or signals over the same fiber, wherein each of the signals is carried over a different wavelength. In this way, the utilized bandwidth of the fiber can be extended. For instance, the C band (roughly, the wavelengths between 1530-1565 nm) may be used. Other bands, such as the S band (below the C band) and the L band may be used in the future, with the aim to further increase the capacity of optical transmission systems.

In such optical transmission systems, amplifiers may be used to periodically amplify the signal power of each signal during its transmission. This may enable compensating signal loss incurred during the transmission of the signals over the optical fiber.

SUMMARY

In view of the above, this disclosure provides a reliable multi-band optical transmission system. For example, an objective is to ensure the quality of the signals, which are carried on various bands of the optical transmission system. For instance, if there is a signal loss on one of the bands, at least the quality of the signals on the other bands should be maintained. For this purpose the disclosure aims for an improved amplification device, which may be used at an amplification site of the optical transmission system. For example, if an amplifier on one band downstream of the amplification device fails, the amplification device should be able to compensate for the loss of signal power on the band associated with the failed amplifier. These and other objectives are achieved by this disclosure, for instance, as described in the independent claims. Advantageous implementations are further described in the dependent claims.

A first aspect of this disclosure provides an amplification device for a multi-band optical transmission system, the amplification device comprising: a first port connectable to a first bidirectional optical transmission line and a second port connectable to a second bi-directional optical transmission line; a set of first amplifiers for amplifying a set of first signals received at the first port and providing the amplified set of first signals to the second port, wherein each first signal is carried on a different band, of a set of bands, than the other first signals, and wherein each first amplifier is associated with one of the bands and configured to amplify the first signal carried on that band; a set of second amplifiers for amplifying a set of second signals received at the second port and providing the amplified set of second signals to the first port, wherein each second signal is carried on a different band, of the set of bands, than the other second signals, and wherein each second amplifier is associated with one of the bands and configured to amplify the second signal carried on that band; wherein the amplification device is at least configured to power feed one first amplifier with power from one second amplifier associated with the same band as the first amplifier.

The amplification device according to the first aspect enables a very reliable multi-band optical transmission system. For example, the amplification device may help to ensure the quality of the signals carried on the various bands used by the optical transmission system. For instance, if there is a signal loss on one of the bands (the band associated with the power-fed first amplifier), then the quality of the signals on the other bands can be maintained by power feeding the first amplifier. Some explanation about this effect is provided in the following.

During multi-band transmission over an optical transmission line, like an optical fiber, the Stimulated Raman Scattering (SRS) effect leads to a transfer of power from signals at shorter wavelengths to signals at longer wavelengths. The optical signal to noise ratio (OSNR), which is the ratio of signal power to the Amplified Spontaneous Noise (ASE) noise power, will increase at the longer wavelengths and will decrease at the shorter wavelengths. Additionally, “nonlinear” noise is generated by the Kerr effect during propagation over the optical transmission line, which is exacerbated with optical power. Conversely, the combination of the SRS effect and the Kerr effect will make the signal to nonlinear noise ratio (SNNR) decrease at the longer wavelengths and increase at the shorter wavelengths.

The magnitude of the SRS-induced power transfer depends on the total optical power launched on the optical transmission line. For example, the SRS is exacerbated for larger optical powers. As more transmission bands are used, more optical power will be launched, which will increase the SRS and thus the power transfer across the bands, thereby impacting both the OSNR and the SNNR. The SNR, which combines the OSNR and the SNNR (1/SNR=1/OSNR+1/SNNR, when all noises are measured/estimated within the band of the signal), is impacted by the SRS differently, depending on the wavelength of the considered signal.

In a multi-band transmission system, at each of one or more amplification sites, amplifiers are provided and are set (e.g., through their gain) to compensate for the SRS effect and to ensure an optimum SNR of the signals on all bands.

However, if there is a loss of light power, for example caused by an amplifier fail, for a single band at a given amplification site, the launch power on the optical transmission line span following the failure will change. Thereby, also the SRS for signals on the other bands will change. Hence, their SNR is decreased, which possibly leads to disruptions. In fact, a disruption on one band (e.g., through a failure of an amplifier) may affect the quality (e.g., SNR) of the signals carried on the other bands, even if the amplifiers on the other bands are not impacted by the aforementioned failure.

For example, when a component located on one band of a multi-band optical transmission system fails, the signals on the other bands may fail as well. An amplifier downstream of the failure could be used to compensate for the loss of power on the failed band, however, its output optical power may be too low to compensate for the power loss on the next fiber span. If the power loss is not compensated, the SRS will change, and the signals on other bands will be impacted.

The amplification device of the first aspect thus is configured for the power feeding of at least one of the first amplifiers, in order to compensate such power loss on the band associated with the first amplifier, and thus to avoid that the SRS changes and affects the signals on the other bands. Notably, both the first port and the second port are bi-directional ports, e.g. physically in case that the bi-directional optical transmission lines each comprise two fibers (fibers are typically unidirectional). The first and the second bi-directional transmission optical transmission lines may thus each comprise two fibers (one per direction). Accordingly, each port may comprise two sub-ports, namely one sub-port per direction and/or per fiber of each optical transmission line. In other words, at the first port of the amplification device, the first signals may be received by a first sub-port and the second signals may be output by a second sub-port. Likewise, at the second port of the amplification device, the second signals may be received by a third sub-port and the first signals may be output by a fourth sub-port.

In an implementation form of the first aspect, the amplification device is configured to power feed each first amplifier with power from the respective second amplifier associated with the respective same band as that first amplifier.

In this way, a light power loss on each band in a first transmission direction can be compensated.

In an implementation form of the first aspect, the amplification device is further at least configured to power feed one second amplifier with power from one first amplifier associated with the same band as the second amplifier.

In this way, a light power loss can also be compensated in a second transmission direction opposite to the first transmission direction.

In an implementation form of the first aspect, the amplification device is configured to power feed each second amplifier with power from the respective first amplifier associated with the respective same band as that second amplifier.

In this way, a light power loss on each band in the second transmission direction can be compensated.

In an implementation form of the first aspect, the amplification device is configured to selectively enable and disable the power feeding of the at least one first amplifier and/or of the at least one second amplifier. For example, the amplification device can enable the power feeding only when it is needed. The enabling and disabling, and optionally determining whether power feeding is needed, may occur automatically in the amplification device.

In an implementation form of the first aspect, the amplification device is further configured to enable the power feeding of a particular first amplifier, if there is a loss of light power of the same band as the particular first amplifier is associated with on the first bi-directional transmission line; and/or enable the power feeding of a particular second amplifier, if there is a loss of light power of the same band as the particular second amplifier is associated with on the second bi-directional transmission line.

Thus, only if there is a light power loss, the power feeding may be enabled on the correct bands. Else, a regular operation without power feeding may be performed by the amplification device.

In an implementation form of the first aspect, the amplification device is further configured to: enable the power feeding of a particular first amplifier, if a second amplification device, from which the set of first signals is received over the first bi-directional transmission line at the first port, has a failed amplifier associated with the same band as the particular first amplifier; and/or enable the power feeding of a particular second amplifier, if a third amplification device, from which the set of second signals is received over the second bi-directional transmission line at the second port, has a failed amplifier associated with the same band as the particular second amplifier.

In this way, a failure of equipment upstream of the amplification device, in the first and/or the second transmission direction, can be compensated.

In an implementation form of the first aspect, the amplification device is further configured to at least one of determine, whether there is a loss of light power of at least one band on the first bi-directional transmission line; determine, whether there is a loss of light power of at least one band on the second bi-directional transmission line; determine, whether the second amplification device has a failed amplifier; determine, whether the third amplification device has a failed amplifier. Thus, the amplification device can itself take the decision to power feed or not. Thus, this decision can be taken locally at an amplification site where the amplification device is located.

In an implementation form of the first aspect, the amplification device is configured to determine whether there is a loss of light power of at least one band on the first and/or the second bi-directional transmission line, and/or whether the second and/or the third amplification device has a failed amplifier, based on light power monitoring on, respectively, the first bi-directional transmission line and/or on the second bi-directional optical transmission line.

This provides an easy but reliable way to detect failures, which may be compensated by the power feeding.

In an implementation form of the first aspect, the amplification device comprises at least one photodiode, wherein each photodiode is located at the input of one of the first amplifiers or of one of the second amplifiers, and is configured to detect a light power of the same band as the one of the first amplifiers or the one of the second amplifiers is associated with on, respectively, the first bi-directional transmission line or the second bi-directional transmission line.

In an implementation form of the first aspect, the amplification device comprises an optical gate, which is arranged between the particular first amplifier and the second amplifier associated with the same band as the particular first amplifier, and is configured to selectively enable and disable the power feeding of the particular first amplifier.

The optical gate provides a simple but efficient solution for implementing the selective power feeding.

In an implementation form of the first aspect, the optical gate includes a variable optical attenuator.

Thus, the optical gate is not just on/off, but its attenuation can be tuned. A variable optical attenuator may include a semiconductor optical amplifier (SO A). In an implementation form of the first aspect, the second amplifier associated with the same band as the particular first amplifier comprises an optical coupler for redirecting a part of the light power at its output through the optical gate to an input of the particular first amplifier.

In an implementation form of the first aspect, a ratio of the redirected light power to nonredirected light power is in a range of 20:80 to 1 :99.

In an implementation form of the first aspect, the second amplifier associated with the same band as the particular first amplifier comprises an optical switch for redirecting a part or all of the light power at its output to an input of the particular first amplifier.

In an implementation form of the first aspect, the particular first amplifier comprises an optical switch for redirecting a part or all of the light power at an output of the second amplifier associated with the same band as the particular first amplifier to an input of the particular first amplifier.

The optical switches may replace one or both of the optical couplers described above. The optical switches may be implemented with or without the optical gate.

In an implementation form of the first aspect, the optical gate or the optical switch are configured to enable the power feeding of the particular first amplifier or to redirect a part or all of the light power to the input of the particular first amplifier, respectively, if the photodiode at the input of the particular first amplifier detects a loss of light power of the same band as the particular first amplifier is associated with.

In an implementation form of the first aspect, the amplification device is located at one of a plurality of amplification sites of the multi-band optical transmission system.

A second aspect of this disclosure provides a multi-band optical transmission system comprising at least a first amplification device and a second amplification device, wherein at least the first amplification device is configured according to the first aspect or any implementation form thereof; wherein the first port of the first amplification device is connected to the second amplification device by the first bi-directional transmission line, wherein the second amplification device is configured to transmit the set of first signals over the first bi- directional transmission line to the first port of the first amplification device; and wherein the first amplification device is configured to power feed a particular first amplifier with power from the second amplifier associated with the same band as the particular first amplifier, if there is a loss of light power of the same band as the particular first amplifier is associated with on the first bi-directional transmission line.

The system of the second aspect provides the same advantages as the amplification device of the first aspect.

A third aspect of this disclosure provides a method for a multi-band optical transmission system, the multi-band optical transmission system comprising at least a first amplification device and a second amplification device, wherein at least the first amplification device is configured according to the first aspect or any implementation form thereof; wherein the first port of the first amplification device is connected to the second amplification device by the first bidirectional transmission line, wherein the second amplification device is configured to transmit the set of first signals over the first bi-directional transmission line to the first port of the first amplification device; and wherein the method comprises power feeding a particular first amplifier with power from the second amplifier associated with the same band as the particular first amplifier, if there is a loss of light power of the same band as the particular first amplifier is associated with on the first bi-directional transmission line.

The method of the third aspect provides the same advantages as the amplification device of the first aspect.

In summary, the aspects and implementation forms of this disclosure are based on the fact that in a multi-band optical transmission system, multiple amplifiers may be installed at dedicated locations called amplification sites. Transmission lines between different nodes (e.g., different amplification sites and/or other optical devices) of the optical transmission system are typically bi-directional. For example, at an amplification site, there may be an amplifier per transmission direction. In addition, there may be one amplifier per band (per direction).

In order to ensure that an amplifier can output sufficient power, this disclosure proposes to power feed it with optical power from the amplifier on the same band, but installed for the other transmission direction. This disclosure provides the hardware mechanism to perform such power feeding (power redirection) from one transmission direction to the other. The redirection may occur upon an upstream failure on the same band, wherein the failure causes a loss of signal (i.e., light as it is an optical transmission system) power on that band. The mechanism may be disabled in normal conditions, and may be enabled in failed conditions (where there is a loss of light power). Detection of such failures, and the decision whether the power feeding should be used or not, may be easily done through power monitoring at one or more or each amplifier of the amplification device.

The disclosure applies to point-to-point systems (single transmission line), and also to networks comprising optical routing nodes such as reconfigurable add-drop multiplexers (ROADM) interconnected by an optical multiplex section (OMS).

It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof.

BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

FIG. 1 shows an amplification device according to this disclosure.

FIG. 2 shows a bi-directional OMS including an amplification device according to this disclosure.

FIG. 3 shows an example of an optical network comprising optical routing nodes, such as ROADM nodes, which are interconnected by an OMS including an amplification device according to this disclosure.

FIG. 4 shows an exemplary amplification device according to this disclosure.

FIG. 5 shows an exemplary amplification device according to this disclosure. FIG. 6 shows an exemplary amplification device according to this disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an amplification device 100 according to this disclosure. The amplification device 100 is suitable for a multi-band optical transmission system.

The amplification device 100 comprises a first port 101, which is connectable to a first bidirectional optical transmission line 102, wherein the first bi-directional optical transmission line 102 may comprise two optical fibers, one optical fiber per transmission direction. The amplification device 100 also comprises a second port 103, which is connectable to a second bi-directional optical transmission line 104, wherein the second bi-directional optical transmission line 104 may also comprise two optical fibers, one optical fiber per transmission direction.

The amplification device 100 further comprises a set of first amplifiers 105 (FIG. 1 shows as example two such first amplifiers 105a and 105b). The first amplifiers 105 are for amplifying a set of first signals 106 (FIG. 1 shows as example two such first signals 106a and 106b), which are received at the first port 101. The first amplifiers 105 are further for providing the amplified set of first signals 107 (FIG. 1 shows two such amplified first signals 107a and 107b) to the second port 103. Each first signal 106 is carried on a different band, of a set of bands, than the other first signals 106. Each first amplifier 105 is associated with one of the bands and is configured to amplify the first signal 106 carried on that band. The first amplifiers 105 are accordingly used for receiving and amplifying signals transmitted in a first transmission direction.

Similarly, the amplification device 100 further comprises a set of second amplifiers 108 (FIG. 1 shows two such first amplifiers 108a and 108b). The second amplifiers 108 are for amplifying a set of second signals 109 (FIG. 1 shows two such first signals 109a and 109b, which are received at the second port 104. The second amplifiers 108 are further for providing the amplified set of second signals 110 (FIG. 1 shows two such amplified second signals 110a and 110b) to the first port. Each second signal 109 is carried on a different band, of the set of bands, than the other second signals 109. Each second amplifier 108 is associated with one of the bands, and is configured to amplify the second signal 109 carried on that band. The second amplifiers 108 are accordingly used for receiving and amplifying signals transmitted in a second transmission direction.

The amplification device 100 is at least configured to power feed one first amplifier 105 with power from one second amplifier 108, which is associated with the same band as the first amplifier 105. For instance, the amplification device 100 may be configured to power feed more than one or even each first amplifier 105 with power from the respective second amplifier 108, which is associated with the respective same band as that first amplifier 105. In this way, the amplification device 100 is configured to compensate for a loss of light power on at least one, or on more, or even on each, of the bands for at least the first transmission direction. Similarly, for the second transmission direction, the amplification device 100 may be at least configured to power feed one second amplifier 108 with power from one first amplifier 105, which is associated with the same band as the second amplifier 108. For instance, the amplification device 100 may be configured to power feed more than one, or even each second amplifier 108 with power from the respective first amplifier 105, which is associated with the respective same band as that second amplifier 108. In this way, the amplification device 100 may also be configured to compensate for a loss of light power on at least one, or on more, or even on each, of the bands for the second transmission direction.

Notably, the amplification device 100 may selectively enable and disable the power feeding of each first amplifier 105 and/or of each second amplifier 108, for which it is in principle configured to do the power feeding. Any power feeding may thereby be selectively enabled/disabled independent from the others. For instance, the power feeding of each first amplifier 105, for which power feeding is generally possible, can be enabled if there is a loss of light power of the same band as said first amplifier 105 on the first bi-directional transmission line 102, and can be disabled if there is no loss of light power. Likewise, the power feeding of each second amplifier 105, for which power feeding is generally possible, can be enabled if there is a loss of light power of the same band as said second amplifier 105 on the second bidirectional transmission line 104, and can be disabled if there is no loss of light power. The determination, whether there is such a loss of light power or not, can be made by the amplification device 100 itself. For instance, the amplification device 100 may perform one or more measurements, for example by using one or more photodiodes, on the light power per band and/or per transmission line 102, 104. FIG. 2 shows part of a multi-band optical transmission system 200 according to this disclosure, in which one or more amplification devices 100 as described above can be included. Each amplification device 100 according to this disclosure may be located at one of multiple in-line amplification sites of the optical transmission system 200. In particular, FIG. 2 shows an OMS of the optical transmission system 200, which connects nodes 210a, 210b by means of transmission lines and amplification sites (including amplification devices 100a and 100b).

The optical transmission system 200, as shown in FIG. 2, includes at least two RO ADM nodes, namely a first RO ADM node 210a and a second RO ADM node 210b, and includes the two amplification devices 100 according to this disclosure, namely a first amplification device 100a and a second amplification device 100b. The first amplification device 100a is connected to the first RO ADM node 210a via the first bi-directional transmission line 102, which comprises a first fiber 202a for the first transmission direction from the first RAODM node 210a to the first amplification device 100a, and a second fiber 202b for the second transmission direction from the first amplification device 100a to the first RAODM node 210a. The first port 101 of the first amplification device 100a comprises a first demultiplexer 201a, which is connected to the first RO ADM node 210a by the first fiber 202a, and comprises a first multiplexer 201b, which is connected to the first RO ADM node 210a by the second fiber 202b.

The first amplification device 100a is further connected to the second amplification device 100b by the second bi-directional transmission line 104, which comprises a third fiber 204a for the first transmission direction from the first amplification device 100a to the second amplification device 100b, and a fourth fiber 204b for the second transmission direction from the second amplification device 100b to the first amplification device 100a. The second port 103 of the first amplification device 100a comprises a second multiplexer 203a, which is connected to the second amplification device 100b by the third fiber 204a, and comprises a second demultiplexer 203b, which is connected to the second amplification device 100b by the fourth fiber 204b.

Similarly, the first port 101 of the second amplification device 100b comprises a demultiplexer 201c, which is connected to the first amplification device 100a by the third fiber 204a, and comprises a multiplexer 20 Id, which is connected to the first amplification device 100a by the fourth fiber 204b. The second port 103 of the second amplification device 100b comprises a multiplexer 203c, which is connected to another amplification device or to the second RO ADM node 210b by a fifth fiber, and comprises a demultiplexer 203d, which is connected to another amplification device or to the second RO ADM node 210b by a sixth fiber.

The first RO ADM 210a is configured to transmit the set of first signals 106 over the first fiber 202a to the first amplification device 100a. The first amplification device 100a is configured to amplify the set of first signals 106 and to provide the amplified set of first signals 107 over the third fiber 204a to the second amplification device 100b. As described with respect to FIG. 1, each first signal 106 is carried on a different band, of the set of bands, than the other first signals 106, and each first amplifier 105 of the first amplification device 100a is associated with one of the bands and is configured to amplify the first signal 106 carried on that band.

Further, the second amplification device 100a is configured to transmit the set of second signals 109 over the fourth fiber 204b to the first amplification device 100a. The first amplification device 100a is configured to amplify the set of second signals 109 and to provide the amplified set of second signals 110 to the first ROADM node 210a over the second fiber 202b. As described with respect to FIG. 1, each second signal 109 is carried on a different band, of the set of bands, than the other second signals 109, and each second amplifier 108 of the first amplification device 100a is associated with one of the bands and configured to amplify the second signal 109 carried on that band.

Further, the second amplification device 100b is configured to amplify the amplified set of first signals 107 again, and to provide the again amplified set of amplified first signals 107 over the fifth fiber to the another amplification device or to the second ROADM node 210b. Similarly, the set of second signals 109, which the second amplification device 100b sends to the first amplification device 100a over the fourth fiber 204b, may be already amplified second signals 109, which the second amplification device 100b has previously received, for example, from the another amplification device or the second ROADM node 210b and amplified.

The first amplification device 100a is configured to power feed at least one of the first amplifiers 105 with power from one of the second amplifiers 108, which is associated with the same band as the one of the first amplifiers 105. The first amplification device 100a may also be configured to power feed at least one of the second amplifiers 108 with power from one of the first amplifiers 105, which is associated with the same band as the one of the second amplifiers 108. This may be as described for the amplification device 100 of FIG. 1. That is, also more than one or each first amplifier 105 and/or second amplifier 108 of the first amplification device 100a may be power-fed like this. Further, this may be likewise true for the second amplification device 100b, which is also an amplification device of this disclosure. That is, also one or more or each first amplifier 105 and/or second amplifier 108 of the second amplification device 100b may be power-fed like this.

In this way, in the optical transmission system 200, a loss of light power of one, or more, or even all bands of the set of bands can be compensated, in particular, in case of an upstream light power loss. For instance, in the first transmission direction, if there is light loss between the first RO ADM node 210a and the first amplification device 100a, the power feeding of the at least one first amplifier 105 of the first amplification device 100a may compensate this light power loss. Further, if there is light power loss between the first amplification device 100a and the second amplification device 100b, the power feeding of the at least one first amplifier 105 of the second amplification device 100b may compensate this light power loss. In the second transmission direction, if there is light loss between the second RO ADM node 210b and the second amplification device 100b, the power feeding of the at least one second amplifier 108 of the second amplification device 100b may compensate this light power loss. Further, if there is light power loss between the second amplification device 100b and the first amplification device 100a, the power feeding of the at least one second amplifier 108 of the first amplification device 100a may compensate this light power loss.

Notably, as shown in FIG. 2, the different bands of the set of bands may comprise a low wavelength band, a medium wavelength band, and a high wavelength band. The nodes 210a, 210b may transmit (Tx) or receive (Rx) on these bands. Moreover, also the RO ADM nodes 210a and 210b may comprise multiplexers and demultiplexers, respectively, depending on whether a set of signals is received over a certain fiber and needs to be demultiplexed by the node 201a, 201b, or whether a set of signals is to be multiplexed for transmission on a fiber by the node 210a, 210b.

FIG. 2 shows part of a multi-band optical transmission system 300 according to this disclosure, in which at least one amplification device 100 of this disclosure, as described above, can be included. The amplification device 100 may be located between a second amplification device 301, which is not configured according to this disclosure, and a receiving (Rx) device, for instance, a RO ADM node 210 (could also be another amplification device). The first port 101 of the amplification device 100 of FIG. 3 is connected to the second amplification device 301 by the first bi-directional transmission line 102. The second amplification device 301 is configured to transmit the set of first signals 106 over the first bidirectional transmission line 102 to the first port 101 of the first amplification device 100. The first amplification device 100 is configured to power feed at least one particular first amplifier 105 of its set of first amplifiers 105 with power from the second amplifier 108 of its set of second amplifiers 108, which is associated with the same band as the particular first amplifier 105. For instance, if there is a loss of light power of the same band, as the particular first amplifier 105 is associated with, on the first bi-directional transmission line 102.

Else, the amplification device 100 works as described above, and can amplify the set of first signals 106 into the amplified set of first signals 107 and provide them to the receiving device, for instance, the other RO ADM node 210. The path of the signals is illustrated by the dashed line in FIG. 3. Other paths through the optical transmission system 300, which may comprise an optical network of many nodes (e.g., RO ADM nodes 210, amplification devices 100, 300), are possible.

FIG. 4 shows an exemplary amplification device 100 according to this disclosure. Same elements in FIG. 4 and FIG. 1 and 2, respectively, are labelled with the same reference signs.

In particular, FIG. 4 shows, as an example, a particular first amplifier 105 of the amplification device 100 and a particular second amplifier 108 associated with the same band as the particular first amplifier 105, but installed for the opposite transmission direction. These particular amplifiers 105 and 108 are connected for implementing the power feeding described above. The amplification device 100 is in this case configured to enable the power feeding of the particular first amplifier 105 with power from the particular second amplifier 108, if there is a loss of light power of the same band, as the particular first amplifier 105 is associated with, on the first bi-directional transmission line 102. For instance, if a second amplification device 301, from which the set of first signals 106 should be received over the first bi-directional transmission line 102 by the amplification device 100, has a failed amplifier that is associated with the same band as the particular first amplifier 105, as it is illustrated in FIG. 4, the power feeding can be used. The amplification device 100 of FIG. 4 also comprises an optical gate 402, which is arranged between the particular first amplifier 105 and the particular second amplifier 108, and is configured to selectively enable and disable the power feeding of the particular first amplifier 105 by the particular second amplifier 108. The optical gate 402 is labelled as a device ‘S’ in FIG. 4. The optical gate 402 is configured to act as an on-off switch. The optical gate 402 can be implemented by a VOA, SOA, or any other component that is able to switch light on and off, selectively.

A first optical coupler 401a may connect the optical gate 402 and the input of the particular first amplifier 105, and a second optical coupler 401b may connect the output of the particular second amplifier 108 and the optical gate 402. The first optical coupler 401b at the output of the particular second amplifier 108 (denoted Ampk ,i in FIG. 4) may be adapted to redirect part of the light power to the input of the particular first amplifier (denoted Ampk,i in FIG. 4) through the optical gate 402. In a regular operation, the optical gate 402 may be off and configured to cut all light between the two amplifiers 105 and 108. When, for instance, the amplifier of the second amplification device 301 (denoted Ampk-i.i in FIG. 4) fails - wherein the loss of light may be detected by a photodiode located at the input of the particular first amplifier 105 (optional and not depicted) - the optical gate 402 may be configured to open, such that the input power of the particular first amplifier (coming from the output of the particular second amplifier 108 via a path through the optical coupler 401b, the optical gate 402, and the second optical coupler 401a) is amplified by the particular first amplifier 105, and the new output power of the particular first amplifier 105 is restored to the pre-failure level.

The optical couplers 401b and 401a are respectively configured to tap light from an optical fiber of the first bi-directional optical transmission line 102, and to inject it into another fiber of the first bi-directional transmission line 102. A ratio of tapped power over the total power can be selected at amplifier installation time. Ideally, the input power in the particular first amplifier 105 after a failure should be the same as before the failure. During the failure, the input power Pini of the particular first amplifier is the product of the output power P _ou ti’ of the particular second amplifier 108, the attenuation X2 of the second optical coupler 401b, the attenuation A _s of the optical gate 402, and the attenuation xi of the first optical coupler 401a. In particular, according to the following formula:

Pinl = Pouti’ (1-X2) As (1 -X1) (1) For instance, assuming that the optical gate 402 is an on-off switch with no attenuation in the ON state (i.e., A _s =l), the optical couplers 401a and 401b may be 90%/10% couplers (i.e., XI=X2=0.9), and the output power of the particular second amplifier 108 may be P _ou ti’=100 mW. Then the input power of the particular first amplifier 105 is

Pini = 100 x 0.1 x 1 x O.l ~= 1 mW (2)

If the particular first amplifier 105 is set to amplify light by 100, then its output power is Pouti=100 mW.

In a case that the optical gate 402 is implemented as a variable switch, the attenuation of the optical gate 402 may be tuned, such that the new input power level of the particular first amplifier 105 after a failure matches exactly the pre-failure power.

FIG. 5 shows an exemplary amplification device 100 according to this disclosure. Same elements in FIG. 5, and FIG. 4 and FIG. 1, respectively, are labelled with the same reference signs.

In particular, FIG. 5 builds on FIG. 4 and shows again, as an example, the particular first amplifier 105 and the particular second amplifier 108 associated with the same band as the particular first amplifier 105, but installed in the opposite transmission direction. In FIG. 5 the particular second amplifier 108 comprises an optical switch 501 (instead of the optical coupler 401b shown in FIG. 4) for redirecting a part or all of the light power at its output to the input of the particular first amplifier 105.

The optical switch 501 may selectively direct light from one input to either one of two outputs (it may be a 1x2 optical switch). A possible implementation is a micro-electro-mechanical system (MEMS). In regular operation, the optical switch 501 may be configured to direct light from its input to its one output connected to the second transmission direction (“direction 2”). When, for instance, the amplifier of the second amplification device 301 fails, the optical switch 501 is configured to direct light from its input to its other output connected to the optical gate 402. In formula (1) above, (I-X2) is replaced by the attenuation of the optical switch 501. The amplification device 100 of FIG. 5 can be advantageous over that of FIG. 4, when the loss of the optical switch 501 is small, in which case in regular operation (almost) all optical power at the output of the particular second amplifier 108 is redirected to the next amplifier (of the second amplification device 301) along the second transmission direction. In addition, in the failed mode, all optical power at the output of particular second amplifier 108 may be redirected to the input of the particular first amplifier 105. This allows using a first optical coupler 401a with a higher xi value, which benefits the SNR of signals going through the particular first amplifier 105 (the higher the input power, the lower the penalty on OSNR due to ASE noise generation).

As an example, assuming a perfect optical switch 501 (no attenuation, corresponding to X2=0), one can now select a 99%/l % coupler for the first optical coupler 401a to achieve the same Pini as for the amplification device 100 of FIG. 4:

Pirn = Poutr (l-x ₂ ) As (1-xi) = 100 x 1 x 1 x 0.01 ~= 1 mW (3)

FIG. 6 shows an exemplary amplification device 100 according to this disclosure. Same elements in FIG. 6, and FIG. 4 and FIG. 1, respectively, are labelled with the same reference signs.

In particular, FIG. 6 builds on FIG. 4 and shows again, as an example, the particular first amplifier 105 and the particular second amplifier 108 associated with the same band as the particular first amplifier 105, but installed in the opposite transmission direction. In FIG. 6 the particular first amplifier 105 comprises an optical switch 601 for redirecting a part or all of the light power at an output of the particular second amplifier 108 to the input of the particular first amplifier 105.

That is, the first optical coupler 401a shown in FIG. 4 and FIG. 5 is replaced by the optical switch 601 in FIG. 6, but not the second optical coupler 401b as replaced in FIG. 5. The optical switch 601 may be like the optical switch 501 shown in FIG. 5, for example, it may be an 1x2 optical switch.

Notably, also both optical couplers 401, i.e. the first optical coupler 401a and the second optical coupler 401b, may be replaced by the optical switches 501 and 601, respectively. In the case that at least one of the optical switches 501 and 601 is present in the amplification device 100, the optical gate 402 may also be removed. In this case, the one or two optical switches 501 and/or 601 are configured to selectively enable the power feeding of the particular first amplifier 105 with the power of the particular second amplifier 108.

Of course, in the amplification devices 100 shown in FIG. 4, FIG. 5, and FIG. 6, more than one pair of particular first amplifier 105 and particular second amplifier 108 may be configured for the power feeding. That is, other pairs of first and second amplifiers 105, 108 on the same band may be connected by an optical gate 402, optical couplers 401a, 401b, and/or optical switches 501, 601.

This disclosure in general, and the amplification devices 100 described above specifically, provide several advantages. For example, feedback optical power from at least one second amplifier 108 in one transmission direction to a first amplifier 105 on the same band in the other transmission direction (and optionally vice versa) allows to prevent the loss of signal (light power) on other bands, for instance, when equipment on a single band fails. Further, variable optical tuning on the path(s) between the first amplifier(s) 105 and the second amplifier(s) 108 on the same band than the first amplifier(s) 105 may allow tuning the optical input power of the power-fed first amplifier(s) 105, so that there is no need to reconfigure these first amplifier(s) 105 (e.g., by gain or output power) upon a failure. Further, since the amplification device 100 may be located at a single amplification site of the optical transmission system 200, 300, a fast recovery in case of a failure is enabled, and there is no need for management plane support. In addition, protection or restoration or rerouting can be avoided.

The present disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed matter, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.