Abstract:
An optical communication system, a method and a network device for an optical network are provided, wherein the device comprises a first port coupled with a first optical fiber link, a second port coupled with a second optical fiber link, the first port and the second port being configured to be coupled with respect to each other in case of a failure of the first optical fiber link or in case of a failure of the second optical fiber link.

Description:
RELATED APPLICATIONS 
     This application is a 35 U.S.C. 371 national stage filing of International Application No. PCT/EP2012/061561, filed on Jun. 18, 2012, which claims priority to and benefit of European Patent Application No. 11171624.7, filed Jun. 28, 2011, the contents of each of which are hereby incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates to a method, a system and a device for data processing in an optical network. In particular, the invention relates to connection interruptions of optical networks. 
     BACKGROUND OF THE INVENTION 
     This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section. 
     Optical amplifiers are employed in the field of optical transmission technology for amplifying the optical signals transmitted in an optical network. The optical signals in many cases propagate over long links measuring several hundred kilometers and more in an optical fiber, being attenuated in the process. It is therefore necessary to amplify the optical signals when they have been transmitted over a long distance. 
     Optical links and networks of this type frequently employ Wavelength Division Multiplexing (WDM), a technique whereby a plurality of channels is transmitted in an optical fiber simultaneously at various wavelengths. 
     Erbium-doped fiber amplifiers (EDFAs) are largely employed in WDM transmission systems. An EDFA operates using an erbium-doped fiber into which the light from an optical pump, for example a laser diode, is coupled. The optical signal launched into the doped fiber is therein amplified by means of stimulated photon emission. 
     The optical signals are transmitted from one network node (cross-connect, XC) to another network node over a chain of transmission fibers interrupted by inline optical amplifiers. 
       FIG. 1  is a diagrammatic representation of an optical network  11  with five cross-connects (XC), namely XC 1 , XC 2 , XC 3 , XC 4 , and XC 5 . In the cross-connects, signals from many cross-connects are routed to different directions. This can be seen in  FIG. 1  as an example: cross-connect XC 4  routes the optical signals λ 1  and λ 2  from the cross-connect XC 5  to the cross-connect XC 3  and backwards from XC 3  to XC 5  along the paths  14  and  15 ; similarly XC 4  routes the optical signals λ 3  and λ 4  from the cross-connect XC 5  to the cross-connect XC 1  and backwards from the cross-connect XC 1  to XC 5  along the paths  12  and  15 ; in a similar way the cross-connect XC 4  routes the optical signal λ 5  from XC 1  to XC 3  and backwards from XC 3  to XC 1  along the paths  12  and  14 ; in a similar fashion XC 4  routes the optical signal λ 6  from XC 2  to XC 3  and backwards from XC 3  to XC 2  along the paths  13  and  14 , and the optical signal λ 7  from XC 1  to XC 2  and backwards from XC 2  to XC 1  along the paths  12  and  13 . The paths  12 ,  13 ,  14  and  15  described in  FIG. 1  may include two different optical fibers. 
     The inline amplifier&#39;s gain depends on the defined output power P out , on the properties of the preceding fiber and strongly on the number of channels. As an example, the gain factor G of an inline amplifier  16  located between XC 4  and XC 5  is G=P out /P in , where P out  is the desired sum signal power of the signals λ 1 , λ 2 , λ 3  and λ 4  and P in  is the sum input signal power of the signals λ 1 , λ 2 , λ 3  and λ 4  at the amplifier&#39;s input. The sum signal power P out  is defined by the network planner and can be normally a fixed value. The amplifier  16  measures the incoming sum signal power P in  and adapts the gain factor G so that the desired sum output power P out  can be obtained. 
       FIG. 2  is a diagrammatic representation of an optical network  21 , which is similar to the optical network  11  of  FIG. 1  but, unlike the optical network  11 , it has an interrupted link  28  so that no signal can be transmitted anymore between the cross-connects XC 3  and XC 4  along the path  24 . As a consequence, cross-connect XC 4  cannot route the optical signals λ 1  and λ 2  from the cross-connect XC 5  to the cross-connect XC 3  and backwards from XC 3  to XC 5  along the paths  24  and  25 , or the optical signal λ 5  from XC 1  to XC 3  and backwards from XC 3  to XC 1  along the paths  24  and  22 , or the optical signal λ 6  from XC 2  to XC 3  and backwards from XC 3  to XC 2  along the paths  24  and  23 . As a further consequence, along the path  25  and in particular on the fiber  251 , from the cross-connect XC 4  to the cross-connect XC 5 , only the two backwards signals λ 3  and λ 4  can be transmitted and not λ 1  and λ 2 . In this way, the amplifier  26  measures the sum power of λ 3  and λ 4 , and this may results in a reduction of the sum input and sum output power of, for example, 3 dB. Although the amplifiers may try to keep the gain constant there could be some overshoots generated by the population of the third energy level and by an imperfect prediction of the required pump power. 
     The temporary increase of the signal power may result in dynamic impairments, the so-called transients, while static impairments are mainly due to spectral reconfiguration, which may include, for example, non-linear effects, spectral hole burning, intra-band Raman effects and Brillouin scattering. 
     As a consequence, the planned optical performance cannot be guaranteed anymore as the channel power is increased. Moreover, the signal power at the receiver can be too high, so that high optical power may destroy sensitive components of the receiver, such as, for example, the photo-diode. 
     A known way to suppress transients is adding additional lasers which replace the signal power of the lost signals, so that the sum input power in an amplifier is kept constant. These lasers are usually continuous wave signals at defined wavelengths. However, this solution may require a high number of additional lasers which reduce the number of wave length channels. Moreover, due to the high optical power of the fill lasers high non-linear effects might occur especially on low dispersion fibre types. 
     The problem to be solved is to overcome the disadvantages stated above and in particular to provide a solution that in case of a connection interruption suppress transients efficiently without influencing the transmission performance of other channels. 
     SUMMARY OF THE INVENTION 
     In order to overcome the above-described need in the art, the present invention discloses a network device for an optical network comprising a first port coupled with a first optical fiber link, a second port coupled with a second optical fiber link, wherein the first port and the second port are configured to be coupled with respect to each other in case of a failure of the first optical fiber link or in case of a failure of the second optical fiber link. 
     In a next embodiment of the invention the first port is configured to be shunted with the second port in case of a failure of the first optical fiber link or in case of a failure of the second optical fiber link. 
     In other embodiments of the present invention, the network device is a cross-connect element. 
     The problem stated above is also solved by a system for an optical network comprising a network device which includes a first port coupled with a first optical fiber link and a second port coupled with a second optical fiber link and a coupling unit configured to couple the first port with the second port of the network device in case of a failure of the first optical fiber link or in case of a failure of the second optical fiber link. 
     It is also an embodiment, that the coupling unit is a shunting element. 
     In a further embodiment, the network device of the system is a cross-connect element. 
     In a next embodiment the system for an optical network further comprises a detecting unit configured to detect a failure of the first or of the second optical fiber link. 
     According to an alternative embodiment of the invention, the detecting unit is a photo diode. 
     It is also an embodiment, that the network device includes the detecting unit. 
     In a further embodiment, the coupling unit includes the detecting unit. 
     In a next embodiment, the coupling unit comprises an actuating element configured to couple the first port with the second port in case of a failure of the first optical fiber link or in case of a failure of the second optical fiber link, the actuating element being a Variable Optical Attenuator (VOA) or a switching element. 
     In a further embodiment, the coupling unit further comprises a controlling unit connected with the detecting unit and with the actuating element configured to instruct the actuating element to couple the first port with the second port in case the detecting unit detects a failure of the first optical fiber link or a failure of the second optical fiber link. 
     In a next embodiment, the coupling unit comprises a first amplifier connected with the first port, with the detecting unit, with the controlling unit and with the actuating element; a second amplifier connected with the second port; a first coupler connected with the first amplifier and a second coupler connected with the second amplifier. 
     It is also an embodiment, that the first amplifier comprises a fist pump laser and the second amplifier comprising a second pump laser, the coupling unit further comprising a third pump laser connected with the first and with the second amplifier, the fist and the second pump lasers being configured to be turned off and the third pump laser being configured to be turned on in case of a failure of the first optical fiber link or of the second optical fiber link is detected. 
     The problem stated above is also solved by a method for an optical network, the optical network comprising network device having a first port coupled with a first optical fiber link and a second port coupled with a second optical fiber link, the method comprising coupling the fist port with the second port in case of a failure of the first optical fiber link or of the second optical fiber link. 
     The method, the device and the system provided, in particular, bear the following advantages:
         a) They can be cost-effectively implemented in existing networks elements employing a limited number of additional components.   b) They do not involve additional power consumption.   c) Remarkable performance improvement can be achieved, in particular the “switching” can be fast enough to suppress transients efficiently without influencing the transmission performance of other channels.   d) They do not involve a reduced number of available channels.   e) In case of a fiber break, the power launched into the transmission fiber can be reduced to an allowed level within a predefined time interval, thereby preserving the safety of the optical components.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained by way of example in more detail below with the aid of the attached drawings. 
         FIG. 1  is a diagrammatic representation of an optical network. 
         FIG. 2  is a diagrammatic representation of an optical network having an interrupted link. 
         FIG. 3  is a diagrammatic representation of an optical network including the shunt according to an embodiment of the present invention. 
         FIG. 4  is a diagrammatic representation of an illustrative example of realization of the shunt according to an embodiment of the invention. 
         FIG. 5  is a diagrammatic representation of an illustrative example of realization of the shunt according to an alternative embodiment of the invention. 
         FIG. 6  is a diagrammatic representation of an illustrative example of realization of the shunt according to an alternative embodiment of the invention. 
         FIG. 7 a    is a diagrammatic representation of the signal propagation before the fiber break according to an embodiment of the invention. 
         FIG. 7 b    is a diagrammatic representation of the signal propagation after the fiber break according to an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     As regards the description of  FIGS. 1 and 2 , reference is made to the background of the invention. 
     Illustrative embodiments will now be described with reference to the accompanying drawings to disclose the teachings of the present invention. While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
       FIG. 3  is a diagrammatic representation of an optical network  31 , which, similarly to the optical network  21  of  FIG. 2 , has an interrupted link  34  so that no signal can be transmitted between the cross-connects XC 3  and XC 4  along the path  34 . However, differently from the optical network  21  of  FIG. 2 , in the optical network  31 , according to an embodiment of the present invention, a “shunt” has been added to the output of cross-connect XC 4 , so that the outgoing signal  342  is coupled to the cross-connects input  341 . The forward and backward signals between two cross-connects are transmitted over two separated fibers, but signals have the same wavelengths. In the case of a fiber break  34  the lost channels of the incoming signals are replaced by the outgoing signals. Using this shunt, exactly the same wavelengths are replaced, so that the signals of the remaining network are not affected. They act like optimized fill lasers. 
     As a consequence, due to the fiber break  38 , cross-connect XC 4  cannot route the optical signals λ 1  and λ 2  from the cross-connect XC 5  to the cross-connect XC 3  along the path  34 . However, thanks to the shunt  37 , the two backwards signals λ 1  and λ 2  can be transmitted from the cross-connect XC 4  to the cross-connect XC 5  along the path  35  and in particular on the fiber  351 . In this way, the amplifier  36  measures the sum power of λ 1 , λ 2 , λ 3  and λ 4 , and therefore no reduction of the sum input and sum output power occurs. As an effect, overshoots can be avoided and an optimum transient suppression can be avoided. 
     Similarly, due to the fiber break  38 , cross-connect XC 4  cannot route the optical signal λ 5  from XC 1  to XC 3  along the path  34 , however, thanks to the shunt  37 , the backward signal λ 5  can be transmitted from the cross-connect XC 4  to the cross-connect XC 5  along the path  32 . 
     In a similar way, due to the fiber break  38 , cross-connect XC 4  cannot route the optical signal λ 6  from XC 2  to XC 3  along the path  34 , however, thanks to the shunt  37 , the backward signal λ 6  can be transmitted from the cross-connect XC 4  to the cross-connect XC 2  along the path  33 . 
     The shunt can be made by means of a shunting device, which may be integrated in the cross-connect XC 4  or installed as an external network card coupled with the cross-connect XC 4 . 
       FIG. 4  is a diagrammatic representation of an illustrative example of realization of the shunt according to an embodiment of the invention. The embodiment  41  of  FIG. 4  is realized as an extra card. However, according to an alternative embodiment of the invention, the shunting device can be included in the cross connect. In particular  FIG. 4  shows that the switching can be realized by splitting the outgoing signals (for example, signals λ 1 , λ 2 , λ 3  and λ 4 ) at the output  48  of the cross-connect  42  by a coupler  45  and coupling them again to the incoming signal  49  by another coupler  44  at the input of the cross-connect  42 . A Variable Optical Attenuator (VOA)  47  (αs) or alternatively an optical switch may be configured to activate the signal replacement realizing the shunt if a fiber break is detected on the path  491 . The loss of the signals can be detected by a photo diode  46 , which may be read by a controller  43  (D), which can decide to shunt the outgoing signals or not. The photo diode  46  in the illustrative embodiment shown in  FIG. 4  is included in the cross connect  42  and can be read out by the extra card. Alternatively a photo diode may be implemented on the extra shunt-card. 
       FIG. 5  is a diagrammatic representation of an illustrative example of realization of the shunt according to an embodiment of the invention. In particular in  FIG. 5  the shunt is implemented into the pre-amplifier  511  (amplifier of the incoming signals  59 ) and into the booster amplifier  522  (amplifier of the outgoing signals  59 ). A Dispersion-Compensating Fiber module DCF 1  and an additional loss  592  (α 1 ) are placed between the two amplifier stages of the pre-amplifier  511 . A Dispersion-Compensating Fiber module DCF 2  and an additional loss  593  (α 2 ) are placed between the two amplifier stages of the booster amplifier  522 . A coupler  56  can split the signal, and the loss α 2  then can be reduced by the loss of the coupler  56 , so that no additional loss is inserted. The (outgoing) signal  58  is coupled into the amplifier of the incoming signal  511  by a coupler  54 . A Variable Optical Attenuator (VOA)  57  (αs) or alternatively an optical switch may be configured to activate the signal replacement realizing the shunt if a fiber break is detected on the path  591 . 
     The loss of the signals can be detected by a photo diode  53  or generally by a photo detector, which may be read by a controller  55  (D), which can decide to shunt the outgoing signals or not. The photo diode  53  in the illustrative embodiment shown in  FIG. 5  is included in pre-amplifier  511 . Alternatively a photo diode or photo detector may be implemented externally. Alternatively to the employment of couplers, switches could be used to realize the shunting device described in  FIG. 4  and  FIG. 5 . The optical shunt could also be realized by simply using the cross-connects available input and output ports. 
       FIG. 6  is a diagrammatic representation of an illustrative example  61  of realization of the shunt according to an embodiment of the invention. In particular in  FIG. 6  the shunt is implemented into the pre-amplifier  63  (amplifier of the incoming signals) and into the booster amplifier  62  (amplifier of the outgoing signals) in a common housing. Although the pre-amplifier  63  and the booster amplifier  62  can be realized in separate housing, the setup  61  is particularly cost efficient. 
     Without fiber break, the first pump laser  67  and the second pump laser  65  are turned on, whereas the third pump laser  66  is turned off. Since the erbium-doped fiber (EDF)  69  connecting the two amplifiers is not pumped, the signal is largely attenuated and only a negligible part of the signal power in the upper path is launched into the preamplifier  63 . 
     In case of a fiber break, the amplifier  61  shuts down the first pump laser  67  and the second pump laser  65 . Thus, there is no power launched any more into the broken fiber. The speed of this mechanism is high enough to comply with current laser safety rules. However, at the same time, the third pump laser  66  is turned on. In this way, the erbium-doped fiber (EDF) coil  68  of the booster  62  as well as the connecting erbium-doped fiber (EDF) coil  69  are pumped and the signals are redirected. The attenuation of the additional amplifier can been adjusted in a turn up procedure to a value guaranteeing that the total signal power in the preamplifier remains almost constant. In this way, the powers of the lightwaves coupled back into the preamplifier may correspond to the power values without fiber break. 
       FIG. 7 a    is a diagrammatic representation of the signal propagation before the fiber break according to an embodiment of the invention. In particular  FIG. 7 a    schematically illustrates the setup  61  shown in  FIG. 6  with the pre-amplifier  712 , the booster amplifier  711 , the third pump laser  713 , the signal propagation  714  within the booster amplifier  711  and the signal propagation  714  within the pre-amplifier  712  before the fiber break. 
       FIG. 7 b    is a diagrammatic representation of the signal propagation after the fiber break according to an embodiment of the invention. In particular  FIG. 7 b    schematically illustrates the setup  61  shown in  FIG. 6  with the pre-amplifier  722 , the booster amplifier  721 , the third pump laser  723  and the signal propagation  723  after the fiber break.