Abstract:
A new transmission link configuration with remote Er post- and pre-amplifiers where pump power is shared between a pair of fibers carrying traffic in opposite directions is proposed. A budget increase of &gt;4 dB is demonstrated.

Description:
FIELD OF THE INVENTION 
     This invention relates to remotely optically pumped amplifiers (ROPA) used in optical fiber transmission links. 
     BACKGROUND OF THE INVENTION 
     The longest unrepeatered links utilize a remotely-pumped Er post-amplifier (Tx ROPA), located ˜30-70 km from the transmit terminal, in addition to a remote pre-amplifier (Rx ROPA), typically positioned ˜100-140 km from the receive terminal. Pumping of the ROPAs can be accomplished either by directly launching high power at ˜1480 nm (first-order pumping) or through the generation of 1480-nm pump power inside the transmission fiber (or dedicated pump-delivery fibers) via high-order cascaded Raman processes as described in the following articles: F. Boubal, J-P. Blondel, E. Brandon, L. Buet, V. Havard, L. Labrunie, P. Le Roux, “Recent unrepeatered WDM 10 Gbit/s experiments in the range 300 km to 450 km,” Suboptic 2001, Kyoto, Paper P3.6., S. Papernyi, V. Karpov, W. Clements, “Third-Order Cascaded Raman Amplification,” OFC2002, Anaheim, postdeadline paper FB4., L. Labrunie, F. Boubal, P. Le Roux, E. Brandon, “500 km WDM 12×10 Gbit/s CRZ repeaterless transmission using second order remote amplification,” Electronic Letters, Vol 39, No 19, pp 1394-1395, 2003., V. Karpov, S. Papernyi, V. Ivanov, W. Clements, T. Araki, Y. Koyano, “Cascaded pump delivery for remotely pumped Erbium doped amplifiers,” Suboptic 2004, Paper We.8.8. A Rx ROPA can be pumped through the transmission fiber and/or dedicated delivery fibers. On the other hand, in the case of Tx ROPAs, the transmission fiber cannot be used for pump delivery since Raman interactions between the high pump power and the co-propagating signals would cause excess noise generation and limit the pump power delivered to the ROPA. As a result, all systems with a Tx ROPA have to date utilized one or two pump sources with each connected to a dedicated pump-delivery fiber. Although utilizing two dedicated fibers and Tx ROPA pump sources allows optical budget increases of ˜9-12 dB, the improvement comes at the expense of a substantial increase in system cost and complexity as mentioned in the above-cited articles by, Labrunie et al, and Karpov et al. 
     SUMMARY OF THE INVENTION 
     According to some embodiments of the invention, there is provided a new transmission link configuration with remote Er post- and pre-amplifiers where pump power is shared between a pair of fibers carrying traffic in opposite directions is proposed. 
     According to some embodiments of the invention, the new Tx+Rx ROPA link architecture can potentially provide up to 5 dB of budget improvement compared to the best achievable with a Rx ROPA alone, but does not require dedicated delivery fibers nor dedicated Tx ROPA pump sources. The concept applies to the most common type of link, one consisting of a pair of fibers carrying traffic in opposite directions, and is based on the idea of power sharing between the two fibers carrying traffic “West” and “East”. In the paper by C. R. S Fludger, V. Handerek, N. Jolley, R. J. Mears, titled “Inline loopbacks for improved OSNR and reduced double Rayleigh Scattering in distributed Raman Amplifiers,” presented at OFC 2001, in Baltimore, Md., USA, paper MI1-1., the application of a power sharing concept for providing distributed Raman amplification was theoretically explored. However, the principle was not experimentally investigated and, in fact, our detailed calculations have shown that using power sharing for distributed Raman amplification cannot provide any significant budget improvement. On the other hand, we will show that when applied to the remote pumping of Er amplifiers, it provides a cost-effective means of realizing substantial budget increases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by way of the following detailed description of embodiments of the invention with reference to the appended drawings, in which: 
         FIG. 1  is a schematic diagram of the transmission link layout according to one embodiment; 
         FIG. 2  is a graph of calculated budget improvements, over the best achievable with a Rx ROPA alone, as a function of the distance of the Tx ROPA from the near end terminal for First- and Third-Order ROPA Pumping according to the invention; and 
         FIG. 3  is a plot of BER measurements vs. link losses in which reference  1  is for the case without ROPAs,  2  is the case with only a Rx ROPA located at the optimal position, and  3 , 4  are for the cases with an added Tx ROPA placed at 35.75 and 50.75 km from the pump source, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The power sharing concept is illustrated in the experimental set-up  100  shown in  FIG. 1 . Part of the pump power travelling “East” towards a Rx ROPA  104   a  (amplifying signals travelling “West”) is split off at the location of a Tx ROPA  106   b  in the fiber carrying data “East”  108  and used for pumping the Tx ROPA  106   b . The splitting of the pump light can be by power splitting at the single or at all pump wavelengths, or can be by WDM splitting, the latter being desirable when, for example, the Tx ROPA  106   b  would be pumped by different wavelengths than the Rx ROPA  104   a . Thus, the “West-East” transmission fiber  110  is used for pump delivery to the “East-West” Tx ROPA  106   b  (and vice versa) and therefore, co-propagating Raman interactions are eliminated. The link consists of four fiber lengths: L 1 , the distance from the pump source  102   a  (receiver side) to a point where part of the pump power is split between fibers (S-point)  112   a ; L 2 , the distance from the S-point  112   a  to the Rx ROPA  104   a ; L 3 , the distance between the Tx and Rx ROPAs ( 106   a  and  104   a ); and L 4 =L 1  the distance from the transmitter  114   a  to the Tx ROPA  106   a  (i.e. the total link length L=L 1 +L 2 +L 3 +L 4 ). 
     The conditions for maximum link length are that L 4 +L 1 +L 2  be as large as possible consistent with the requirements that: 1) the pump power reaching the Tx ROPA  106   b  be sufficient to ensure the signal output power is at the limit (P nl ) imposed by nonlinear effects and 2) the pump power reaching the Rx ROPA  104   a  be sufficient for optimal gain and noise figure performance of the amplifier  104   a . Obviously, for a given fiber loss, the maximum distance between ROPA ( 104   a  or  106   b ) and pump launch point  102   a  increases with increasing launch power. However, the maximum value of the pump power that can be launched (P 0 ) is limited by pump depletion by Raman noise amplification and ultimately by random spike generation induced by high Raman gain. 
     For first-order pumping, the optimal lengths L 4 +L 1 +L 2  can be calculated starting from the following equation for the pump power delivered to the Rx ROPA  104   a : 
                     P   Rx     =       [         P   0     ⁢     ⅇ       -     α   p       ⁢   L   ⁢           ⁢   1         -       P   nl     η       ]     ⁢     ⅇ       -     α   p       ⁢   L   ⁢           ⁢   2                 (   1   )               
where P Rx  is the delivered pump power required at the Rx ROPA  104   a  for optimal gain and noise figure performance of the amplifier  104   a, α   p  is the fiber loss at the pump wavelength and η is the efficiency of the Tx ROPA  106   a . It is fair to assume that P 0 , P Rx  and P nl  as well as the distance L 3  should be constants for a particular fiber type and signal modulation format.
 
     Under these assumptions, the optimal location of the Tx ROPA (L 1   opt )  106   b  that provides the longest link can be derived from equation (1): 
     
       
         
           
             
               
                 
                   
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                       1 
                       opt 
                     
                   
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                           p 
                         
                       
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                     × 
                     
                       ln 
                       ⁡ 
                       
                         ( 
                         
                           η 
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                               P 
                               0 
                             
                             
                               2 
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     From (1) and (2) we find that, for first-order pumping, the optimum split ratio is 50/50%. The maximum budget improvement (not accounting for any nonlinear penalties introduced by the Tx ROPA  106   b  compared to the best achievable with a Rx ROPA  104   a  alone is given by:
 
Δ B=α   S [(2 L 1 opt   +L 2+ L 3)−( L 3 R   +L 5)]=α S   ×L 1 opt   (3)
 
where α s  is the fiber loss at the signal wavelength and L 3   R  and L 5  are the optimal distances from the transmitter  114   a  to the Rx ROPA  104   a  and from the Rx ROPA  104   a  to the receiver  118   a  (pump laser  102   a ) for the case of no Tx ROPA  106   b . Under our assumptions, it is clear that L 3 =L 3   R  and L 5 =(1/α p )×ln(P 0 /P Rx ).
 
       FIG. 2  illustrates the estimated budget improvement vs. the distance from the pump source  102   a  to the S-point  112   a  for both first- and third-order pumping for a realistic Tx ROPA  106   b  efficiency of 65% and a fiber with parameters close to those of pure silica core fiber (i.e. α s =0.17 dB/km and α p =0.2 dB/km). For third-order pumping, it not possible to calculate the budget improvement in a closed form and the curve shown in  FIG. 2  is the result of numerical modeling. The numerically calculated improvement was found to be ˜2 dB greater than that for first-order pumping because the 1480-nm pump power achieves its maximum value ˜25 km from the launch point and the “effective” 1480-nm launch power is ˜2 dB higher. 
     Transmission Experiment 
     A transmission experiment was carried out in Corning Vascade EX1000 fiber with average losses of 0.169 dB/km at 1552 nm and 0.195 dB/km at 1485 nm (these values include splice and connector losses averaged through the fiber length). A single 2.5 GHz signal, appropriately dithered for stimulated Brillouin scattering suppression, was amplified in an Er-doped booster amplifier having a saturated output power up to 21 dBm. It was found that the nonlinear limit for the signal launch power P nl  was 20 dBm. A third-order cascaded Raman pump scheme was used for ROPA pumping. The pump source consisted of a 1276-nm high-power Raman laser with a maximum output power of 4 W, plus a seed LD at 1485 nm with a power output up to 100 mW. Two fiber Bragg gratings, reflecting incoming Raman ASE at 1360 nm and 1427 nm back into the span were spliced between the Raman laser and the entrance of the transmission link to provide feedback for the build-up of the first- and second-order Stokes powers out in the span. 
     Two tests were carried out with the setup shown in  FIG. 1 . In the first test, the distance to the S-points  112   a  and  112   b  (Tx ROPAs  106   b  and  106   a ) was 35.75 km, in the second it was increased to 50.75 km. The split ratio in the first case was 30/70% with 30% of the power used for pumping the Tx ROPAs  106   b  and  106   a  and 70% propagating on towards the Rx ROPAs  104   a  and  104   b . In the second test, the split ratio was 60/40% with the 60% being used for Tx ROPA ( 106   b  and  106   a ) pumping. In order to keep the Rx ROPA ( 104   a  and  104   b ) pumping constant in both tests, the fiber length from the S-points ( 112   a  and  112   b ) to the Rx ROPAs ( 104   a  and  104   b ) was changed from 89 km to 63 km. In both tests, the pump power delivered to each ROPA was the same: 135 mW for the Tx ROPAs ( 106   a  and  106   b ) and 6.6 mW for the Rx ROPAs ( 104   a  and  104   b ). These values were found to be optimal and provided a Tx ROPA ( 106   a  and  106   b ) efficiency of 63% (including losses in WDMs and isolators) and a Rx ROPA ( 104   a  and  104   b ) gain of 18 dB and noise figure of 5.6 dB. A VOA ( 116   a  and  116   b ) located between the Tx and Rx ROPAs ( 106   a  and  104   a ;  106   b  and  104   b ) was used for changing the total link losses. 
     It will be appreciated that the illustration in  FIG. 1  is schematic and details are not shown. For example, the pump light can be coupled  120   b  into the active fiber of the Tx ROPA  106   b  to be counter-propagating with respect to the transmission signals. This provides maximum amplification near the output of the ROPA that can be near the nonlinear power limit of the transmission fiber. Likewise, at the Rx ROPA  104   a , pump light can be coupled out  122   a  of the receive fiber and into  124   a  the active fiber of the Rx ROPA to be co-propagating with respect to the incoming signals. This provides for the maximum gain at the beginning of the Rx ROPA, thus optimizing the noise performance of the Rx ROPA. 
     A direct comparison with the “Rx ROPA alone”  104   a  configuration was made by simply taking out the Tx ROPA  106   a  and the splitter  112   a , reconnecting the link and appropriately adjusting the VOA settings. Of course, the losses in the pump-power WDM  120   a  were taken into account when calculating the total link budget. 
     As can be seen in  FIG. 3 , the addition of the Tx ROPAs and pump power sharing between the pair of fibers ( 108  and  110 ) provides up to 4 dB of margin improvement for a total link loss of ˜90 dB. The measured improvement values are approximately 1 dB smaller than predicted. This could be attributed to a nonlinear penalty caused by the Tx ROPA. In other words, the nonlinear limit of the Tx ROPA output power was found to be ˜19.3 dBm as compared to the P nl  of 20 dBm at the booster output when the link did not include the Tx ROPAs (though we do not have a clear explanation for this fact). 
     In conclusion, we have proposed and demonstrated a new link configuration that allows a budget increase &gt;4 dB with only one ROPA pump source per transmission fiber and no dedicated pump delivery fibers.