Abstract:
Improved lumped Raman amplification systems are disclosed. A lumped Raman amplification structure provides optimized efficiency, low noise figure over a range of gain settings, and a high saturation threshold. Responsibility for amplifying different portions of the spectrum is divided among multiple stages. In one particular implementation, two outer stages amplify a first band and two inner stages amplify a second band. The two inner stages also apply a small amount of amplification to the first band. A modification improves noise figure in the second band by adding some amplification in the second band to the first stage.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/419,380, filed on May 19, 2006 now abandoned, which in turn is a continuation of U.S. patent application Ser. No. 10/437,496, filed May 14, 2003 now U.S. Pat. No. 7,054,059, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to optical networking and more particularly to systems and methods for optical amplification. 
     Available optical fiber bandwidth has been only partially utilized in high capacity optical transmission due to lack of applicable optical amplifier technologies to overcome fiber losses in the wavelength range 1300 nm to 1650 nm. Lumped Raman amplifiers allow the designer to adjust the spectral position and spectral width of the amplification region to optimally account for channel capacity, channel spacing, and transmission fiber type. Lumped Raman amplification also provides important advantages in that the same fiber can provide gain and dispersion compensation simultaneously while maintaining low noise figure in comparison to standard EDFA (Erbium-doped fiber amplifier) technology. 
     A disadvantage of Raman amplification is poor power conversion efficiency in comparison to EDFAs. For example, a typical lumped Raman amplifier that also provides dispersion compensation may have an efficiency of 2-3% as compared to 8-10% for an EDFA module that incorporates a variable optical attenuator that balances saturation and chromatic dispersion compensation loss. This lower efficiency can be justified only for large bandwidth/high capacity applications where at least 2 EDFAs must be used. For example where both the 1530-1560 nm and 1570-1600 nm bands are used, each band generally requires a two-stage EDFA composed of a preamplifier and a booster. The use of multiple EDFAs raises the cost of that approach to an unacceptable level. 
     When bandwidths are very wide, (e.g., &gt;50 nm) and the number of channels is also large, input amplifier power will be very large giving rise to saturation difficulties with EDFA technology. However, the saturation advantage of lumped Raman amplifier technology in this situation is limited due to interactions among multiple pump signals that are used to establish the very broad amplification bandwidth. In particular, for bandwidths greater than 60 nm, the combination of pump-to-pump interactions and high input power leads to uncontrollable saturation effects unless effective countermeasures are implemented. 
     Schemes have been developed to employ pump reuse to provide improved saturation performance, power conversion efficiency, and also noise figure. One approach adds a pump reflector to a counter-propagating pump structure. Another approach adds an optical feedback scheme to a counter-propagating pump structure. These approaches add undesired complexity. 
     The above-mentioned problems are exacerbated in the context of a metropolitan optical network, e.g., a network where spans range from zero to 80 km. There may be rapid changes in optical network configuration over time due to traffic variations leading to addition and deletion of channels from the channel grid. Also, the number of channels may vary greatly from node to node as channels are added and dropped. Low noise figure, sufficient gain, and appropriate levels of saturation have to be maintained over a wide range of channel counts and system span lengths. EDFA technology can maintain the needed gain and chromatic dispersion compensation over the range of input powers inherent in the variability of channel counts and span length only at the expense of excessive noise figure (e.g., typically from 6 to 12 dB for gain from &gt;20 dB to 12 dB.). If an EDFA is used in a low gain regime the noise figure can reach very high values (e.g., at 5 dB gain, the noise figure can increase up to 25 dB). 
     Improved systems and methods for optical amplification are needed that can provide appropriate gain over a large bandwidth and broad range of channel counts and input power levels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a general optical WDM system architecture to which embodiments of the present invention may be applied. 
         FIG. 2  depicts a lumped Raman amplifier structure according to one embodiment of the present invention. 
         FIG. 3  depicts an alternative lumped Raman amplifier structure according to one embodiment of the present invention. 
         FIG. 4  depicts gain over wavelength for the lumped Raman amplifier structure of  FIG. 2  according to one embodiment of the present invention. 
         FIG. 5  depicts noise figure over wavelength for the lumped Raman amplifier structure of  FIG. 2  according to one embodiment of the present invention. 
         FIG. 6  depicts saturation over wavelength for the lumped Raman amplifier structure of  FIG. 2  according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will be described with reference to a representative optical wavelength division multiplexing (DWM) system architecture. In the representative architecture there are 100 channels or wavelengths spaced 100 GHz apart. The channels are found in the wavelength range of 1530 nm to 1610 nm. Each channel carries a 10 Gbps transmission. The optical signal to noise ratio (OSNR), at the receiver, should be greater than 14 dB over 0.5 nm bandwidth. 
     Single mode fiber (SMF) is used. The link consists of a transmitter site, a receiver site, a series of optical amplification sites, and a series of fiber spans between the sites. Some possible span configurations include seven spans of 14 dB loss (7×14) 5×20, 2×26, 1×31. In each case the amplifier sites provide gain to compensate for the losses of the preceding span. The overall maximum link length before regeneration of the optical signal from recovered data is considered in this example to be approximately 400 Km. 
       FIG. 1  depicts such an architecture. At a transmitter site, there are 100 optical transmitters  102 , one for each wavelength. Each modulated optical signal is fed into a first stage multiplexer  104 . A final multiplexer  106  combines all the wavelengths into the WDM signal to be transmitted. A booster optical amplifier  108  increases the WDM signal power prior to injection into the link. 
     The link includes a series of fiber spans  110 . Three are depicted although it would be understood that the link may extend for a larger number of spans. Between the spans are found line optical amplifiers  112 . Two are shown. Each line optical amplifier  112  includes two lumped Raman amplifier modules  114 . In the representative application, each lumped Raman amplifier module  114  has a noise figure of less than 5.5 dB. Also the fiber used to implement the Raman amplifier module is also used to implement chromatic dispersion compensation. Although the amplifier modules will typically have the same gain, the amount of chromatic dispersion compensation may be varied in order to match needs at particular points in the link. 
     Also, at each amplifier site between the lumped Raman amplifier modules  114  there is an access loss stage  116  that allows for, e.g., 16 dB of loss. For example, an optical add-drop multiplexer (OADM) may be implemented at this point to add and drop particular optical channels for local use. Other signal conditioning devices can be substituted for an OADM. 
     At the receiver end of the link, there is a demultiplexer  118  followed by parallel optical pre-amplifiers  120  and additional demultiplexing stages  122  which provide the individual wavelengths to receivers  124 . There is one receiver  124  for each wavelength. 
       FIG. 2  depicts a lumped Raman amplifier structure according to one embodiment of the present invention. There are four lumped Raman amplification stages  202 ,  204 ,  206 , and  208 . Stages  202  and  208  mainly amplify channels in the L-band while stages  204  and  206  mainly amplify channels in the C-band. Here L-band may refer to the range between 1570 and 1610 nm, a range that is somewhat larger than that customarily referred to as L-band. The C-band refers to the range between 1530 and 1570 nm. This configuration allows one to avoid pump-to-pump interactions that occur when both bands are amplified together. These interactions would occur due to depletion of the C-band pumps resulting from power transfer from the C-band pumps to the L-band pumps. 
     To induce Raman amplification in the C-band, two laser pumps  210  and  212  are provided that output pump energy at 1430 nm and 1443 nm respectively. There are three pumps  214 ,  216 , and  218  to induce Raman amplification of the L-band wavelengths. These three pumps operate at 1458 nm, 1475 nm, and 1503 nm, respectively. The various pumps can consist of two standard Fabry-Perot lasers for each wavelength that are polarization multiplexed together and may include gratings for wavelength stabilization as known in the art. Alternatively, a single Fabry-Perot laser with a depolarizer, e.g., a Lyot type depolarizer may be used. For optimal power consumption, newly developed high power hybrid pumps that provide two polarization multiplexed chip lasers in the same substrate may also be used. 
     The C-band pumps are combined by a wavelength division multiplexer  220  and input to a 2×2 splitter  222 . From splitter  222 , the C-band pump light is counter-propagated into lumped Raman amplification stages  204  and  206  via 3-port circulators  224  and  226 , respectively. The L-band pump light is combined in a wavelength division multiplexer  228 . A 1×2 splitter  230  then divides the L-band pump light between lumped Raman amplification stages  202  and  208 . The L-band pump light is injected into the lumped Raman amplification stages in a counter-propagating direction via three-port circulators  232  and  234 . 
     Although stages  204  and  206  act primarily as C-band amplifiers they also provide a certain amount of L-band amplification. This is accomplished by transferring residual counter-propagating L-band pump light from the signal input of stage  202  out through a 3-port circulator  236  and then back into stages  204  and  206  via splitter  222  and circulators  224  and  226 . The L-band pump reuse in stages  204  and  206  results in relatively little pump-to-pump interaction because of the low power of the L-band pumps relative to the C-band pumps. 
     By providing some L-band amplification in stages  204  and  206  these stages may then become transparent to the L-band wavelengths. One can then readily provide the capability of modular addition of bands. During a first phase of implementation, only the L-band wavelengths (or a portion thereof) may be included in the WDM link. The C-band pumps  210  and  212  may be omitted during this phase. Since stages  204  and  206  are transparent to the L-band signal, these wavelengths are unaffected by the as yet unimplemented C-band capability. Then when system needs justify the addition of the C-band wavelengths (or a portion thereof) these may be added by inserting the C-band pumps  210  and  212 . 
     The fiber used in the lumped Raman amplifier stages is preferably a Highly Non-Linear Fiber (HNLF) with a small effective area, high Raman gain coefficient, and low pump loss to optimize efficiency. Some representative parameters for this fiber are given in the following Table 1: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Parameter 
                 HNLF (typical value) 
               
               
                   
                   
               
             
             
               
                   
                 Effective area (μm2) 
                 9   
               
               
                   
                 Loss @ 1550 nm (dB/Km) 
                 0.49 
               
               
                   
                 Loss @ 1455 nm (dB/Km) 
                 0.69 
               
               
                   
                 Loss @ 1485 nm (dB/Km) 
                 0.58 
               
               
                   
                 Raman gain coefficient (m/W) 
                  6 × 10 −14   
               
               
                   
                 Chromatic Dispersion (ps/nm/Km) 
                 range: 0 to −100 
               
               
                   
                 Backscattering coefficient (l/m) 
                 7.5 × 10 −7   
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Pump 
                 Pump power in fiber (mW) 
               
             
          
           
               
                 wavelength 
                 Stage 1 
                 Stage 2 
                 Stage 3 
                 Stage 4 
               
               
                 (nm) 
                 (2 Km) 
                 (1.1 Km) 
                 (1.1 Km) 
                 (2 Km) 
               
               
                   
               
               
                 1430 
                 — 
                 225 
                 225 
                 — 
               
               
                 1443 
                 — 
                 155 
                 155 
                 — 
               
               
                 1458 
                 100 
                 11 (residual) 
                 11 (residual) 
                 100 
               
               
                 1475 
                 124 
                 23 (residual) 
                 23 (residual) 
                 124 
               
               
                 1503 
                 125 
                 52 (residual) 
                 52 (residual) 
                 125 
               
               
                 Total pump 
                 349 
                 380 
                 380 
                 349 
               
               
                 (mW) 
               
               
                   
               
             
          
         
       
     
     In one particular implementation, there are five spans of 20 dB, each lumped Raman amplifier  114  has a gain of 18 dB, and 16 dB of loss is allocated for MAL  116 . For this example, Table 2 gives pump power values and fiber lengths for each of the stages. 
     Chromatic dispersion compensation may be provided by every amplifier stage or by only a subset of the stages. In general, all these stages can be made pluggable so that one can use Raman fiber having the correct amount of chromatic dispersion compensation needed at that point in the link. For example, each of the dispersion compensating stages could have one of four fiber dispersion values between 0 and −100 ps/nm/Km. The chromatic dispersion slope of the HNLF used in the lumped Raman amplification stages can be tailored to match that of the single mode fiber used in the spans. Also, if needed, a dispersion slope compensator may be used. 
       FIG. 4  depicts the spectral gain profile of lumped Raman amplifier module  114  of  FIG. 2 , as it would be configured for three different span budgets, 20 dB loss, 22 dB loss, and 26 dB loss.  FIG. 5  depicts noise figure profiles across spectrum for lumped Raman amplifier module  114  of  FIG. 2 .  FIG. 6  depicts the saturation behavior across spectrum for lumped Raman amplifier module  114  of  FIG. 2 . 
     In lumped Raman amplifier module  114  as depicted in  FIG. 2 , C-band noise figure is impacted by the lack of any C-band amplification in the first stage  202 .  FIG. 3  depicts an alternative implementation of lumped Raman amplifier module  114  according to one embodiment of the present invention. The structure depicted in  FIG. 3  provides improved C-band noise figure and improved overall amplifier efficiency. A pair of couplers  302  and  304  directs residual C-band pump light into stage  202  in a co-propagating direction relative to the optical signal. Couplers  302  and  304  also direct second-order residual L-band pump light into stage  202 . The addition of C-band gain in stage  202  improves the C-band noise figure. 
     Further improvement in efficiency can be provided by inserting an L-band pump reflector  306  between circulator  226  and stage  208 . Also, a C+L-band pump reflector  308  may be placed between circulator  224  and stage  206 . This preserves a greater amount of L-band pump light within stage  208  and a greater amount of both kinds of pump light within stage  206 . The structure of  FIG. 3  brings the C-band noise figure down to 4.5 dB. Also, overall efficiency is improved. 
     Many variations on the embodiments described above may be implemented. For example, parameters of the HNLF including effective area, length, and fiber loss may be varied. The wavelengths and powers of the pumps may be varied. Also, for example, an additional C-band pump wavelength may be added to increase bandwidth to 100 nm or more. Further increases in bandwidth may be accomplished by adding further pump wavelengths for C-band or L-band. 
     The band amplification sequence in  FIGS. 2-3  may be changed while achieving the same performance. For example, if a capital letter represents the principal band of amplification and a lower case letter refers to amplification due to residual pump power, other possible configurations include, e.g., C+1/L/C+1/L or C+1/L/L/C+1. Also the amplification bands themselves may be shifted by varying the pump wavelengths. One can also use a smaller (as few as two) or larger number of stages. One can also add co-propagating pump power where desired. 
     Many advantages are provided by the above-described lumped Raman amplification structure. For example, consider a situation where there are seven spans of 14 dB loss each and the power per channel launched in the transmission fiber is −3 dBm/ch for a total power of 17 dBm (i.e., 100 channels, each modulated with 10 Gbps). The gain for each amplifier module should be at least 15 dB (assuming 16 dB access loss for an OADM is included between the two amplifier modules). For an EDFA-based system that employs only 32 channels, the same span reach would require a launch power of +2 dBm/ch due to the higher equivalent noise figure of the EDFA system. Due to the higher launch power, the EDFA system exhibits reduced linearity. Consequences of reduced linearity include a need for greater precision in chromatic dispersion compensation at the expense of deployment flexibility and also greater difficulty in upgrading to 50 GHz spacing due to four-wave mixing (FWM) effects. 
     Lumped Raman amplifier module  114  also has relatively low amplifier saturation. This makes gain control easier to implement and facilitates a highly variable number of channels. 
     There is also low noise figure without the use of a co-propagating pump. Optimal power efficiency is provided. Even without the use of pump reflectors there is a similar pump power per channel efficiency as would be found in an equivalent EDFA. 
     If needed, gain flatness can be improved using known flattening filter technologies, for example, fiber long period gratings This type of filter can be placed in any position depending on system architecture optimization. 
     The lumped Raman amplification structure is robust to non-linear effects such as four wave mixing (FWM) and interference noise. Double Rayleigh Scattering (DRS) noise for both lumped Raman amplifier modules is approximately 42 dB below the signal and there is negligible four-wave mixing cross talk. 
     The lumped Raman amplification structure that has just been described also has highly beneficial modularity properties. As has been discussed, C-band pumps may be added only when those channels are actually needed to carry revenue bearing services. Also, the Raman fibers used for the various stages may be made modular so that one can vary the amount of chromatic dispersion as desired while preserving the same gain. 
     Furthermore, the structure provides cost advantages. This is due to the relatively high power efficiency, the use of embedded dispersion compensation, and the overall modularity. 
     It is understood that the examples and embodiments that are described herein are for illustrative purposes only and that various modifications and changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims and their full scope of equivalents.