Abstract:
Methods and apparatus for providing amplification to coarse wave division multiplexing channels or signals are disclosed. According to one aspect of the present invention, an arrangement that adds gain to a set of signals that may be divided into a first band including signals of lower wavelengths and a second band including signals of higher wavelengths includes a multiplexer, first and second optical amplifiers, and a processing arrangement. The multiplexer multiplexes the set of signals. The first optical amplifier has a first gain peak and provides amplification to the set of signals, while the second optical amplifier has a second gain peak and provides amplification to the second band but not to the first band. The processing arrangement passes the second band from the first optical amplifier to the second optical amplifier, and substantially prevents the first band from passing from the first optical amplifier to the second optical amplifier.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates generally to optical networks. More particularly, the present invention relates to extending the span distances associated with coarse wavelength division multiplexing systems. 
     2. Description of the Related Art 
     In response to the ever-growing demand for fiber networks, coarse wave division multiplexing (CWDM) has been developed as an alternative to dense wavelength division multiplexing (DWDM). CWDM systems use uncooled lasers, and therefore allow a greater spacing between wavelengths or channels than DWDM systems. In general, CWDM is a relatively low cost solution that provides connection flexibility and increased throughput for metropolitan networks. 
     CWDM combines up to sixteen or eighteen wavelengths onto a single fiber, although many CWDM systems combine four or eight wavelengths onto a single fiber. CWDM technology uses an ITU standard approximately 20 nanometer (nm) spacing between wavelengths or channels. The wavelengths are typically between approximately 1270 nm and approximately 1610 nm. 
       FIG. 1  is a diagrammatic representation of an overall network system that includes a system which communicates using CWDM. A network  120  that is in communication with an overall Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) transport network  104  includes various nodes  124   a - d  that are in communication over links  128 . Network  120  may be a metro network that uses CWDM. Any of nodes  124   a - d , as for example node  124   c , may be in communication with other nodes  124   e ,  124   f  across links  132 . Nodes  124   c ,  124   e ,  124   f  may be part of a local area network. 
     When CWDM is used in network  120 , the distances covered by links  128  are typically limited to being less than approximately 100 kilometers (km). That is, CWDM systems are generally limited by an optical span budget of approximately 100 km. To increase the distances over which CWDM communications may occur, optical amplifiers may be incorporated into network  120 . 
     Optical amplifiers such as semiconductor optical amplifiers (SOAs) provide relatively high-speed switching capability and also provide the ability to add gain to optical signals. As will be understood by those skilled in the art, an SOA uses technology that is similar to that of a Fabry-Perot laser diode. In some CWDM systems, SOAs may be used as amplifiers to provide gain to optical signals relatively inexpensively. Adding gain to optical signals enables the optical signals to traverse a greater distance. That is, amplifying optical signals allows the span budget associated with optical signals to be increased. 
       FIG. 2  is a block diagram representation of a four channel CWDM system that uses an SOA for amplification. Four CWDM channels  204   a - d  are provided as input into an SOA  208 . SOA  208  amplifies channels  204   a - d  to produce amplified channels  204   a ′-d′ as an output. 
     An eight channel CWDM system, on the contrary, may utilize two SOAs which each amplify four channels. Two SOAs may be used due to the limited amplication bandwidth of SOAs. With reference to  FIG. 3 , a system which utilizes two SOAs in conjunction with a band splitter and a band combiner will be described. CWDM channels  304 ,  306  are provided to a band splitter  314  which substantially splits channels  304 ,  306  into low band channels  304  and high band channels  306 . Low band channels  304  are then amplified by a first SOA  318 , while high band channels  306  are amplified by a second SOA  316 . The outputs of SOAs  316 ,  318  are provided to a band combiner  320  that recombines amplified low band channels  304 ′ and amplified high band channels  306 ′. 
     Although the use of filters such as band splitter  314  and band combiner  320 , in conjunction with SOAs  316 ,  318 , is effective in providing amplification for eight channel CWDM systems, the use of the filters introduce additional loss that impacts and reduces the span budget. Filters such as band splitter  314  and band combiner  320  may occupy a significant amount of physical space. 
     Therefore, what is needed is a method and an apparatus which enables the maximum span budget in a CWDM system to be efficiently increased. That is, what is desired is a system that allows CWDM channels to be efficiently amplified such that the distances which may be traversed by signals traveling on the channels may be increased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a diagrammatic representation of an overall network system that includes a system which communicates using coarse wavelength division multiplexing (CWDM). 
         FIG. 2  is a block diagram representation of a four channel CWDM system that uses one semiconductor optical amplifier (SOA). 
         FIG. 3  is a block diagram representation of an eight channel CWDM system that uses a band splitter, two SOAs, and a band combiner. 
         FIG. 4A  is a block diagram representation of a CWDM system which includes two SOAs and a four channel add/drop demultiplexer in accordance with an embodiment of the present invention. 
         FIG. 4B  is a block diagram representation of a CWDM system which includes two SOAs as well as a four channel add/drop demultiplexer, and input and output channels associated with the CWDM system in accordance with an embodiment of the present invention. 
         FIG. 5A  is a graphical representation of gain peaks for two SOAs in accordance with an embodiment of the present invention. 
         FIG. 5B  is a graphical representation of gain peaks for two SOAs and corresponding CWDM channel gains in accordance with an embodiment of the present invention. 
         FIG. 6A  is a block diagram representation of a multiplexer and an SOA with a gain peak that occurs at approximately 1500 nanometers (nm) in accordance with an embodiment of the present invention. 
         FIG. 6B  is a block diagram representation of an add/drop demultiplexer in accordance with an embodiment of the present invention. 
         FIG. 6C  is a block diagram representation of an SOA with a gain peak that occurs at approximately 1580 nm and a demultiplexer in accordance with an embodiment of the present invention. 
         FIG. 7  is a block diagram representation of a network node within an optical network that is arranged to add gain to CWDM signals using a cascaded SOA arrangement with an add/drop demultiplexer in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The span budget associated with a coarse wavelength division multiplexing (CWDM) system may generally be increased through the use of systems that include semiconductor optical amplifiers (SOAs). However, conventional CWDM systems that use SOAs to amplify, for example, eight channels typically include filters in addition to the SOAs which each amplify only four channels. The use of filters, while effective, generally has an adverse impact on the total span budget. 
     By utilizing an SOA arrangement in which a plurality of SOAs are cascaded, the span budget of a CWDM system may be efficiently increased. As will be appreciated by those skilled in the art, SOAs have asymmetrical gain curves that have bandwidths of approximately eighty nanometers (nm). To accommodate a bandwidth of approximately 160 nm for an eight channel CWDM system, the gains curves of the SOAs may be shifted such that substantially the entire 160 nm bandwidth is accommodated. Sending all wavelengths through a first SOA that is substantially optimized for a lower wavelength band and then sending only a higher wavelength band through a second SOA that is substantially optimized for the higher wavelength band, gain may be added to all wavelengths substantially without significant power absorption due to negative gains associated with the SOAs. 
       FIG. 4A  is a block diagram representation of a system that includes two cascaded SOAs and an add/drop demultiplexer in accordance with an embodiment of the present invention. Input channels  402  are provided to a multiplexer  404 . Input channels  402  are associated with CWDM transmissions, and generally are associated with eight wavelengths. It should be understood that the wavelengths are associated with optical signals carried in channels  402 . The eight wavelengths are typically wavelengths of approximately 1470 nm, approximately 1490 nm, approximately 1510 nm, approximately 1530 nm, approximately 1550 nm, approximately 1570 nm, approximately 1590 nm, and approximately 1610 nm. 
     Multiplexer  404  is typically an 8:1 multiplexer, or an eight channel multiplexer. The output of multiplexer  404  is provided to a first SOA  408  that is configured to have a peak gain at approximately 1500 nm, i.e., the largest amount of gain for SOA  408  occurs at a wavelength of approximately 1500 nm. The gain curve associated with first SOA  408  will be discussed below with respect to  FIGS. 5A and 5B . First SOA  408  is arranged to provide amplification to channels  402 . It should be appreciated that first SOA  408  generally provides amplification to channels  402  with lower wavelengths as well as at least some channels  402  with higher wavelengths. Although channels  402  are described as being amplified for ease of discussion, as is known in the art, it is the optical signals carried within channels  402  are actually amplified. 
     The output of first SOA  408  is provided to an add/drop demultiplexer  412 , which is typically a four channel add/drop demultiplexer, that is arranged to effectively drop lower band amplified channels  424 , or channels  424  which have lower wavelengths. The remaining ones of channels  402 , i.e., the four channels that are not dropped by add/drop the demultiplexer  412 , are provided to a second SOA  416 . 
     Second SOA  416  is arranged to provide amplifications to remaining channels  402 , and is configured to have a different peak gain than first SOA  408 . In the described embodiment, the peak gain associated with second SOA  416  occurs at approximately 1580 nm. After being amplified by second SOA  416 , the amplified remaining channels are demultiplexed by a demultiplexer  420 , e.g., a 1:4 or four channel demultiplexer. Then, amplified channels  428  are provided as output from demultiplexer  420 . 
     With reference to  FIG. 4B , the disposition of actual signals which are processed by SOAs  408 ,  416  and add/drop demultiplexer  412  will be discussed in accordance with an embodiment of the present invention. Input CWDM channels or wavelengths  452  are provided as inputs to multiplexer  404 . In the described embodiment, channels  452  include lower band channels  452   a - d  and higher band channels  452   e - h . Channels  452   a - d  generally all have lower wavelengths than channels  452   e - h.    
     Channels  452  are all multiplexed by multiplexer  404 , and the output of multiplexer  404 , which contains channels  452 , is provided to first SOA  408 . First SOA  408 , which is arranged to provide amplification to channels  452   a - d , also provides some amplification to channels  452   e - h . Add/drop demultiplexer  412  then drops amplified channels  452   a ′-d′ and, as a result, allow channels  452   a ′-d′ to be provided to their intended destination. Channels  452   e - h , which have not been dropped, are provided to second SOA  416  which is arranged to provide amplification. The output of second SOA  416 , which contains channels  452   e - h  after amplification, are demultiplexed by demultiplexer  420 . Amplified channels  452   e ′-h′, which are the output of demultiplexer  420 , are then ready to be provided to their intended destination. 
     To substantially control the channels that are amplified by each SOA  408 ,  416 , SOAs  408 ,  416  may be configured to have peak gains that occur at wavelengths which are appropriate for the channels that each SOA  408 ,  416  is intended to amplify. As will be appreciated by those skilled in the art, the bandwidth associated with the gain of an SOA is typically approximately ±40 nm around the wavelength that corresponds to a peak gain amount. Hence, by centering the peak associated with an SOA that is intended to amplify lower band channels, it may have a peak gain substantially in the middle of the lower band; this way, substantially all lower band channels may be amplified without causing attenuation in the higher band channels. Similarly, an SOA that is intended to amplify higher band channels may have a peak gain substantially in the middle of the higher band. However, to avoid causing attenuation due to negative gains in lower band channels with an SOA that is intended to amplify higher band channels, the lower band channels may be prevented from being processed by the SOA that is intended to amplify the higher band channels. 
     Referring next to  FIG. 5A , gain curves associated with cascaded SOAs will be described in accordance with an embodiment of the present invention. A graphical representation  500  includes a gain axis  508  and a wavelength axis  504 . Gain axis  508  is arranged to indicate a gain in decibels (dB), while wavelength axis  504  is arranged to indicate a wavelength in nm. A first gain curve  512  is associated with an SOA that is arranged to provide gain for lower band channels or wavelengths. In the described embodiment, the lower band wavelengths are wavelengths of approximately 1470 nm, approximately 1490 nm, approximately 1510 nm, and approximately 1530 nm. Hence, an approximate peak  520  of first gain curve  512  is arranged, e.g., shifted or otherwise manipulated, to occur at approximately 1500 nm such that each of the lower band wavelengths may be amplified. 
     A second gain curve  516  is associated with an SOA that is arranged to provide gain for higher or upper band wavelengths. The upper band wavelengths, in the described embodiment, are wavelengths of approximately 1550 nm, approximately 1570 nm, approximately 1590 nm, and approximately 1610 nm. An approximate peak  524  of second gain curve  516  is arranged to occur at approximately 1580 nm. The location of approximate peak  524  at approximately 1580 nm allows substantially all of the upper band wavelengths to be amplified. 
     To prevent negative gains in second gain curve  516  from adversely affecting lower band wavelengths by causing gain absorption, the lower band wavelengths are effectively prevented from being amplified by second gain curve  516 . As previously discussed, a low band demultiplexer or add/drop demultiplexer may be used to prevent lower band wavelengths from being affected by second gain curve  516 .  FIG. 5B  is a graphical representation  500 ′ of first gain curve  512 , second gain curve  516 , and the amplified gains  560   a - h  associated with channels. The amplified gains  560   a - d  for lower band channels are such that the gains are approximately reflected by first gain curve  512 , i.e., gains  560   a - d  for lower band channels substantially coincide with first gain curve  512 . 
     Negative gains on gain curves  512 ,  516  generally have an adverse affect on amplified gains  560   a - h  associated with channels. As such, lower band channels are not amplified by an SOA associated with second gain curve  516 , i.e., amplified gains  560   a - d  associated with channels are affected substantially only by first gain curve  512  and not by second gain curve  516 . Gain curve  512  has negative gains for wavelengths above approximately 1600 nm. Hence, for a higher band channel with a wavelength of approximately 1610 nm, gain curve  512  may cause some attenuation of or gain absorption. However, the effect of second gain curve  516  on higher band channels is such that the overall amplified gain associated with the channel associated with a wavelength of approximately 1610 nm is positive as indicated by amplified gain  560   h , albeit slightly less than the value on second gain curve  516  at approximately 1610 nm. 
     As first gain curve  512  and second gain curve  516  both have a positive effect on higher band channels with wavelengths of approximately 1550 nm and 1570 nm, amplified gains  560   e ,  560   f  have gain values that are greater than the values associated with both first gain curve  512  and second gain curve  516 . Amplified gain  560   g , which corresponds to a higher band channel with a wavelength of approximately 1590 nm, has a value that is close to the gain associated with second gain curve  516 . 
     Using a cascaded SOA arrangement as shown in  FIGS. 4A and 4B  allows the gain associated with CWDM channels in a band with a width of approximately 140 nm to be increased by at least five dB, as indicated in  FIG. 5B . Using two SOAs in a cascaded arrangement to achieve a gain increase is efficient and relatively easy to implement. With reference to  FIGS. 6A-6C , a cascaded SOA arrangement such as the cascaded SOA arrangement of  FIG. 4B  will be described in terms of an eight channel CWDM band with wavelengths in the range between approximately 1470 nm and approximately 1610 nm in accordance with an embodiment of the present invention. An eight channel CWDM band includes channels or wavelengths  602   a - h , as shown in  FIG. 6A . Lower band channels  602   a - d  have wavelengths of approximately 1470 nm, approximately 1490 nm, approximately 1510 nm, and approximately 1530 nm, respectively. Higher band channels  602   e - h  have wavelengths of approximately 1550 nm, approximately 1570 nm, approximately 1590 nm, and approximately 1610 nm. 
     Channels  602   a - h  are provided as inputs to a multiplexer  604  which multiplexes channels  602   a - h , and creates a signal  606  that includes information associated with channels  602   a - h . Signal  606  is then provided as an input into an SOA  608  that has a peak gain at approximately 1500 nm. SOA  608  amplifies signal  606  and, as a result, effectively amplifies at least some channels  602   a - h . Some channels  602   a - h , e.g., lower band channels  602   a - d , are amplified while others, e.g., higher band channel  602   h , may be subjected to attenuation or gain absorption. 
     A signal  610  generated by SOA  608  is passed as an input into an add/drop demultiplexer  612 , as shown in  FIG. 6B . Add/drop demultiplexer  612  is arranged to demultiplex and drop amplified lower band channels contained in signal  610 . As shown, amplified lower band channels  602   a ′-d′ are dropped by add/drop demultiplexer  612 . A signal  614  which contains information pertaining to higher band channels is allowed to substantially pass through add/drop demultiplexer  612 . 
     Signal  614  is provided as input into an SOA  616  that is arranged to effectively amplify higher band channels, as shown in  FIG. 6C . As higher band channels have wavelengths in a band between approximately 1550 nm and approximately 1610 nm, SOA  616  is arranged to have a peak gain at approximately 1580 nm. SOA  616  generates a signal  618  that is passed into a demultiplexer which demultiplexes amplified higher band channels  602   e ′-h′ from signal  618 . 
     A cascaded SOA arrangement as shown, for example, in  FIGS. 4A and 4B  is often implemented as part of a network element or node within an optical network.  FIG. 7  is a block diagram representation of a network node within an optical network that is arranged to add gain to CWDM signals using a cascaded SOA arrangement with an add/drop demultiplexer in accordance with an embodiment of the present invention. Within a network that includes multiple network nodes or elements, a node  704  includes a cascaded SOA arrangement  716 . Node  704  may generally also include a processor  720  and memory  724  which stores data and code devices which may be processed by processor  720 . 
     An optical light transmitter  712  or light source in node  704  produces CWDM signals  732  of approximately eight wavelengths. As discussed above, signals  732  may include a lower band wavelengths and higher band wavelengths. Signals  732  are processed by cascaded SOA arrangement  716  which increases the span budget associated with signals  732  by, in one embodiment, at least approximately six or eight dB. By way of example, the span budget associated with signals  732  may be approximately 38 dB. A first SOA of cascaded SOA arrangement  715  adds gain to signals  732  of lower band wavelengths and a second SOA of cascaded SOA arrangement  715  cooperates with the first SOA to add gain to signals  732  of higher band wavelengths. Hence, cascaded SOA arrangement  715 , which includes an add/drop demultiplexer that prevents gain absorption of a second SOA from affecting signals  732  with lower band wavelengths, creates amplified signals  732 ′. 
     Amplified signals  732 ′ may be sent over distances that exceed approximately 100 kilometers to a receiving node  708  that includes an optical light receiver  728 . The gain added to signals  732  to create amplified signals  732 ′ allow amplified signals  732 ′ to be transmitted over distances that are greater than approximately 100 kilometers. 
     Although only a few embodiments of the present invention have been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or the scope of the present invention. By way of example, while an eight channel CWDM band has been described as having wavelengths in the range between approximately 1470 nm and approximately 1610 nm spread out by approximately 20 nm, the wavelengths in an eight channel CWDM band may generally vary widely. In other words, a CWDM band may include various ranges of wavelengths. When different ranges of wavelengths are to be processed by a cascaded SOA arrangement, the peak gains for the SOAs in the SOA arrangement may be shifted to compensate for the different ranges of wavelengths. 
     It should be appreciated that CWDM generally combines up to approximately sixteen or eighteen channels or wavelengths onto a single fiber that are spaced apart by approximately 20 nm. For instance, 18 channels or wavelengths may be spaced 20 nm apart between approximately 1270 nm and approximately 1610 nm. Hence, although the eight channels described as being amplified by a cascaded SOA arrangement are spaced 20 nm apart between approximately 1470 nm and approximately 1610 nm, the wavelengths of the eight channels may be widely varied without departing from the spirit or the scope of the present invention. Additionally, a cascaded arrangement of SOAs may include more than two SOAs when more than eight channels are to be amplified. 
     Further, the number of channels processed or amplified using a cascaded SOA arrangement may vary. By way of example, while amplifying eight channels using a cascaded SOA arrangement has been described, it should be appreciated that fewer than eight channels may also be amplified using a cascaded SOA arrangement. In addition, an SOA has been described as having an approximately 80 nm gain bandwidth. For an embodiment in which SOAs in a cascaded SOA arrangement have a larger bandwidth than approximately 80 nm, it may be possible to amplify more than eight channels using the cascaded SOA arrangement. 
     In general, although the components of a cascaded SOA arrangement and supporting multiplexers and demultiplexers are generally hardware components, the components may include any suitable combination of hardware and software components. The software components may include program code devices arranged to perform various functions of the present invention. Further, some components may be implemented as application specific integrated circuits which may be at least partially programmed with code devices that cause various functions to be performed. 
     The components used to allow multiple channels to be amplified may vary. For instance, although a four channel add/drop demultiplexer has been described as being used to allow lower band channels to be dropped before being provided to a second SOA, substantially any device which allows lower band channels to drop while allowing higher band channels to pass therethrough may generally be used. Additionally, it should be appreciated that SOAs are only one example of optical amplifiers which are suitable for use in a cascaded arrangement to amplify CWDM channels. In general, any suitable optical amplifiers may be used in a cascaded arrangement to amplify CWDM channels. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.