Abstract:
The present invention provides a system and method for multiplexing DWDM channels on top existing CWDM infrastructure. An erbium-doped fiber amplifier amplifies DWDM signals in the DWDM domain to compensate for 10 G optics power budget limitations without blocking the CWDM signals. A passive WDM infrastructure allows the CWDM and DWDM signals to be multiplexed and de-multiplexed on the same fiber and allows seamless integration with existing infrastructure avoiding the need to sacrifice CWDM channels.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The present invention relates to optical networking and more specifically, to a system and method for integrating CWDM and DWDM technologies on a common fiber optics infrastructure.  
         [0003]     2. Description of Related Art  
         [0004]     Coarse wavelength division multiplexing (CWDM) is a method of combining multiple signals on laser beams at various wavelengths for transmission along fiber optic cables. CWDM systems are a popular choice for metro access networks and major telecoms have a significant capital investment in the CWDM infrastructure. Although the number of channels in a CWDM system is fewer than in a dense wavelength division multiplexing (DWDM) system, CWDM remains widely deployed in metro access networks where the distance is limited to about 80 kilometers (km). As used herein, CWDM refers to an ITU (International Telecommunications Union) standard which includes the specification of the particular channel wavelengths and the spacing between these channels. DWDM refers to an ITU standard in which the channel spacing is tighter so more wavelength channels are packed into an optical fiber.  
         [0005]     With the continued growth in network traffic, telecoms are motivated to upgrade the capacity of their network to meet customer expectations. This means that telecoms need to increase channel density by adding additional wavelengths. However, CWDM is effectively limited to about eight different wavelengths on common ITU-T G.652 fiber (type A and B). The G.652a and G.652b specifications define the optical fiber specifications. These optical fibers are typically found in extended length LAN, MAN and access network systems. Clearly, rather than ripping out the CWDM network and replacing it with a DWDM network, telecoms need a cost effective solution that can increase channel density by adding wavelengths in a seamless, non-evasive manner to the CWDM network.  
         [0006]     On approach that has been suggested is to cannibalize a portion of the CWDM wavelengths to route DWDM channels. Because, DWDM has tighter channel spacing, replacing 25% to 50% of the CWDM channels with DWDM channels results in an overall increase in channel density. Unfortunately, this approach has several shortcomings. For example, the telecom loses a significant portion of their CWDM bandwidth, which is clearly undesirable. Further, because of the optical characteristics of the 1 GbE and 10 GbE, performance is degraded and the network is limited to much less than 80 kilometers (km). Thus, the telecom would have to redesign the entire network to take into account the degraded performance. Further still, the DWDM channels undergo significantly more attenuation than the CWDM channels which is a critical limitation for 10 G application already suffering a power budget gap with ½ GbE CWDM channels.  
         [0007]     Notwithstanding the problems with adding addition channels, telecoms are also motivated to upgrade the data rates of their network to meet customer expectations. This means that telecoms need to increase data rates on at least part of the channel capacity. Since most of the installed CWDM networks already include the technology infrastructure to support 1 Gigabit Ethernet (GbE), the natural progression would be to upgrade the CWDM infrastructure to handle 10 GbE. However, because DWDM technology dominates the 10 GbE market, there is only limited market opportunity for 10 GbE CWDM technology and the acquisition price for that technology too high. Thus, telecoms are being forced to upgrade their infrastructure to 10 GbE DWDM.  
         [0008]     The upgrade to 10 GbE DWDM means that telecoms either have to string new fiber or mix DWDM with CWDM optical technologies on the same fiber. Unfortunately, because the 10 G optics has a reduced power budget compared to the lower speed GbE optics, it is not possible to simply insert 10 G optics on existing CWDM installations because any optical amplification of the DWDM would block the CWDM wavelengths that are outside the pass band of the amplifiers.  
         [0009]     Accordingly, most telecoms resort to leaving the traditional CWDM network intact and stringing a separate fiber to handle the DWDM network traffic. Not only is this an expensive alternative, laying new fiber is intrusive and potentially disruptive to the existing network as new power supplies and other infrastructure is swapped out to handle the new network.  
         [0010]     What is needed is a system and method that increases the channel density of CWDM networks and migrates the CWDM networks to 10 GbE in a seamless and non-invasive manner.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  illustrates an exemplary and simplified CWDM optical network  10  that includes both CWDM and DWDM channels in accordance with an embodiment of the present invention.  
         [0012]      FIG. 2  illustrates the typical channel spacing in a CWDM system together with the DWDM channel overlay in accordance with an embodiment of the present invention.  
         [0013]      FIG. 3  illustrates a DWDM multiplexer for multiplexing DWDM channels onto the CWDM network in accordance with an embodiment of the present invention.  
         [0014]      FIG. 4  illustrates the receiver portion of demultiplexer for recovering DWDM channels from the CWDM network in accordance with an embodiment of the present invention.  
         [0015]      FIG. 5  illustrates a transceiver having a DWDM multiplexer and a DWDM demultiplexer in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     The present invention relates to optical networking and more specifically, to a system and method for integrating CWDM and DWDM technologies on a common fiber optics infrastructure that provides an increases channel density and migrates current lower speed GbE/OC-48 CWDM infrastructure to 10 GbE in a seamless and non-invasive manner.  
         [0017]     In the following description of embodiments of the present invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention.  
         [0018]     Further, in the following description of embodiments of the present invention, numerous specific details are presented to provide a complete understanding of the embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention. Wherever possible, the same reference numbers will be used throughout in the drawings to refer to the same or like components.  
         [0019]     Refer now to  FIG. 1 , which illustrates an exemplary and simplified CWDM optical network  10  that includes both CWDM and DWDM channels. More specifically, optical network  10  includes an optical multiplexer  12  receiving a plurality of CWDM channels, generally indicated at  13 . In the illustrated embodiment, the channels have wavelengths spaced 20 nanometers (nm) apart at eight defined wavelengths: 1611 nm, 1591 nm, 1571 nm, 1551 nm, 1531 nm, 1511 nm, 1491 nm, and 1471 nm. As is well understood, the energy from the lasers in a CWDM system is spread out over a larger range of wavelengths than is the energy from the lasers in a DWDM system. The tolerance or extent of wavelength imprecision or variability in a CWDM laser is much looser compared to the tighter tolerance required of a DWDM laser. Because of the use of lasers with lower precision, a CWDM system is less expensive and consumes less power than a DWDM system.  
         [0020]     The output of multiplexer  12  is routed to a DWDM multiplexer  14  where the CWDM signal is multiplexed onto fiber  15  with a plurality of DWDM signals, generally indicated at  16 . Advantageously, none of the CWDM channels are sacrificed to provide the plurality of DWDM channels.  
         [0021]     Before the DWDM signals are multiplexed with the CWDM signals, they are amplified in the DWDM domain by erbium doped amplifiers  17  to compensate for the reduced power budget of 10 G optics.  
         [0022]     The combined CWDM and DWDM signals are demultiplexed by DWDM demultiplexer  20  that separates the DWDM from the CWDM signals. The DWDM signals are provided as outputs as generally indicated at  21 . The CWDM signal is routed to a second demultiplexer, CWDM demultiplexer  22  that provides the plurality of CWDM signals, as generally indicated at  23 .  
         [0023]      FIG. 2  illustrates the typical channel spacing in a CWDM system together with the DWDM channel overlay. As is well understood in the art, common CWDM filter design calls for eight channels aligned on the ITU-T G.694.1 grid (1471, 1491, 1511, 1531, 1551, 1571, 1591, 1611 nm). One skilled in the art will understand that other channel alignments are feasible (for example, the grid may begin at  1470  with a channel spacing of 20 nm. Further, there may be more than eight CWDM channels depending on the specific implementation of a network.  
         [0024]     Each CWDM filter bandwidth is about 12 nm wide. This means that the 1551 nm filter channel extends as high as 1557 nm, as indicated at  25 . The 1571 nm filter channel extends as low as 1565 nm, as indicated at  26 . Similarly, the 1531 channels extends up to 1537 nm, as indicated at  27 , while the 1551 channels extends down to 1545 nm, as indicated at  28 .  
         [0025]     The CWDM filter design leaves two spectral regions, in between CWDM channels, that overlap with the C-band where DWDM wavelengths reside. The C-band, or conventional band, refers to the spectral window from about 1525 nm to 1565 nm. This window also corresponds to the amplifying range of erbium-doped fiber amplifiers  17 . With the ITU-T G.692 DWDM grid and appropriate CWDM passive filters, eight additional DWDM channels are inserted in between existing CWDM channels. In one embodiment, additional band filters are used to provide about 30 dB isolation between CWDM and DWDM channels.  
         [0026]     The eight additional DWDM channels are divided into two groups. One group in the 1538.98 to 1542.94 spectral window is inserted between the 1531 and 1551 CWDM channels as indicated at  29 . The second group in the 1548.98 to 1560.61 spectral window are inserted between the 1551 and the 1570 CWDM channels as indicated at  30 .  
         [0027]      FIG. 3  illustrates DWDM multiplexer  14  in greater detail. Specifically, DWDM  14  comprises a plurality of DWDM inputs, indicated generally at  31 . In one embodiment, two groups of DWDM channels are defined, one group having three wavelength and one group having five wavelengths for a total number of eight DWDM channels in addition to eight CWDM channels received from CWDM multiplexer  12 . The multiplexed CWDM channels are received on input  32 .  
         [0028]     Each of the two groups of DWDM signals are passed through an appropriate channel filter, indicated generally at  33 , and combined. The combined channels from each group are then routed to an edge filter  35  to ensure adequate isolation from the CWDM signals.  
         [0029]     The output of edge filter  35  is routed to erbium doped amplifiers  17  to compensate for the reduced power budget of 10 G optics. The present invention deploys amplification on the 10 G DWDM channels alone because these channels are the ones short of power budget. The challenge is to deploy optical amplification in a manner that does not affect the CWDM channels because most optical amplifiers will cut-off any wavelength outside their amplification range (C-band for common metro devices). Because it is not possible to pass CWDM wavelengths through DWDM amplifiers, it is not possible to place an amplifier on the fiber where both CWDM and DWDM channels co-exist. The addition of amplifiers  17  advantageously amplifies the DWDM channels before combination with the CWDM signals.  
         [0030]     To illustrate, consider that a typical CWDM 1 GbE packaged in a Small Form factor Pluggable (SFP) that has a 29 dB power budget over 80 km and assume that the dispersion penalty is negligible. The maximum fiber loss will be 29 dB-4.4 dB (which is the worst case loss through the multiplexer  12  and demultiplexer  22 )-2.2 dB (DWDM filter pass-through in DWDM multiplexer  14 )=22.4 dB  
         [0031]     10GE DWDM Xenpak has 20 dB of power budget over 80 km with approximately 3 dB of dispersion penalty. The maximum fiber loss is thus 20 dB-4.7 dB (DWDM multiplexer  14  and demultiplexer  20 )=15.3 dB. Thus, there is a 7.1 dB difference in fiber budget which means approximately 25 to 30 km less distance for the 10 GE channels.  
         [0032]     Amplifier  17  may be a mini-erbium-doped fiber amplifier (EDFA), which is a very low cost optical device with several meters of glass fiber doped with erbium ions that boosts an optical signal when the erbium ions are excited to a high energy state or an erbium-doped waveguide amplifier (EDWA) which is an optical amplifier similar to an EDFA, but which derives a higher gain through a small waveguide rather than several meters of fiber. Amplifier is put on the MUX path to bump up the power of the eight 10 GbE DWDM channels by about +7 dB.  
         [0033]     The enclosure housing multiplexer  14  includes two additional customer accessible ports to couple the DWDM signals to optical amplifier  17 . Amplifier  17  amplifies the DWDM channels before they get multiplexed with the CWDM signals on the fiber trunk.  
         [0034]     In this manner, amplifier  17  provides enough power to the eight DWDM 10 GbE channels overlaying the CWDM cloud to transport the DWDM 10 GbE channels at least 80 km. This eliminates the need to develop special 10 G CWDM lasers. Further, there is no need to sacrifice any of the existing CWDM channels when the DWDM 10 GbE channels are added to the same fiber infrastructure.  
         [0035]     The output of amplifier  17  is routed to a second edge filter  36  to re-shape the DWDM signals. Filter  36  feeds one of the two groups, for example channels  21 - 23 , to a band filter  37  where the DWDM are multiplexed onto the CWDM signal (from input  32 ) as it is reflected by filter  37 . The combined signal is then reflected at band filter  38  where the second of the two groups of DWDM channels are multiplexed onto the combined signal. The output of filter  38  is sent out of output port  39  to the receiver along fiber  15 .  
         [0036]      FIG. 4  illustrates the receiver portion of demultiplexer  20 . More specifically, the combined CWDM and DWDM signal is received from fiber  15  at input port  42 . The combined signal is reflected at a first band filter  43  where the second of the two DWDM groups are demultiplexed from the combined signal. The demultiplexed signal is reflected through a series of band pass filters, indicated generally at  44 , to recover each of the five channels CH43 (1542.94), CH44 (1542.14), CH46 (1540.56), CH47 (1539.77) and CH48 (1538.98).  
         [0037]     The second group of DWDM signals are then reflected at a second band pass filter  45  where channels CH21 (1560.61), CH22 (1559.79) and CH23 (1558.98) are filtered from the combined signal. Channels  21 - 23  are then separated by band pass filters, indicated generally at  46 . The DWDM channels are provided as outputs at output ports  47 . The CWDM signal is then routed through two additional band filters, indicated generally at  48 , to further improve isolation and then from output port  49  to the CWDM de-multiplexer  22  (see  FIG. 1 ) where CWDM signals  1471 - 1611  are recovered.  
         [0038]     In one embodiment of the present invention, a separate DWDM multiplexer and amplifier are provided on the transmit side of network  15  and a separate DWDM de-multiplexer is provided on the receive side of network  15 . In another embodiment, each side of network  15  comprises a module that includes the DWDM multiplexer, amplifier, and DWDM de-multiplexer in a single enclosure.  
         [0039]     Advantageously, the present invention increases channel capacity in a CWDM networks without sacrificing existing wavelengths. From the telecom&#39;s perspective, the present invention does not require any network re-engineering because the module is simply cascaded with an existing CWDM filter.  
         [0040]      FIG. 5  illustrates an alternative embodiment for a module  50  that implements two additional DWDM 10 GbE channels in a CWDM network. This low cost embodiment, includes input port  51  for receiving the CWDM signals and input ports  52  for receiving DWDM channels. The DWDM channels are combined by filtering channel  22  with pass band filter  53  and then reflecting channel  22  at pass band filter  54  where it is combined with channel  21 . Amplifier  17  then amplifies the DWDM channels before routing the signals to band pass filter  55 . The DWDM channels are combined with the CWDM channel when it is reflected by filter  55 . The combined DWDM and CWDM signals are then routed to the network  15  through port  56 . Advantageously, there is no requirement for edge filters due to the low density of DWDM channels.  
         [0041]     On the receive side, the combined DWDM and CWDM signal is received on port  58  with the CWDM signal reflected by band filter  59  where the DWDM signals are removed. The CWDM signal is then reflected by band filter  60  to improve isolation before being routed to the CWDM de-multiplexer  22  (see  FIG. 1 ) by way of output port  61 . The individual DWDM signals, channel  22  and channel  21 , are recovered by filter  62  and  63 , respectively. Channels  22  and  21  are then available at output ports  64 .  
         [0042]     In other embodiments, a transceiver  50  that implements four additional DWDM 10 GbE channels in a CWDM network. In yet another embodiment, a transceiver  50  that implements six additional DWDM 10 GbE channels in a CWDM network. In yet another embodiment, a plurality of additional DWDM 10 GbE channels are provided in the CWDM network.  
         [0043]     The present invention solves the problem of growing existing CWDM networks to 10 GbE using DWDM  10 G optics. By amplifying the DWDM signals in the DWDM domain, the present invention addresses the reduced power budget of 10 G optics in a fashion that is totally transparent to the CWDM network.  
         [0044]     Therefore, while the description above provides a full and complete disclosure of the preferred embodiments of the present invention, various modifications, alternate constructions, and equivalents will be obvious to those with skill in the art. Thus, the scope of the present invention is limited solely by the metes and bounds of the appended claims.