Abstract:
Flexible optical spectrum management systems and methods in an optical network including a plurality of interconnected network elements include determining an associated frequency/wavelength center and one or more bins for each of one or more traffic carrying channels on each of a plurality of optical fibers in the optical network; and managing the one or more traffic carrying channels on the plurality of optical fibers using the one or more bins of bins and the associated frequency/wavelength center, wherein at least one of the one or more traffic carrying channels comprises a coherent optical signal occupying a flexible spectrum on the plurality of optical fibers.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present non-provisional patent application is a continuation of U.S. patent application Ser. No. 13/218,759, filed Aug. 26, 2011, and entitled “CONCATENATED OPTICAL SPECTRUM TRANSMISSION SYSTEM,” which claims priority to U.S. Provisional Patent Application Ser. No. 61/377,290, filed Aug. 26, 2010, and entitled “CONCATENATED OPTICAL SPECTRUM TRANSMISSION SYSTEM,” each is incorporated in full by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to optical transmission systems and methods. More particularly, the present invention relates to concatenated optical spectrum transmission systems and methods that allocate optical spectrum of groups of channels to reduce or eliminate deadbands or guardbands (i.e., unused optical spectrum) between channels. 
     BACKGROUND OF THE INVENTION 
     Fiber optic transmission systems and methods are widely recognized as the most efficient way to transmit large amounts of data over long distances. An important metric for these systems and methods is called spectral efficiency. Spectral efficiency is a measure of the rate at which data can be transmitted in a given amount of optical spectrum, usually expressed in bits/s/Hz. The size of the available optical spectrum is determined by factors such as the wavelength of low attenuation in transmission fiber, bandwidth of optical amplifiers, and availability of suitable semiconductor lasers and detectors. For example, the C-band may generally include optical spectrum of 1530-1565 nm which corresponds to the amplification range and bandwidth of erbium-doped fiber amplifiers (EDFAs). Given that there is a finite useable spectral range, the spectral occupancy and a given bit-rate of channel, the spectral efficiency of a dense wave division multiplexing (DWDM) system then determines the maximum information carrying capacity. As data demands increase, there is consistently a need for more information carrying capacity given the finite constraints on the useable spectral range. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment, a flexible optical spectrum management method in an optical network including a plurality of interconnected network elements includes determining an associated frequency/wavelength center and one or more bins for each of one or more traffic carrying channels on optical fibers in the optical network; and managing the one or more traffic carrying channels on the optical fibers using the one or more bins of bins and the associated frequency/wavelength center, wherein at least one of the one or more traffic carrying channels includes a coherent optical signal occupying a flexible spectrum on the optical fibers. A size of each of the one or more of bins can be smaller than or equal to a smallest required roll off of a wavelength selective component in the optical network. A plurality of the one or more traffic carrying channels can each be managed by concatenating a number of the one or more of bins together. Different baud rate channels are allocated a different number of bins. The method can be performed by one of a network management system (NMS), an element management system (EMS), a network controller, and a module in a network element. Each of a plurality of traffic carrying channels with a same A-Z path in the optical network can be in a concatenated number of bins together without a deadband between any of the plurality of traffic carrying channels. A guardband can be configured using one or more bins for traffic carrying channels with a different A-Z path in the optical network. 
     In another exemplary embodiment, a management system configured for flexible optical spectrum management in an optical network including a plurality of interconnected network elements includes a processor configured to determine an associated frequency/wavelength center and one or more bins for each of one or more traffic carrying channels on optical fibers in the optical network, and manage the one or more traffic carrying channels on the optical fibers using the one or more bins of bins and the associated frequency/wavelength center, wherein at least one of the one or more traffic carrying channels includes a coherent optical signal occupying a flexible spectrum on the optical fibers. A size of each of the one or more bins can be smaller than or equal to a smallest required roll off of a wavelength selective component in the optical network. A plurality of the one or more traffic carrying channels can each be managed by concatenating a number of the one or more of bins together. Different baud rate channels are allocated a different number of bins. The management system can be one of a network management system (NMS), an element management system (EMS), a network controller, and a module in a network element. Each of a plurality of traffic carrying channels with a same A-Z path in the optical network can be in a concatenated number of bins together without a deadband between any of the plurality of traffic carrying channels. A guardband can be configured using one or more bins for traffic carrying channels with a different A-Z path in the optical network. 
     In a further exemplary embodiment, an optical network configured for flexible optical spectrum management includes a plurality of network elements interconnected by optical fibers; and a management system configured to determine an associated frequency/wavelength center and one or more bins for each of one or more traffic carrying channels on the optical fibers in the optical network, and manage the one or more traffic carrying channels on the optical fibers using the one or more bins of bins and the associated frequency/wavelength center, wherein at least one of the one or more traffic carrying channels includes a coherent optical signal occupying a flexible spectrum on the optical fibers. A size of each of the one or more bins can be smaller than or equal to a smallest required roll off of a wavelength selective component in the optical network. A plurality of the one or more traffic carrying channels can each be managed by concatenating a number of the one or more bins together. Different baud rate channels are allocated a different number of bins. The management system can be one of a network management system (NMS), an element management system (EMS), a network controller, and a module in a network element. Each of a plurality of traffic carrying channels with a same A-Z path in the optical network are in a concatenated number of bins together without a deadband between any of the plurality of traffic carrying channels, and wherein a guardband can be configured using one or more bins for traffic carrying channels with a different A-Z path in the optical network. 
     In an exemplary embodiment, an optical network includes a first network element; a second network element communicatively coupled to the first network element by a first optical fiber; a first plurality of optical channels over the first optical fiber; and a second plurality of optical channels over the first optical fiber; wherein each of the first plurality of optical channels are located on an optical spectrum with substantially no spectrum between adjacent channels, and wherein each of the second plurality of optical channels are located on the optical spectrum with substantially no spectrum between adjacent channels. A guardband may be defined on the optical spectrum between one end of the first plurality of optical channels and one end of the second plurality of optical channels. The optical network may include a first flexible spectrum wavelength selective switch at the second network element, the first flexible spectrum wavelength selective switch configured to drop the first plurality of optical channels. The guardband may be spectrally defined based on a roll-off associated with the first flexible spectrum wavelength selective switch. The optical network may further include, at the second network element, a first power splitter coupled to a drop port of the first wavelength selective switch; and a first plurality of Common Mode Rejection Ratio (CMRR) coherent optical receivers coupled to the power splitter, wherein each of the first plurality of CMRR coherent optical receivers receives each of the first plurality of optical channels and selectively receives one of the first plurality of optical channels. The first flexible spectrum wavelength selective switch may include a continuous spectral response on adjacent actuated portions of the optical spectrum when pointed to the same port. 
     The optical network may include a third network element communicatively coupled to the second network element by a second optical fiber; wherein the second plurality of optical channels is communicated over the second optical fiber and the first optical fiber. The optical network may include a second flexible spectrum wavelength selective switch at the third network element, the second flexible spectrum wavelength selective switch configured to drop the second plurality of optical channels. The optical network may further include, at the third network element, a second power splitter coupled to a drop port of the second flexible spectrum wavelength selective switch; and a second plurality of Common Mode Rejection Ratio (CMRR) coherent optical receivers coupled to the power splitter, wherein each of the second plurality of CMRR coherent optical receivers receives each of the second plurality of optical channels and selectively receives one of the second plurality of optical channels. The optical network may include a plurality of bins defined on the optical spectrum, wherein each of the first plurality of optical channels and each of the second plurality of optical channels are assigned to one or more of the plurality of bins. At least two of the first plurality of optical channels may include a different baud rate. 
     In another exemplary embodiment, an optical system includes a flexible spectrum wavelength selective switch receiving a wavelength division multiplexed (WDM) signal and including at least one drop port configured to receive a first group of optical channels from the WDM signal with adjacent channels spectrally spaced substantially next to one another; a power splitter coupled to the at least one drop port; and a plurality of Common Mode Rejection Ratio (CMRR) coherent optical receivers coupled to the power splitter, wherein each of the CMRR coherent optical receivers receives each of the first group of optical channels and selectively receives one of the first group of optical channels. The flexible spectrum wavelength selective switch may include a continuous spectral response on adjacent actuated portions of an optical spectrum when pointed to the at least one drop port. The flexible spectrum wavelength selective switch may further include at least one express port configured to receive a second group of optical channels from the WDM signal, wherein an optical spectrum of the WDM signal may include a guardband between the first group of optical channels and the second group of optical channels. The guardband may be spectrally defined based on a roll-off associated with the flexible spectrum wavelength selective switch. 
     In yet another exemplary embodiment, a method includes defining a plurality of bins on an optical spectrum associated with an optical fiber; dropping a first optical signal to a first coherent optical receiver, wherein the first optical signal uses one or more bins of the plurality of bins; and dropping a second optical signal to a second coherent optical receiver, wherein the second optical signal uses one or more of adjacent bins to the one or more bins used by the first optical signal. The method may further include defining guardbands in one or more of the plurality of bins, wherein the guardbands are spectrally defined based on a roll-off associated with a flexible spectrum wavelength selective switch. A number of bins for each of the first optical signal and the second optical signal may be based on a format and baud rate associated therewith. The method may further include utilizing a management system to provision the first optical signal in the one or more bins. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated and described herein with reference to the various drawings of exemplary embodiments, in which like reference numbers denote like method steps and/or system components, respectively, and in which: 
         FIG. 1  is a spectral diagram of wavelength channel spacing constrained by a demultiplexer filter response; 
         FIG. 2  is a table of an exemplary standard set of channels and frequencies for optical spectrum from the International Telecommunication Union (ITU); 
         FIG. 3  is diagram of an exemplary micro-electromechanical system (MEMS)-based wavelength-selective switch (WSS); 
         FIG. 4A  is a drop section of an exemplary coherent augmented optical add/drop multiplexer (OADM); 
         FIG. 4B  is an add section of the exemplary coherent augmented OADM; 
         FIG. 5  is a nodal configuration using the coherent augmented OADM for guardband minimization in concatenated optical spectrum transmission systems and methods; 
         FIG. 6  is a network diagram of an exemplary network of plurality of optical network elements configured with the nodal configuration of  FIG. 5 ; 
         FIG. 7  is a spectral diagram of wavelength channel spacing utilizing a range, group, or bin of channels for the concatenated optical spectrum transmission systems and methods; and 
         FIG. 8  is a spectral diagram of wavelength channel spacing utilizing a bin of channels with a guardband contained within bins for the concatenated optical spectrum transmission systems and methods. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In various exemplary embodiments, the present disclosure relates to concatenated optical spectrum transmission systems and methods that allocate optical spectrum of groups of channels to reduce or eliminate deadbands or guardbands (i.e., unused optical spectrum) between channels. The concatenated optical spectrum transmission systems and methods include various techniques for using optical spectrum such as over the C-band or any other frequency bands. In particular, the concatenated optical spectrum transmission systems and methods provide a balance between fixed channel systems such as provided for by the International Telecommunication Union (ITU) and a more flexible system enabled by coherent optical detection. In an exemplary embodiment, the concatenated optical spectrum transmission systems and methods may utilize a Wavelength Selective Switch (WSS) and a plurality of moderate Common Mode Rejection Ratio (CMRR) coherent receivers in combination to achieve a concatenated optical spectrum. 
     Referring to  FIG. 1 , in an exemplary embodiment, a spectral diagram  10  illustrates wavelength channel spacing constrained by a demultiplexer filter frequency response  12 . The spectral diagram  10  is a diagram of optical frequency versus transmission intensity, and the exemplary spectral diagram  10  illustrates an optical channel. Prior to the advent of commercially available coherent optical transponders, all receivers required the DWDM system to filter out all but one optical channel before it was presented to a photodetector. Any practical filter has a passband shape response  12  which has a range of frequencies it can pass efficiently, and then a gradual decrease in transmission efficiency until the point where an acceptable isolation is achieved. For example, the shape response  12  shows an example shape which may be encountered in such a system. The shape response  12  includes a passband portion  14  and roll off portions  16 . The limiting factor for channel spacing in these systems is the roll-off portions  16  of practical filters. In particular, the optical spectrum in the spectral diagram  10  may include various frequency portions including a useable channel passband  18  defined by the passband portion  14 , unusable spectrum  20  where the roll off portions  16  extends to a minimum receiver isolation level  22 , and a closest useable adjacent channel portion  24 . The passband portion  14  may generally be referred to as having guardbands, i.e. the passband portion  14  is spaced apart by the unusable spectrum  20  or guardbands. The best way to overcome this waste of spectrum previously was to create filters with a sharper roll off portion  16  thereby minimizing the unusable spectrum  20 . Disadvantageously, such systems and methods introduce extra complications and expense. 
     Referring to  FIG. 2 , in an exemplary embodiment, a table  30  illustrates an exemplary standard set of channels for optical spectrum. In particular, the table  30  is defined by the ITU and provides a standard set of channels offset by equal frequency spacing, e.g. 12.5 GHz, 25 GHz, 50 GHz, and 100 GHz in the table  30 . For example, the standardization of optical spectrum are described in ITU-T Recommendation G.694.1 (June 2002) Spectral grids for WDM applications: DWDM frequency grid and ITU-T Recommendation G.698.2 (November 2009) Amplified multichannel DWDM applications with single channel optical interfaces, the contents of each are incorporated by reference herein. The channels in the table  30  are defined to provide an acceptable channel passbands and guardbands. One advantage of standardizing channels is to allow a common set of laser sources to be provided in the market. Another advantage is to allow management systems to have a common way of enumerating channels so as to keep track of them regardless of the manufacturer. In an exemplary embodiment, the concatenated optical spectrum transmission systems and methods may utilize the channels in the table  30  but in a modified fashion to adjust for application requirements such as with coherent optical detection. 
     Referring to  FIG. 3 , in an exemplary embodiment, an exemplary micro-electromechanical system (MEMS)-based wavelength-selective switch (WSS)  40  is illustrated. In this exemplary WSS  40 , an input fiber including multiple wavelengths λ 1 , λ 2 , . . . λ n  of optical signals is input into a de-multiplexer  42 , such as a diffraction grating or the like. The de-multiplexer  42  separates each wavelength from the common input, and optionally a variable optical attenuator (VOA)  44  can be included following the de-multiplexer  42 . VOAs  44  are configured to provide variable attenuation to the wavelength, and the VOAs  44  can be remotely and dynamically set to a range of values. The WSS  40  includes a pixel array  46  for each of the wavelengths λ 1 , λ 2 , . . . λ n . The pixelarray  46  includes a plurality of pixels per channel that deflect the optical signal to an appropriate output port  48 . Advantageously, the WSS  40  is fully reconfigurable for adding, dropping, and expressing through optical signals. Since there is the pixel array  46  for each of the optical signals, any signal can be dropped to any of the output ports  48 . Additionally, multiple wavelengths including all wavelengths can be dropped to a single port  48 , such as an express port. To indicate device fan out, these WSS devices are often classified as “1×N” devices, e.g., a “1×9” WSS means a ten port device, with one common input and nine output ports. 
     The WSS  40  may be a flexible spectrum WSS which can switch and attenuate on arbitrary widths of spectrum based on the pixel array  46 . In particular, the flexible spectrum WSS  40  is utilized in the concatenated optical spectrum transmission systems and methods to provide arbitrary spectral widths to avoid or reduce guardbands on optical spectrum. In an exemplary embodiment, the pixel array  46  may include a Liquid crystal on silicon (LCOS) device with thousands of columns, pixels, etc. with hundreds of pixels per optical channel. In another exemplary embodiment, the pixel array  46  may include a Digital Light Processing (DLP) (available from Texas Instruments Inc.) device. Specifically, the pixel array  46  may include any pixelated device which enables flexible spectrum for each optical channel. For example, the flexible spectrum WSS  40  may include a granularity of 1 GHz or less per channel. 
     Referring to  FIGS. 4A and 4B , in an exemplary embodiment, an exemplary coherent augmented optical add/drop multiplexer (OADM) is illustrated with a drop section  52  and an add section  54 . In  FIG. 4A , the drop section  52  includes an n-channel Wavelength Division Multiplexed (WDM) signal  56  input to a drop section WSS  40   d  having a set of p ports  58  (p being a positive integer), which are allocated between q (where q is an integer greater than or equal to zero) express ports  58   e  and  m  (where m is an integer an equal to p minus q) drop ports  58   d . The drop section WSS  40   d  may include the WSS  40  in  FIG. 3  or any other implementation that is generally configured to route any given channel from the input WDM signal  56  to any of the p ports  58  in a dynamic and reconfigurable manner. Using the drop section WSS  40   d , a respective set of w (where w is an integer) wavelength channels from the WDM signal  56  may be supplied to each drop port  58   d . The number of wavelength channels supplied to any given drop port  58   d  may be the same, or different from the number of wavelength channels supplied to another one of the drop ports  58   d.    
     A 1:s power splitter  60  (where s is an integer) may be connected to each drop port  58   d  then supplies the respective set of channels to each one of a corresponding set of s coherent optical receivers (cRx)  62 . The power splitter  60  is configured to receive an output from each of the drop ports  58   d  and perform a splitting function providing a split copy of the output to s outputs. As described herein, the drop section WSS  40   d  may be a conventional WSS. In an exemplary embodiment, the WDM signal  56  may be formatted to conform with a standard spectral grid, for example an ITU-T grid having a 100 GHz channel spacing illustrated in the table  30 . In exemplary embodiments, the WDM signal  56  may have between n=32 and n=96 wavelength channels, and the WSS  40   d  may have p=20 ports  48 . The number (m) of drop ports  58   d , and the number (q) of express ports  58   e  may be selected as appropriate. For example, in a mesh network node requiring eight-degree branching, a set of q=7 express ports  58   e  is required, leaving m=13 ports available for use as drop ports  58   d.    
     Each coherent receiver (cRx)  62  may be tunable, so that it can receive a wavelength channel signal centered at a desired carrier wavelength (or frequency). In an exemplary embodiment in which tunable coherent receivers are used, the frequency range of each coherent receiver (cRx)  62  may be wide enough to enable the coherent receiver (cRx)  62  to tune in any channel of the WDM signal  56 . In other exemplary embodiments, the dynamic range of each coherent receiver (cRx)  62  may be wide enough to enable the coherent receiver (cRx)  62  to tune in anyone of a subset of channels of the WDM signal  56 , such as w channels associated with the particular drop port  58   d . With the arrangement of  FIG. 4A , each of the coherent receivers (cRx)  62  must be designed having a Common Mode Rejection Ratio (CMRR) which enables the coherent receiver (cRx)  62  to tune in and receive a selected one channel while rejecting each of the other s−1 channels presented to it by the power splitter  60 . Because s&lt;n, the CMRR requirement for the coherent receivers (cRx)  62  is significantly lower than that which would be required to support all n channels. This relaxed CMRR requirement means that lower cost coherent receivers may be used. However, it will be seen that, even with the lower CMRR of each coherent receiver  62 , a total drop count of d=m*s is achieved. For example, consider a network system in which the WDM signal  56  has n=96 wavelength channels, and the WSS  40   d  has m=6 drop ports, each of which receives a respective set of s=16 channels. In this case, the total drop count is d=6*16=96 channels. In an exemplary embodiment, the coherent receiver  62  may be configured to support all of the n wavelengths. Here, the channels presented to the coherent receiver  62  from the power splitter  60  do not have to be adjacent, but could be scattered anywhere across the n wavelengths. In another exemplary embodiment, the coherent receiver  62  is configured to accept a subset of the n wavelengths, e.g. s wavelengths per receiver  62 . In  FIG. 4B , the add section  54  of the coherent augmented OADM operates in a manner that is effectively the reciprocal of the drop section  52  of  FIG. 4B . Thus, an add section WSS  40   a  is provided with a set of ports  58 , which are designated as either add-ports  58   a  or express ports  58   e . The add section WSS  40   a  operates to add the channels received through each port  58   a  into an outbound WDM signal  66  which is launched into a downstream optical fiber medium. Each express port  40   e  receives a respective WDM optical signal from upstream optical equipment such as, for example, the drop section  52  of the same (or a different) OADM. Each add port  58   a  is connected to an s:1 power combiner  66  (where s is an integer) which combines the channel signals generated by a respective set of coherent optical transmitters (cTx)  64 . Some or all of the coherent optical transmitters (cTx)  64  connected to a given power combiner  66  may be operating at any given time, so each add port  58   a  will receive a respective set of w (where is an integer less than or equal to s) wavelength channels. The number of wavelength channels received by any given add port  58   a  may be the same, or different from the number of wavelength channels received by another one of the add ports  58   a . With this arrangement, the total number of transmitters that can be supported is t=m*s. For example, consider a network system having a capacity of n=96 wavelength channels, and the add section WSS  40   a  has m=6 add ports, each of which is coupled to a power combiner  66  that supports a respective set of s=16 transmitters. In a case where all of the transmitters are generating a respective wavelength channel, each add port  58   a  will receive a set of s=16 channels, and the total add count is t=6*16=96 channels. 
     In an exemplary embodiment, each coherent optical transmitter (cTx)  64  is tunable so that it can selectively generate a wavelength channel signal centered at a desired carrier wavelength (or frequency). In exemplary embodiments in which tunable coherent optical transmitters (cTx)  64  are used, the dynamic range of each transmitter (cTx)  64  may be wide enough to enable the transmitter (cTx)  64  to generate any channel of the WDM signal  56 . In other exemplary embodiments, the dynamic range of each transmitter (cTx)  64  may be wide enough to enable the transmitter (cTx)  64  to generate anyone of a subset of channels of the WDM signal  56 , such as one of s signals. The coherent optical receivers (cRx)  62  and the coherent optical transmitters (cTx)  64  may be configured to use any of duo-binary, quadrature amplitude modulation (QAM), differential phase shift keying (DPSK), differential quadrature phase shift keying (DQPSK), orthogonal frequency-division multiplexing (OFDM), polarization multiplexing with any of the foregoing, and any other type of coherent optical modulation and detection technique. It is understood that for electronic channel discrimination, a tunable Rx is required. In nQAM and nPSK it is achieved using a linear receiver, i.e. a receiver where frequency mixing is taking place between a local oscillator and the incoming signal. The local oscillator needs to be tuned at the right frequency such that the mixing product can be at base band where all the necessary filtering will occur. If a receiver is not operating like above, it requires a tunable optical filter prior to the optical detector. 
     Generally, the WSS  40 ,  40   a ,  40   d  and other types of WSSs are essentially a polychrometer device with multiple output/input ports. Individual channels (i.e., wavelengths) can be switched by such a device and sharp roll-offs can be achieved. That is, the WSS  40 ,  40   a ,  40   d  may be utilized to provide s demultiplexer function such as illustrated by the demultiplexer filter shape response  12  in  FIG. 1 . The flexible spectrum WSS  40 ,  40   a ,  40   d  can provide significantly improved roll-off portions  16  from other technologies such as arrayed waveguide gratings (AWGs) or thin film filters (TFFs). In an exemplary embodiment, the concatenated optical spectrum transmission systems and methods may utilize the coherent augmented OADM in  FIG. 4  to eliminate individual channel filtering at the drop ports  58   d . Thus, without the demultiplexer, individual channels may be arranged or spaced closer together only limited by the significantly improved roll-off portions  16  associated with the WSS  40 ,  40   a ,  40   d . Advantageously, through such a configuration, deadbands or guardbands may be reduced or eliminated. The coherent augmented OADM of  FIGS. 4A and 4B  may include additional components which are omitted for simplicity. For example, one of ordinary skill in the art will recognize that there may be optical amplifiers added in these configurations to overcome the losses of WSS&#39;s  40   a ,  40   d  and splitters  60 , e.g. in location  58   d , and  58   a.    
     Referring to  FIG. 5 , in an exemplary embodiment, a nodal configuration  70  illustrates guardband minimization in the concatenated optical spectrum transmission systems and methods. The nodal configuration  70  includes the drop section WSS  40   d  and the add section WSS  40   a  associated with the coherent augmented OADM. To provide concatenated optical spectrum, the WSSs  40   a ,  40   d  provide a continuous spectral response on adjacent actuated portions of the spectrum when they are pointed to the same port  58 . The selection of the number of these WSSs  40   a ,  40   d  and their respective spectral widths is arbitrary and can be chosen according to system need and device design convenience. An advantage of the concatenated optical spectrum transmission systems and methods is the ability to use the WSSs  40   a ,  40   d  which have roll-offs which require small guardbands, but some portion of spectrum is concatenated in such a way that there is no need for guardbands on adjacent channels within a sub-section of the spectrum. 
     The nodal configuration  70  receives a WDM signal  72  at an ingress point and optionally may include an optical amplifier  74  to amplify the received WDM signal  72 . The received WDM signal  72  includes a plurality of channels (i.e. wavelengths) in a concatenated structure with respect to the optical spectrum. For example, the received WDM signal  72  may include drop channels  76  and express channels  78  with a guardband  80  therebetween. Specifically, each adjacent channel in the drop channels  76  and the express channels  78  may abut adjacent channels with little or no spectral space therebetween. The only unused spectrum in the received WDM signal  72  may include the guardband  80 . In terms of network-wide functionality, the express channels  78  are configured to transit the nodal configuration  70  whereas the drop channels  76  are configured to be dropped and added at the nodal configuration  70 . One of ordinary skill in the art will recognize the nodal configuration  70  may be repeated at other nodes in a network with the express channels  78  from the perspective of the nodal configuration  70  being drop channels  76  at another node. Furthermore, this functionality of the nodal configuration  70  applies as well to the coherent augmented OADM of  FIGS. 4A and 4B . 
     The received and optionally amplified WDM signal  72  is input into a 1:z power splitter  82  where z is an integer. For example, z may be the number of ports in the nodal configuration  70  with one port per degree and one port for add/drop traffic. Alternatively, in a 1:2 node, the 1:z power splitter  82  may be omitted. The power splitter  82  is configured to split the WDM signal  72  in a plurality of copies on output connections coupled to a drop section WSS  40   d  and an add section WSS  40   a . The drop section WSS  40   d  provides functionality similar to that described in  FIG. 4A , namely there is no demultiplexer or filter in line to separate the individual channels of the drop channels  76 . Rather, a power splitter  60  is configured to split the drop channels  76  into plural copies each of which is sent to coherent receivers (cRx)  62  which are configured with CMRR as described herein. In particular, the drop section WSS  40   d  is configured to perform a drop filtering function on the drop channels  76  prior to the power splitter  60 . In such configuration with the CMRR coherent receivers (cRx)  62 , the drop channels  76  may be in a concatenated optical spectrum where there is little or no guardbands between adjacent channels. Further, the CMRR coherent receivers (cRx)  62  are configured to receive all of the drop channels  76  and to selectively tune to a channel of interest. 
     The add section WSS  40   a  is configured to receive the express channels  78  from the power splitter  82  as well as local add traffic from coherent transmitters (cTx)  64  (not shown in  FIG. 5 ). The add section WSS  40   a  includes an output WDM signal  82  which includes the guardband  80  between adjacent concatenated spectrum portions. With respect to adding channels in a concatenated fashion, the local add traffic may be added with the power combiners  66 . In another exemplary embodiment, the add section WSS  40   a  may have add ports  58   a  for all local traffic, i.e. the WSS  40   a  may be configured to multiplex the locally added channels together with little or no space therebetween in terms of optical spectrum. For example, The WSS  40   a  may require a guardband for some isolation between the channels due to limitations on the flexible spectrum WSS  40   a . Further, as WSS port count increases, it is also contemplated that the WSS  40   d  may drop channels on an individual basis with each channel having little or no spectral space therebetween based on the fact there is no need to provide isolation between ports on the WSS  40   d  in the concatenated optical spectrum transmission systems and methods described herein. Those of ordinary skill in the art will recognize other embodiments are also contemplated which generally will add channels in such a manner as to not have spectral space therebetween. Further, the coherent receivers  62  and the coherent transmitters  64  are illustrated as separate devices in  FIG. 4 , and those of ordinary skill in the art will recognize these may be a single device referred to as a CMRR coherent optical transceiver. 
     Referring to  FIG. 6 , in an exemplary embodiment, a network  90  is illustrated of a plurality of optical network elements  92  configured with the nodal configuration  70  of  FIG. 5 . In this exemplary embodiment, the network elements  92  each may be configured to provide the concatenated optical spectrum transmission systems and methods. The network elements  92  may include any of WDM network elements, optical switches, cross-connects, multi-service provisioning platforms, routers, and the like with the CMRR coherent receivers (cRx)  62  and the coherent transmitters (cTx)  64 . The network elements  92  are communicatively coupled therebetween by optical fiber  94 . For example, the network elements  92  are connected in a mesh architecture in the exemplary network  90 , and those of ordinary skill in the art will recognize the concatenated optical spectrum transmission systems and methods are contemplated for use with any network architecture, such as, for example, mesh, rings (BLSR, VLSR, UPSR, MS-SPRING, etc,), linear (1:1, 1+1, 1:N, 0:1, etc.), and the like. 
     The nodal architecture  70  at each of the network elements  92  is configured to transmit an optical spectrum  96  over the optical fibers  94 . In the network  90 , in an exemplary embodiment, traffic generated at any network element  92  may terminate on another network element  92 . Even though there are a large number of channels in the DWDM band, there is a smaller number of unique A-Z paths. The A-Z path includes an originating network element  92  and a terminating network element  92  with potentially intermediate network elements  92  where the channels are expressed. At the originating network element  92  and the terminating network element  92 , the channels in an A-Z path are added/dropped through the ports  58   a ,  58   d . At the intermediate network elements  92 , the channels in the A-Z path are expressed. Using the concatenated optical spectrum transmission systems and methods, the network  90  may be configured to group A-Z demands together and place channels in the spectrum going on the same path without deadbands between the channels in the same path. For example, the network  90  includes six network elements  92 , and for full connectivity between each network element  90 , the optical spectrum  96  may be segmented into five segments or groups  98 . Within each group  98 , there is little or no unused spectrum, i.e. deadbands, using the nodal configuration  70 . Between the groups  98 , there is the guardband  80 . 
     Referring to  FIG. 7 , in an exemplary embodiment, a spectral diagram  100  illustrates wavelength channel spacing utilizing a range, group, or bin of channels for the concatenated optical spectrum transmission systems and methods. In an exemplary embodiment, the concatenated optical spectrum transmission systems and methods may provide a completely variable spectrum where channels can be placed anywhere. In this embodiment, to avoid the deadbands, A-Z demands are placed in the same groups  90  in the spectrum. The spectral diagram  100  introduces additional constraints to the selection of spectral segments and the frequency range of channels for the concatenated optical spectrum transmission systems and methods. For example, in the concatenated optical spectrum transmission systems and methods, there are no requirements to define channels, frequencies, etc., but such a configuration is burdensome in operation for network operators. The aim of this approach is to constrain the channel placement in such a way as to ease the concerns of operators of such networks, i.e. to produce a set of channels which is manageable similarly to the ITU grids which have become ubiquitous. However, this approach includes flexibility to account for the concatenated optical spectrum transmission systems and methods. 
     In an exemplary embodiment, traffic carrying channels can be fixed to frequency/wavelength centers which are defined by sub-grid elements. Alternatively, the traffic carrying channels can float within a bin  102 . This would allow an optimization of performance of these channels using arbitrary frequency spacing, while at the same time presenting a fixed range of frequency for the bin  102  to the higher level management system which is then un-encumbered of the exact frequency location of the optical carriers, except to know that they are contained within the bin  102 . 
     In particular, the spectral diagram  100  illustrates an example of how concatenated grids may work in the concatenated optical spectrum transmission systems and methods. The spectral diagram  100  may be segmented into a plurality of bins  102  (i.e., groups, ranges, bands, etc.) of spectrum. Each of the bins  102  may occupy an equal amount of spectrum similar or equivalent to the channels in the table  30  of  FIG. 2 . In an exemplary embodiment, each of the bins  102  may include a granularity of spectrum which is smaller than or equal to the width of the narrowest modulated spectrum which is needed. This amount could be chosen, for instance to coincide with the smallest required roll off of the WSS  40  and any other wavelength selective component used in the system. Further, one could also use devices which have slower roll off by simply allocating multiple bins  102  to the filter guardband. Channels can then be defined by concatenating a number of these bins  102  together. If the bins  102  are enumerated, a descriptive and unique identification may be generated of a channel in this system by stating the start and stop bins  102 , for example, channel  1 - 5  could mean the channel which occupies bins  1  through  5  inclusive. 
     In the exemplary spectral diagram  100 , six exemplary bins  102 - 1 - 102 - 6  are illustrated. Those of ordinary skill in the art will recognize that an optical spectrum may include any arbitrary number of bins  102 . The spectral diagram  100  includes a single group  98 . A first bin  102 - 1  is outside the group  98  and represents allocable spectrum for another group  98  or channel. A second bin  102 - 2  is allocated as an unusable guardband such as the guardband  80  in the nodal configuration  70 . Bins  102 - 3 ,  102 - 4 ,  102 - 5 ,  102 - 6  are all a part of the group  98 . As described herein, channels within the group  98  do not require guardbands. Thus, in an exemplary embodiment, a coherent optical signal  104  may be provisioned in the bins  102 - 3 ,  102 - 4 ,  102 - 5 , and the bin  102 - 6  may be useable spectrum for another coherent optical signal. In such a manner, the coherent optical signals  104  may be provisioned on the spectral diagram  100  with little or no unused spectrum. 
     Advantageously, the concatenated optical spectrum transmission systems and methods provide a mechanism for minimizing deadband allocation. The concatenated optical spectrum transmission systems and methods further allows allocating varying widths of spectrum to individual channels such that one can optimize the amount of spectrum which is used. For example, a 10 Gbaud channel and a 40 Gbaud channel can be allocated different numbers of bins, a 100 Gbaud channel can be allocated yet another different number of bins. For example, the concatenated optical spectrum transmission systems and methods enable an optical transmission system with mixed baud rate channels without a loss of spectral efficiency. In effect, the bins  102  enable flexibility in the use of the optical spectrum. That is, each channel may be provisioned to use only the spectrum it needs based on the associated modulation format. Advantageously, the concatenated optical spectrum transmission systems and methods provide a fiber optic transmission system which groups channels for the purpose of reducing or eliminating deadbands between channels. The concatenated optical spectrum transmission systems and methods further provides a fiber optic transmission system which allocates spectrum on a predetermined group of bins to create virtual channels with predictable start and end points in the optical spectrum. Furthermore, the concatenated optical spectrum transmission systems and methods allow more efficient use of optical spectrum in an optical mesh like that in the network  90  by minimizing conflicts for spectrum, and by fixing the start and stop frequencies thereby allowing a simple method to find a common set of sub-bins to bind together for a path from source to destination. 
     Referring to  FIG. 8 , in an exemplary embodiment, a spectral diagram  120  illustrates wavelength channel spacing utilizing a bin of channels  102  with a guardband contained within bins  102 - 2 ,  102 - 3  for the concatenated optical spectrum transmission systems and methods. The spectral diagram  120  includes two coherent optical signals  122 ,  124 . The optical signal  122  occupies the bins  102 - 2 ,  102 - 3 ,  102 - 4 ,  102 - 5 ,  102 - 6  and the optical signal  124  occupies the bins  102 - 0 ,  102 - 1 ,  102 - 2 . In this exemplary embodiment, a guardband  126  is shared between the bins  102 - 2 ,  102 - 3 . Specifically, the guardband  126  is contained within the bins  102 - 2 ,  102 - 3 , such that the bins  102 - 2 ,  102 - 3  start and end at nominally the same frequency. The associated coherent receivers receiving the signals  122 ,  124  may be configured as such and tune accordingly. 
     In an exemplary embodiment, the concatenated optical spectrum transmission systems and methods may be implemented between the WSS  40 , the coherent receivers  62 , and a management system. The management system may include, for example, a network management system (NMS), an element management system (EMS), a network controller, a control module or processor in a network element with the coherent receiver  62 , and the like. In particular, the management system may be configured with the plurality of bins  102  and associated optical signals  104 ,  122 ,  124  configured thereon. The management system variously may be utilized for operations, administration, maintenance, and provisioning (OAM&amp;P) of an optical system. In performing such functionality, the management system may be utilized in the concatenated optical spectrum transmission systems and methods to manage the bins  102  and respective optical signals thereon with the WSS  40 , the coherent receivers  62 , etc. 
     Although the present invention has been illustrated and described herein with reference to preferred embodiments and specific examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples may perform similar functions and/or achieve like results. All such equivalent embodiments and examples are within the spirit and scope of the present invention and are intended to be covered by the following claims.