Patent Publication Number: US-2023135594-A1

Title: Clock Recovery for Digital Subcarriers for Optical Networks

Description:
INCORPORATION BY REFERENCE 
     The entirety of the following patents and patent applications are hereby expressly incorporated herein by reference: U.S. Pat. No. 8,831,439, entitled “Upsampling Optical Transmitter”, which issued Sep. 9, 2014; U.S. Pat. No. 10,014,975, entitled “Channel Carrying Multiple Digital Subcarriers”, which issued Jul. 3, 2018; U.S. patent application Ser. No. 16/155,624, entitled “Individually Routable Digital Subcarriers”, which was filed Oct. 9, 2018; U.S. Provisional Patent Application No. 62/627,712, entitled “Independently Routable Digital Subcarriers for Optical Network”, which was filed Feb. 7, 2018, to which the present application claims priority; and Provisional Patent Application No. 62/668,297, entitled “Spectral Efficiency Improvements using Variable Subcarrier Root-Raised Cosine Shaping”, which was filed May 8, 2018, to which the present application claims priority. 
    
    
     FIELD OF THE DISCLOSURE 
     The disclosure generally relates to methods and apparatuses for the generation and use of subcarriers in optical communication systems. More particularly the disclosure relates to such methods and apparatuses that route or direct individual subcarriers to a different destination, wherein the modulation format, data rate, and/or baud rate, as well as the spectral width and frequency spacing between subcarriers, may be tailored for each subcarrier based on receiver characteristics and/or in accordance with the path or route over which a corresponding subcarrier is transmitted. 
     BACKGROUND 
     Communication systems are known in which optical signals, each being modulated to carry data and having a different wavelength, are transmitted from a first location to a second location. The optical signals may be combined on a single fiber and transmitted to a receiving node that includes circuitry to optically separate or demultiplex each signal. Alternatively, coherent detection techniques may be employed to extract the data carried by each optical signal. 
     In such systems, a plurality of transmitters may be provided, such that each transmitter supplies a respective one of the optical signals. Typically, each transmitter includes a laser, modulator, and associated circuitry for controlling the modulator and the laser. As network capacity requirements increase, however, additional transmitters may be provided to supply additional optical signals, but at significantly increased cost. 
     Moreover, communications systems may include multiple nodes, such that selected optical signals may be intended to transmission to certain nodes, and other signals may be intended for reception by one or more other nodes. Accordingly, optical add-drop multiplexers (“OADMs”) may be provided to drop one or more signals at an intended local receiver, for example, while other optical signals are passed by the OADM to one or more downstream nodes. Further, optical signals may be added by the OADM for transmission to one or more nodes in the communication system. Thus, the optical signals may be transmitted over varying distances and through varying numbers of OADMs. In addition, the data and/or baud rate, and or modulation format is preferably tailored to a particular route, as well as the capacity of the intended receiver. 
     Thus, not only may multiple transmitters be required to provide a required number of optical signals to satisfy network capacity needs, but each transmitter may be required to be customized to generate each optical signal with a desired modulation format, data rate, and/or baud rate. 
     SUMMARY 
     Optical communication network systems and methods are disclosed. The problem of requiring multiple transmitters to provide a required number of optical signals to satisfy network capacity needs, and requiring each transmitter to be customized to generate each optical signal with a desired modulation format, data rate, and/or baud rate is addressed through systems and methods for providing subcarriers that may be routed through a network independently of one another. In addition, each subcarrier may have characteristics, such as baud rate, data rate and modulation format, spectral width, and frequency spacings that may be tailored based on the intended receiver for such subcarrier and the particular optical path or route over which the subcarrier is transmitted. 
     Consistent with an aspect of the present disclosure, a transmitter may comprise a digital signal processor that receives data; circuitry that generate a plurality of electrical signals based on the data; a plurality of filters, each of which receiving a corresponding one of the plurality of electrical signals, a plurality of roll-off factors being associated with a respective one of the plurality of filters; a plurality of digital-to-analog converter circuits that receive outputs from the digital signal processor, the outputs being indicative of outputs from the plurality of filters; a laser that supplies light; and a modulator that receives the light and outputs from the digital-to-analog converter circuits, the modulator supplying a plurality of optical subcarriers based on the outputs of the digital-to-analog converter circuits, such that one of the plurality of optical subcarriers has a frequency bandwidth that is wider than remaining ones of the plurality of optical subcarriers, said one of the plurality of optical subcarriers carrying information for clock recovery. 
     The plurality of optical subcarriers may be Nyquist optical subcarriers. 
     The data received by the digital signal processor may be indicative of a plurality of independent data streams. A number of the plurality of optical subcarriers may be greater than the number of the plurality of independent data streams, and wherein two or more of the plurality of optical subcarriers carry a single one of the plurality of independent data streams. The optical subcarriers may have data with different symbol rates, the same or different data rates, variable spacing between the optical subcarriers, and/or the same or different modulation formats. 
     Consistent with an aspect of the present disclosure, a receiver may comprise a plurality of photodiodes that receive optical signals, the optical signals including a plurality of optical subcarriers, one of the plurality of optical subcarriers having a frequency bandwidth that is wider than remaining ones of the plurality of optical subcarriers, said one of the plurality of optical subcarriers carrying information for clock recovery; a plurality of analog-to-digital converter circuits that receive analog signals indicative of outputs from the photodiodes, the analog-to-digital converters circuits outputting digital samples; a digital signal processor that generates a plurality of electrical signals, each of which corresponding to a respective one of the plurality of subcarriers, one of the plurality of electrical signals being associated with said one of the plurality of optical subcarriers; and a clock recovery circuit that receives said one of the plurality of electrical signals, the clock recovery circuit supplying a clock signal such that the outputting of at least one of the digital samples is based on the clock signal. 
     In one implementation, the clock recovery circuit supplying a clock signal such that the outputting at least two of the digital samples is based on the clock signal. 
     Consistent with an aspect of the present disclosure, an optical network system may comprise a transmitter, comprising: a digital signal processor that receives data; circuitry that generate a plurality of electrical signals based on the data; a plurality of filters, each of which receiving a corresponding one of the plurality of electrical signals, a plurality of roll-off factors being associated with a respective one of the plurality of filters; a plurality of digital-to-analog converter circuits that receive outputs from the digital signal processor, the outputs being indicative of outputs from the plurality of filters; a laser that supplies light; and a modulator that receives the light and outputs from the digital-to-analog converter circuits, the modulator supplying a plurality of optical subcarriers based on the outputs of the digital-to-analog converter circuits, such that one of the plurality of optical subcarriers has a frequency bandwidth that is wider than remaining ones of the plurality of optical subcarriers, said one of the plurality of optical subcarriers carrying information for clock recovery; and a receiver, comprising: a plurality of photodiodes that receive optical signals, the optical signals including the plurality of optical subcarriers; a plurality of analog-to-digital converter circuits that receive analog signals indicative of outputs from the photodiodes, the analog-to-digital converters circuits outputting digital samples; a digital signal processor that generates a plurality of electrical signals, each of which corresponding to a respective one of the plurality of subcarriers, one of the plurality of electrical signals being associated with said one of the plurality of optical subcarriers; and a clock recovery circuit that receives said one of the plurality of electrical signals, the clock recovery circuit supplying a clock signal such that the outputting of at least one of the digital samples is based on the clock signal. 
     In one implementation, the optical network system may further comprise, one or more optical add-drop multiplexer (OADM) configured to do one or more of: drop one or more of the optical subcarriers and add one or more optical subcarrier. 
     In one implementation, the clock recovery circuit may be supplying a clock signal such that the outputting at least two of the digital samples is based on the clock signal. 
     In one implementation, the data received by the digital signal processor may be indicative of a plurality of independent data streams, and a number of the plurality of optical subcarriers may be greater than the number of the plurality of independent data streams, and wherein two or more of the plurality of optical subcarriers carry a single one of the plurality of independent data streams. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more implementations described herein and, together with the description, explain these implementations. In the drawings: 
         FIG.  1    illustrates an optical communication system consistent with an aspect of the present disclosure; 
         FIG.  2    is a diagram illustrating an example of components of an optical transmitter shown in  FIG.  1   ; 
         FIG.  3    is a diagram illustrating example components of a transmitter digital signal processor (Tx DSP) shown in  FIG.  2   ; 
         FIG.  4 A  illustrates an exemplary plurality of subcarriers consistent with an aspect of the present disclosure; 
         FIG.  4 B  illustrates another exemplary plurality of subcarriers consistent with an aspect of the present disclosure; 
         FIG.  4 C  illustrates another exemplary plurality of subcarriers consistent with an aspect of the present disclosure; 
         FIG.  4 D  illustrates another exemplary plurality of subcarriers consistent with an aspect of the present disclosure; 
         FIG.  4 E  illustrates another exemplary plurality of subcarriers consistent with an aspect of the present disclosure; 
         FIG.  5 A  is a diagram illustrating a portion of an exemplary process consistent with an aspect of the present disclosure; 
         FIG.  5 B  is a diagram illustrating a portion of another exemplary process consistent with an aspect of the present disclosure; 
         FIG.  5 C  is a diagram illustrating exemplary variable-spaced subcarriers consistent with an aspect of the present disclosure; 
         FIG.  5 D  is a diagram illustrating exemplary variable-spaced subcarriers consistent with an aspect of the present disclosure; 
         FIG.  5 E  is a diagram illustrating an exemplary subcarrier consistent with an aspect of the present disclosure; 
         FIG.  6    is a diagram illustrating an example of components of an optical receiver shown in  FIG.  1    consistent with an aspect of the present disclosure; 
         FIG.  7    is a diagram illustrating example components of an exemplary receiver digital signal processor (Rx DSP), such as that shown in  FIG.  6   , consistent with an aspect of the present disclosure; 
         FIG.  8 A  is an illustration of a use case example of subcarriers having fixed subcarrier width and variable capacity per subcarrier consistent with an aspect of the present disclosure; 
         FIG.  8 B  is an illustration of another use case example of subcarriers having fixed subcarrier width and variable capacity per subcarrier consistent with an aspect of the present disclosure; 
         FIG.  8 C  is an illustration of another use case example of subcarriers having fixed subcarrier width and variable capacity per subcarrier consistent with an aspect of the present disclosure; 
         FIG.  8 D  is an illustration of another use case example of subcarriers having fixed subcarrier width and variable capacity per subcarrier consistent with an aspect of the present disclosure; 
         FIG.  8 E  is an illustration of another use case example of subcarriers having fixed subcarrier width and variable capacity per subcarrier consistent with an aspect of the present disclosure; 
         FIG.  8 F  is an illustration of another use case example of subcarriers having fixed subcarrier width and variable capacity per subcarrier consistent with an aspect of the present disclosure; 
         FIG.  8 G  is an illustration of another use case example of subcarriers having fixed subcarrier width and variable capacity per subcarrier consistent with an aspect of the present disclosure; 
         FIG.  9 A  is an illustration of a use case example of subcarriers having fixed capacity and variable subcarrier width consistent with an aspect of the present disclosure; 
         FIG.  9 B  is an illustration of another use case example of subcarriers having fixed capacity and variable subcarrier width consistent with an aspect of the present disclosure; 
         FIG.  9 C  is an illustration of another use case example of subcarriers having fixed capacity and variable subcarrier width consistent with an aspect of the present disclosure; 
         FIG.  9 D  is an illustration of another use case example of subcarriers having fixed capacity and variable subcarrier width consistent with an aspect of the present disclosure; 
         FIG.  10    illustrates an exemplary mesh network configuration consistent with a further aspect of the present disclosure; 
         FIG.  11 A  illustrates an exemplary ring network configuration consistent with a further aspect of the present disclosure; 
         FIG.  11 B  illustrates exemplary components of a node of the network of  FIG.  11 A  consistent with a further aspect of the present disclosure; 
         FIG.  12 A  illustrates an exemplary network configuration consistent with a further aspect of the present disclosure; 
         FIG.  12 B  illustrates another exemplary network configuration consistent with a further aspect of the present disclosure; 
         FIG.  13    illustrates an exemplary ring and hub network configuration consistent with a further aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. 
     The mechanisms proposed in this disclosure circumvent the problems described above. The present disclosure describes a system to route or direct individual subcarriers to a different destination, wherein the modulation format, data rate, and/or baud rate, as well as the spectral width and frequency spacing between subcarriers, may be tailored for each subcarrier based on receiver characteristics and/or in accordance with the path or route over which a corresponding subcarrier is transmitted. 
     Definitions 
     If used throughout the description and the drawings, the following short terms have the following meanings unless otherwise stated: 
     ADC stands for analog-to-digital converter. 
     DAC stands for digital-to-analog converter. 
     DSP stands for digital signal processor. 
     OADM stands for optical add-drop multiplexer. 
     PIC stands for photonic integrated circuit. 
     Rx (or RX) stands for Receiver, which typically refers to optical channel receivers, but can also refer to circuit receivers. 
     Tx (or TX) stands for Transmitter, which typically refers to optical channel transmitters, but can also refer to circuit transmitters. 
     DESCRIPTION 
     As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by anyone of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
     In addition, use of the “a” or “an” are employed to describe elements and components of the embodiments herein. This is done merely for convenience and to give a general sense of the inventive concept. This description should be read to include one or more and the singular also includes the plural unless it is obvious that it is meant otherwise. 
     Further, use of the term “plurality” is meant to convey “more than one” unless expressly stated to the contrary. 
     Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise. 
     Also, certain portions of the implementations have been described as “components” or “circuitry” that perform one or more functions. The term “component” or “circuitry” may include hardware, such as a processor, an application specific integrated circuit (ASIC), or a field programmable gate array (FPGA), or a combination of hardware and software. Software includes one or more computer executable instructions that when executed by one or more component cause the component or circuitry to perform a specified function. It should be understood that the algorithms described herein are stored on one or more non-transient memory. Exemplary non-transient memory includes random access memory, read only memory, flash memory or the like. Such non-transient memory can be electrically based or optically based. Further, the messages described herein may be generated by the components and result in various physical transformations. 
     Finally, as used herein any reference to “one embodiment” or “an embodiment” or “implementation: means that a particular element, feature, structure, or characteristic described in connection with the embodiment or implementation is included in at least one embodiment or implementation. The appearances of the phrase “in one embodiment” or “in one implementation” in various places in the specification are not necessarily all referring to the same embodiment or implementation. 
     In accordance with the present disclosure, messages or data transmitted between nodes may be processed by circuitry within the input interface(s), and/or the output interface(s) and/or the control module. Circuitry could be analog and/or digital, components, or one or more suitably programmed microprocessors and associated hardware and software, or hardwired logic. Also, certain portions of the implementations have been described as “components” that perform one or more functions. The term “component,” may include hardware, such as a processor, an application specific integrated circuit (ASIC), or a field programmable gate array (FPGA), or a combination of hardware and software. Software includes one or more computer executable instructions that when executed by one or more component cause the component to perform a specified function. It should be understood that the algorithms described herein are stored on one or more non-transient memory. Exemplary non-transient memory includes random access memory, read only memory, flash memory or the like. Such non-transient memory can be electrically based or optically based. Further, the messages described herein may be generated by the components and result in various physical transformations. 
     Consistent with an aspect of the present disclosure, electrical signals or digital subcarriers are generated in a Digital Signal Processor based on independent input data streams. Drive signals are generated based on the digital subcarriers, and such drive signals are applied to an optical modulator, including, for example, a Mach-Zehnder modulator. The optical modulator modulates light output from a laser based on the drive signals to supply optical subcarriers, each of which corresponding to a respective digital subcarrier. Each of the optical subcarriers may be routed separately through a network and received by optical receivers provided at different locations in an optical communications network, where at least one of the optical subcarriers may be processed, and the input data stream associated with such optical subcarrier(s) is output. 
     Accordingly, instead of providing multiple transmitters, each being associated with a respective optical signal, one transmitter, having, in one example, a laser, may be provided that supplies multiple subcarriers, one or more of which carries data that may be independently routable to a corresponding receiver provided at a unique location. Thus, since fewer transmitters are required consistent with the present disclosure, system costs may be reduced. 
     Since the subcarriers may be transmitted over different transmission paths to receivers having different capacities or other properties, characteristics of each subcarriers may be tuned or adjusted to provide optimal performance. For example, the modulation format, data rate, and/or baud rate may be selected for a given subcarrier based on a particular path through the network and capacity or bandwidth of the intended receiver. These parameters may be selected to be different for other subcarriers that are transmitted over different paths to different receivers in the network. In one example, the modulation format may be selected from binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and m-quadrature amplitude modulation (m-QAM, where m is an integer). 
     Consistent with a further aspect of the present disclosure, efficient clock or phase recovery of a received signals may be made more efficient by sensing data or information associated with one subcarrier having a spectral width that is wider than other subcarriers associated with a particular carrier. 
       FIG.  1    is a diagram of a simplified view of an optical network  200  in which systems and/or methods described herein may be implemented. In one example, optical network  200  may constitute part of a larger network including multiple nodes arranged as a mesh or a ring, as discussed in greater detail below. As illustrated in  FIG.  1   , the optical network  200  may include a transmitter (Tx) module  210 , and/or a receiver (Rx) module  220 . In some implementations, the transmitter module  210  may be optically connected to the receiver module  220  via one or more link  230 . Additionally, the link  230  may include one or more optical amplifiers  240  that amplify an optical signal as the optical signal is transmitted over the link  230 . 
     The transmitter module  210  may include a number of optical transmitters  212 - 1  through  212 -M (where M is greater than or equal to one), waveguides  214 , and/or optical multiplexers  216 . In some implementations, the transmitter module  210  may include additional components, fewer components, different components, or differently arranged components. 
     Each optical transmitter  212  may receive data for one or more data inputs  352 , each of which may include a plurality of client data streams  352 - 1  to  352 - 4  discussed in greater detail below with reference to  FIG.  2   . Based on a respective data input, each transmitter provides a carrier and an associated plurality of subcarriers. The carrier has a wavelength equal to or substantially equal to the wavelength of continuous wave (CW) light output from a laser (see  FIG.  2   ) and each subcarrier may have a frequency or wavelength that is different than the carrier wavelength The transmitter  212  is described in greater detail below in relation to  FIG.  2   . 
     Remaining now with  FIG.  1   , in one implementation, the transmitter module  210  may include 5, 10, or some other quantity of the optical transmitters  212 . In one example, the carrier wavelength of the optical signals supplied by each transmitter  212  may be tuned to conform to a wavelength grid, such as a standard grid published by the Telecommunication Standardization Sector (ITU-T). The carrier wavelengths may also be tuned to conform to a flexible grid in which the spacing between the carrier wavelengths is non-uniform. Moreover, the carrier wavelengths may be tuned to be more tightly packed spectrally to create a super channel. 
     The waveguides  214  may include an optical link or some other link to transmit output optical signals (each including a carrier and a plurality of subcarriers) of the optical transmitters  212 . In some implementations, each optical transmitter  212  may include one waveguide  214 , or multiple waveguides  214 , to transmit output optical signals of the optical transmitters  212  to the optical multiplexer  216 . 
     The optical multiplexer  216  may include a power combiner, an arrayed waveguide grating (AWG) or some other multiplexer device. In some implementations, the optical multiplexer  216  may combine multiple output optical signals, associated with the optical transmitters  212 , into a single optical signal (e.g., a WDM signal). In some implementations, the optical multiplexer  216  may combine multiple output optical signals, associated with the optical transmitters  212 , in such a way as to combine polarization multiplexed signals (e.g., also referred to herein as a WDM signal). A corresponding waveguide may output the WDM signal on an optical fiber, such as the link  230 . The optical multiplexer  216  may include waveguides connected to an input and/or an output. 
     As further shown in  FIG.  1   , the optical multiplexer  216  may receive output optical signals outputted by the optical transmitters  212 , and output one or more WDM signals. Each WDM signal may include one or more optical signals, such that each optical signal includes one or more wavelengths. In some implementations, each optical signal in the WDM signal may have a first polarization (e.g., a transverse magnetic (TM) polarization), and a second, substantially orthogonal polarization (e.g., a transverse electric (TE) polarization). Alternatively, each optical signal may have one polarization. 
     The link  230  may comprise an optical fiber. The link  230  may transport one or more optical signals. The amplifier  240  may include one or more amplification device, such as a doped fiber amplifier and/or a Raman amplifier. The amplifier  240  may amplify the optical signals as the optical signals are transmitted via the link  230 . 
     In addition, one or more OADMs  229  may be provided along the fiber link  230 . The OADMs  229  may be configured to add or drop one or more optical subcarriers included in the optical signals output from each transmitters. For example, as further shown in  FIG.  1   , subcarrier SC 1  of a first optical signal may be added and/or dropped (as indicated by the arrows shown in  FIG.  1   ) at OADM  229 - 1 , and subcarrier SC 2  of another optical signal may be added and/or dropped (as shown by the arrows in  FIG.  1   ) at OADM  229 - 2 . 
     The receiver module  220  may include optical demultiplexer  222 , waveguides  224 , and/or optical receivers  226 - 1  through  226 -N (where N is greater than or equal to one). In some implementations, the receiver module  220  may include additional components, fewer components, different components, or differently arranged components. 
     The optical demultiplexer  222  may include an AWG, a power splitter, or some other demultiplexer device. The optical demultiplexer  222  may supply multiple optical signals based on receiving one or more optical signals, such as WDM signals, or components associated with the one or more optical signals. Additionally, the optical demultiplexer  222  may include waveguides  224 . 
     The waveguides  224  may include an optical link or some other link to transmit optical signals, output from the optical demultiplexer  222 , to the optical receivers  226 . In some implementations, each optical receiver  226  may receive optical signals via a single waveguide  224  or via multiple waveguides  224 . 
     As discussed in greater detail below, the optical receivers  226  may each include one or more photodetectors and related devices to receive respective input optical signals outputted by the optical demultiplexer  222 , detect the subcarriers associated with the input optical signals, convert the subcarriers to voltage signals, convert the voltage signals to digital samples, and process the digital samples to produce output data corresponding to the one or more data streams, such as the input client data streams  352 - 1  to  352 - 4  associated with input data  352  provided to transmitter  212 - 1 , for example. In some implementations, each of the optical receivers  226  may include a local oscillator, a hybrid mixer, a detector, an ADC, an RX DSP, and/or some other components, as described in greater detail below in relation to  FIG.  6   . 
     While  FIG.  1    shows the optical network  200  as including a particular quantity and arrangement of components, in some implementations, the optical network  200  may include additional components, fewer components, different components, or differently arranged components. Also, in some instances, one of the devices illustrated in  FIG.  1    may perform a function described herein as being performed by another one of the devices illustrated in  FIG.  1   . 
       FIG.  2    is a diagram illustrating an example of components of the optical transmitter  212  in greater detail. As shown in  FIG.  2   , the optical transmitter  212  may include a TX DSP  310 , two digital-to-analog converters (DACs)  320 - 1  and  320 - 2  (referred to generally as DACs  320  and individually as DAC  320 ), a laser  330 , modulators  340 - 1  and  340 - 2  (referred to generally as modulators  340  and individually as modulator  340 ), and a splitter  350 . 
     In some implementations, the TX DSP  310  and the DAC  320  may be implemented using an application specific integrated circuit (ASIC) and/or may be implemented on a single integrated circuit, such as a single PIC. In some implementations, the laser  330  and the modulator  340  may be implemented on a single integrated circuit, such as a single photonic integrated circuit (PIC). In some other implementations, the TX DSP  310 , the DAC  320 , the laser  330 , and/or the modulator  340  may be implemented on one or more integrated circuits, such as one or more PICs. For example, in some example implementations, components of multiple optical transmitters  212  may be implemented on a single integrated circuit, such as a single PIC, to form a super-channel transmitter. 
     The TX DSP  310  may comprise a digital signal processor. The TX DSP  310  may receive input data from multiple data sources, each of which supplying a respective one of the plurality of Client Data Streams  352 - 1  through  352 - 4 . In general, “N” number of Client Data Streams  3521  to  352 -N can be used. For explanatory purposes, four Client Data Streams  352  (N=4) are used in relation to  FIG.  2   . The TX DSP  310  may determine the signal to apply to the modulator  340  to generate multiple optical subcarriers. Digital subcarriers may comprise electronic signals generated in the TX DSP  310  that correspond to respective optical subcarriers. 
     In some implementations, the TX DSP  310  may receive streams of data (such as one or more of the Client Data Streams  352 - 1  to  352 - 4 ), map the streams of data into each of the digital subcarriers, independently apply spectral shaping to each of the digital subcarriers, and obtain, based on the spectral shaping of each of the digital subcarriers, a sequence of values to supply to the DAC  320 . In some implementations, the TX DSP  310  may generate the digital subcarriers using time domain filtering and frequency shifting by multiplication in the time domain. The TX DSP  310  will be further described in relation to  FIG.  3   . 
     The DAC  320  may comprise a digital-to-analog converter. The DAC  320  may receive the sequence of values and, based on the sequence of values, generate the analog or voltage signals to apply to the modulator  340 . 
     The laser  330  may include a semiconductor laser, such as a distributed feedback (DFB) laser, or some other type of laser. The laser  330  may provide an output optical light beam to the modulator  340 . 
     The modulator  340  may include a Mach-Zehnder modulator (MZM), such as a nested MZM, or another type of modulator. The modulator  340  may receive the optical light beam from the laser  330  and the voltage signals from the DAC  320 , and may modulate the optical light beam, based on the voltage signals, to generate a multiple subcarrier output signal(s), such as Output TE Signal  342 - 1  and Output TM Signal  342 - 2 . 
     The splitter  350  may include an optical splitter that receives the optical light beam from the laser  330  and splits the optical light beam into two branches: one for the first polarization and one for the second polarization. In some implementations, the two optical light beams may have approximately equal power. The splitter  350  may output one optical light beam to modulator  340  including first and second modulators  340 - 1  and  340 - 2 , each of which may include a Mach-Zehnder modulator. 
     The modulator  340 - 1  may be used to modulate signals of the first polarization. The modulator  340 - 2  may be used to modulate signals of the second polarization. 
     In some implementations, one or more subcarrier may be modulated by the modulator  340  to carry data at different rates (see  FIG.  4 A  illustrating exemplary subcarriers). For example, a first subcarrier SC 1  may carry data at a first rate and subcarrier SC 2  may carry data at a different rate that is higher or lower than the first rate. In addition, one or more subcarrier may be modulated by the modulator  340  to carry data with different baud rates (see  FIG.  4 A  illustrating exemplary subcarriers). For example, the first subcarrier SC 1  may carry data at or have an associated a first baud rate and the second subcarrier SC 2  may carry data at or have an associated second baud rate that is higher or lower (different) than the first baud rate. 
     In some implementations, a first one of a plurality of subcarriers SC 1  may be modulated in accordance with a first modulation format and a second one of the plurality of subcarriers SC 2  may be modulated in accordance with a second modulation format different than the first modulation format (see  FIG.  4 A  illustrating exemplary subcarriers). In one implementation, the first modulation format may be one of BPSK, QPSK, and m-QAM, where m is an integer, and the second modulation format may be another one of BPSK, QPSK, and m-QAM. In one implementation, the first modulation format may be one of BPSK, QPSK, and m-QAM, where m is an integer, and the second modulation format may be an intensity modulation format. 
     In some implementations, a plurality of the subcarriers may have a variety of combinations of modulation and data rates configured by the transmitter and/or by a plurality of transmitters  212 . The particular combination of modulation and data rates of the subcarriers may be configured based on the desired distance of transmission, desired error rate, desired data rate, and/or other requirements and/or restrictions for the optical network  200  and/or the end client. 
     In some implementations, two DACs  320  may be associated with each polarization. In these implementations, two DACs  320 - 1  may supply voltage signals to the modulator  340 - 1 , and two DACs  320 - 2  may supply voltage signals to the modulator  340 - 2 . In some implementations, the outputs of the modulators  340  may be combined back together using combiners (e.g., optical multiplexer  216 ) and polarization multiplexing. 
     While  FIG.  2    shows the optical transmitter  212  as including a particular quantity and arrangement of components, in some implementations, the optical transmitter  212  may include additional components, fewer components, different components, or differently arranged components. The quantity of DACs  320 , lasers  330 , and/or modulators  340  may be selected to implement an optical transmitter  212  that is capable of generating polarization diverse signals for transmission on an optical fiber, such as the link  230 . In some instances, one of the components illustrated in  FIG.  2    may perform a function described herein as being performed by another one of the components illustrated in  FIG.  2   . 
       FIG.  3    shows an example of the digital signal processor (TX DSP)  310  of the transmitter  212  in greater detail. In this example, four of the Client Data Streams  352  are shown. The digital signal processor  310  may include FEC encoders  405 - 1  to  405 - 4  (referred to generally as FEC encoders  405  and individually as FEC encoder  405 ), input bits components  420 - 1  to  420 - 4  (referred to generally as input bits components  420  and individually as input bits component  420 ), four bits-to-symbol components  430 - 1  to  430 - 4  (referred to generally as bits-to-symbol components  430  and individually as bits-to-symbol component  430 ), four overlap-and-save buffers  256   440 - 1  to  440 - 4  (referred to generally as overlap-and-save buffers  440  and individually as overlap-and-save buffer  440 ), four fast Fourier transform functions (FFT) 256 components  450 - 1  to  450 - 4  (referred to generally as FFT components  450  and individually as FFT component  450 ), four replicator components  460 - 1  (referred to generally as replicator components  460  and individually as replicator component  460 ), four pulse shape filters  470  (referred to generally as pulse shape filters  470  and individually as pulse shape filter  470 ), an inverse FFT (IFFT)  2048  component  490 , and a take last 1024 component  495 . Optionally, the TX DSP  310  may further comprise one or more zero-bit-insertion-block circuitry components  475  (referred to generally as zero-bit-insertion-block circuitry components  475  and individually as zero-bit-insertion-block circuitry component  475 ), and a memory  2048  array  480 . Optionally, the TX DSP  310  may further comprise four zero-bit-insertion-block circuitry components  475  (referred to generally as zero-bit-insertion-block circuitry components  475  and individually as zero-bit-insertion-block circuitry component  475 ), and a memory  2048  array  480 . 
     For each of the Client Data Streams  352 , the digital signal processor (TX DSP)  310  of the transmitter  301  may contain one each of the FEC encoders  405 , the input bits components  420 , the bits-to-symbol components  430 , the overlap-and-save buffers  440 , the fast Fourier transform functions (FFT) components  450 , the replicator components  460 , the pulse shape filters  470 , and the zero-bit-insertion-block circuitry components  475 . 
     Each of the FEC encoders  405 - 1  to  405 - 4  may receive a particular one of the plurality of independent input data streams of bits (illustrated as exemplary Client Data Streams  352 - 1  to  352 - 4 ) from a respective one of a plurality of data sources and perform error correction coding on a corresponding one of the input Client Data Streams  352 , such as through the addition of parity bits. The FEC encoders  405 - 1  to  405 - 4  may be designed to generate timing skew between the subcarriers to correct for skew induced by link(s) between the transmitter module  210  and the receiver module  220  in the optical network  200 . 
     Input bits component  420  may process, for example, 128*X bits at a time, where X is an integer. For dual-polarization Quadrature Phase Shift Keying (QPSK), X is four. For higher modulation formats, X may be more than four. For example, for an 8-quadrature amplitude modulation (QAM) format, X may be eight and for a 16 QAM modulation format, X may be sixteen. Accordingly, for such 8 QAM modulation, eight FEC encoders  405  may be provided, each of which may encode a respective one of eight independent input data streams (e.g., eight of the Client Data Streams  352 ) for a corresponding one of eight digital subcarriers corresponding to eight optical subcarriers. Likewise, for 16 QAM modulation, sixteen FEC encoders  405  may be provided, each of which may encode a respective one of sixteen independent input data streams (e.g., sixteen of the Client Data Streams  352 ) for a corresponding one of sixteen subcarriers corresponding to sixteen optical subcarriers. 
     The bits-to-symbol component  430  may map the bits to symbols on the complex plane. For example, the bits-to-symbol components  430  may map four bits or other numbers of bits to a symbol in the dual-polarization QPSK constellation or other modulation format constellation. Accordingly, each of the components or circuits  430  may define or determine the modulation format for a corresponding subcarrier. In addition, components or circuits  405 ,  420 , and  430  may define or determine the baud rate and or data rate for each subcarrier. Therefore, the modulation format, baud rate and data rate may be selected for each subcarrier by these circuits. For example, control inputs may be provided to these circuits so that the desired modulation format, baud rate and data rate may be selected. 
     The overlap-and-save buffer  440  may buffer 256 symbols, in one example. The overlap-and-save buffer  440  may receive 128 symbols at a time from the bits-to-symbol component  430 . Thus, the overlap-and-save buffer  440  may combine 128 new symbols, from the bits-to-symbol component  430 , with the previous 128 symbols received from the bits-to-symbol component  430 . 
     The FFT component  450  may receive 256 symbols from the overlap-and-save buffer  440  and convert the symbols to the frequency domain using, for example, a fast Fourier transform (FFT). The FFT component  450  may form 256 frequency bins, for example, as a result of performing the FFT. Components  440  and  450  may carry out the FFT for each subcarrier based on one sample per symbol (per baud) to thereby convert time domain or data symbols received by FFT component  550  into frequency domain data for further spectral shaping (requiring more than one sample/baud or symbol) by filters  470 . 
     The replicator component  460  may replicate the 256 frequency bins, in this example, or registers to form 512 frequency bins (e.g., for T/2 based filtering of the subcarrier). This replication may increase the sample rate. 
     The pulse shape filter  470  may apply a pulse shaping filter to the data stored in the 512 frequency bins to thereby provide the digital subcarriers with desired spectral shapes and such filtered subcarriers are multiplexed and subject to the inverse FFT  490 , as described below. The pulse shape filter  470  may calculate the transitions between the symbols and the desired spectrum so that the subcarriers can be packed together on the channel. The pulse shape filter  470  may also be used to introduce timing skew between the subcarriers to correct for timing skew induced by links between nodes in the optical network  200 . The pulse shape filters  470  may be raised cosine filters. 
     The pulse shape filter  470  may have a variable bandwidth. In some implementations, the bandwidth of the subcarriers may be determined by the width of the pulse shape filters  470 . The pulse shape filters  470  may manipulate the digital signals of the subcarriers or digital subcarriers to provide such digital subcarriers with an associated spectral width. In addition, as generally understood, the pulse shape filter  470  may have an associated “roll-off” factor (α). Consistent with the present disclosure, however, such “roll-off” may be adjustable or changed in response to different control inputs to the pulse shape filter  470 . Such variable roll-off results in the pulse shape filter  470  having a variable or tunable bandwidth, such that each subcarrier may have a different spectral width. In a further example, one of the subcarriers may have an associated spectral width that is wider than the remaining subcarriers. It is understood that the control inputs may be any appropriate signal, information, or data that is supplied to the pulse shape filter  470 , such that the “roll-off” is changed in response to such signal, information, or data. 
     The four zero-bit-insertion-block circuitry components  475  may comprise circuitry to receive the four digital subcarriers from the four pulse shape filters  470  and may output zeros or other bits in bits between a block of data bits of a first subcarrier and a block of data bits of a second subcarrier to the memory array  480  in order to adjust the frequency spacing or gap between the optical subcarriers, as discussed in greater detail below. 
     The memory array  480  may receive all four of the subcarriers from the zero-bit-insertion-block circuitry components  475  and the zeros from the four zero-bit-insertion-block circuitry components  475 . The memory array  480  may store the outputs of the subcarriers and output an array of the four subcarriers and the zeros from the four zero-bit-insertion-block circuitry components  475  to the IFFT component  490 . 
     The IFFT component  490  may receive the 2048 element vector and return the signal back to the time domain, which may now be at 64 GSample/s. The IFFT component  490  may convert the signal to the time domain using, for example, an inverse fast Fourier transform (IFFT). 
     The take last 1024 component  495  may select the last 1024 samples, for example, from IFFT component  490  and output the 1024 samples to the DACs  320  of the transmitter  212  (such as at 64 GSample/s, for example). 
     While  FIG.  3    shows the TX DSP  310  as including a particular quantity and arrangement of functional components, in some implementations, the TX DSP  310  may include additional functional components, fewer functional components, different functional components, or differently arranged functional components. 
     Returning now to  FIG.  2   , as previously described, the DACs  320  may convert the received samples from the take last component  495  of the TX DSP  310 . The modulator  340  may receive the optical light beam from the laser  330  and the voltage signals from the DAC  320 , and may modulate the optical light beam or CW light from laser  330 , based on the voltage signals, to generate a multiple subcarrier output signal, such as Output TE Signal  342 - 1  and Output TM Signal  342 - 2 . 
       FIGS.  4 A- 4 E  illustrate examples of subcarriers SC 1  to SC 4  that may be output from the transmitter  212  (similar subcarriers may be output from transmitters in transceivers located at other nodes). In one example, the subcarriers SC 1  to SC 4  may not spectrally overlap with one another and may be, for example, Nyquist subcarriers, which may have a frequency spacing equal to or slightly larger than the individual subcarrier baud-rate. 
     As illustrated in  FIG.  4 A , the subcarriers may have one or more spectra or bandwidths, such as, for example, S 3  (subcarrier SC 3 ) and S 4  (subcarrier SC 4 ) above frequency f 0 , which may correspond to a center frequency (f 0 ) of the laser  330  of the transmitter  212 . In addition, the subcarriers may have one or more spectra or bandwidths, such as for S 1  (subcarrier SC 1 ) and S 2  (subcarrier SC 2 ) below frequency f 0 . 
     In one example, the number of subcarriers equals a number of the independent input Client Data Streams  352 . For example, Client Data Streams  352 - 1  to  352 - 4  from  FIG.  3    may be four independent input data streams corresponding to four subcarriers SC 1  to SC 4  in  FIG.  4 A , such that each of the subcarriers carries data or information associated with a respective one of the Client Data Streams. In one example, the number of subcarriers is more than the number of the independent input Client Data Streams  352 . For example, Client Data Streams  352 - 1  to  352 - 3  from  FIG.  3    may be three independent input data streams mapped to four subcarriers SC 1  to SC 4 . Two or more of the subcarriers, such as the subcarriers SC 3  and SC 4 , may carry one of the Client Data Streams  352 - 3 . Such an arrangement allows for more data capacity to be dedicated to a particular Client Data Stream  352 . 
     Frequency bandwidth and roll-off of the subcarriers may be determined by appropriate input to the pulse shape filters  470 . The laser frequency (f 0 ) may be centrally positioned within the frequency (f) of the filters&#39; bandwidth. As illustrated in  FIG.  4 A , the filter bandwidths for each of the four pulse shape filters  470  may be the same, for example, and may all have the same roll-off factor (for example, α=0.3), producing four subcarriers SC 1 -SC 4  each having the same bandwidth. As illustrated in  FIG.  4 B , the filter bandwidths for each of the four pulse shape filters  470  may be the same, for example, and may all have the same roll-off factor (for example, α=0.7) differing from the roll-off factor of the example of  FIG.  4 A , producing four subcarriers SC 1 -SC 4  each having the same bandwidth, but with a larger bandwidth than subcarriers produced by a pulse shape filter  470  having a smaller roll-off factor. Thus, the roll-off factor for each of filters  470  may be controlled or adjusted so that corresponding optical subcarriers have different spectral widths, as noted above. 
       FIG.  4 C  illustrates another example in which the bandwidth of the four subcarriers is the same. In the example of  FIG.  4 C , the baud rate of each subcarrier is 8.039, and the shaping factor is 1/16 (6.25%), making the total width of each subcarrier 8.039*(1+ 1/16)=8.54 GHz. In some implementations, the roll-off factor can be assigned to be very narrow for the majority of the subcarriers, while one subcarrier is given a wider roll-off factor. This allows for channel spacing to be tighter than would be possible with conventional shaping and clock recovery. That is, since the majority of subcarriers in this example, have a narrow bandwidth, more subcarriers can be accommodated within a given amount of spectrum, and, therefore, provide greater data carrying capacity for a given link. Clock and/or phase recovery based on the wider subcarrier is discussed in greater detail below. 
     As illustrated in  FIGS.  4 D and  4 E , in some implementations, the filter bandwidths for one or more of the four pulse shape filters  470  may be different that the filter bandwidths of one or more of the other pulse shape filters  470 , thereby resulting in one or more of the subcarriers having a different bandwidth than one or more of the other subcarriers. Additionally or alternately, one or more of the four pulse shape filters  470  may have a different roll-off factor (α) than one or more of the other pulse shape filters  470 , thereby resulting in one or more subcarriers having a different bandwidth than the other subcarriers. For example, a first, third, and fourth of the pulse shape filters  470 - 1 ,  470 - 3 ,  470 - 4  may have a first roll-off factor (such as α=0.3) while a second of the pulse shape filters  470 - 2  may have a roll-off factor different than the other three (such as α=0.7), such that the first, third, and fourth subcarriers SC 1 , SC 3 , and SC 4 , have a first bandwidth and the second subcarrier SC 2  has a second bandwidth different than the bandwidths of the other subcarriers. In some implementations, the subcarrier with the larger bandwidth than the other subcarriers may be used to carry clock-recovery information for a plurality of the subcarriers, as will be described in relation to  FIG.  7   . 
     In the example shown in  FIG.  4 E , the first subcarrier SC 1  is shaped with 1.5% roll-off factor, for example; the second subcarrier SC 2  is shaped with 6.25% roll-off factor, for example; and the third subcarrier SC 3  and the fourth subcarrier SC 4  are shaped with 1.5% roll-off factor, for example. A 1.5% roll-off factor on 8.039 GBaud maps to 8.16 GHz. In this example, total spectral width is reduced by 800 MHz in comparison to the example illustrated in  FIG.  4 D . 
     Referring now to  FIG.  3    and  FIGS.  5 A- 5 D , in some implementations, the subcarriers may have gaps, or spacing, between the subcarriers created by the zero-bit-insertion-block circuitry components  475 . The zero-bit-insertion-block circuitry components  475  may insert zeros or other bits within certain locations between the data from a first subcarrier and the data associated with one or more second subcarriers into the memory array  480 , which may result in one or more frequency gaps between the optical subcarriers of varying or constant width, as described below. 
     Varying or controlling the frequency gap will next be described in greater detail with reference to  FIG.  5 A , which illustrates memory locations 0.2048 included in memory array  480 . The memory array  480  may include, in one example, an array of such memory locations, whereby selected locations store complex numbers output from filters  470 , as well as, in one example, 0 bits. Such complex numbers constitute filtered frequency domain data associated with each subcarrier. These numbers may then be output to IFFT component  490 , which, in turn supplies a time domain signal, and based, on such time domain signal, analog signals are generated for driving modulators  340  to output the optical subcarriers. Thus, by selecting memory locations that store 0 bits and other locations that store the frequency domain data, the inputs to IFFT component  490  may be set to result in particular frequency assignments and spacings of the optical subcarriers. 
     In the example shown in  FIG.  5 A , filters  470 - 1  to  470 - 4  output frequency domain data to location groupings L 1  to L 4 , respectively in memory  480 . Each of memory location groupings L- 1  to L- 4  may store such frequency domain data as complex numbers, and each such complex number may be stored in a respective location in each grouping. In one example, each of memory location groupings L- 1  to L- 4  may have 256 locations, each of which storing a respective one of 256 complex numbers. In addition, zero-bit-insertion-block circuitry components  475  may provide zero bits or other numbers to location groupings Z 1  to Z 4 , respectively, in memory  480 . Memory location groupings Z 1  to Z 5  including those memory remaining locations in memory  480  other than the locations included in locations L 1  to L 4 . When the resulting combination of numbers in location groupings L 1  to L 4  and the zero bits stored in locations Z 1  to Z 5  of memory  480  are output to the IFFT component  490 , the IFFT component  490  outputs time domain signals, in digital form, that result in optical subcarriers SC 1  to SC 4  having frequencies f 1  to f 4 , respectively, as shown in  FIG.  5 A , and associated frequency gaps G 1 - 1  to G 1 - 3 , as further shown in  FIG.  5 A . 
     As further shown in  FIG.  5   a   , the frequency domain data stored in locations L- 1  is associated with and corresponds to data carried by subcarrier SC 3 ; the frequency domain data stored in locations L- 2  is associated with and corresponds to data carried by subcarrier SC 4 ; the frequency domain data stored in locations L- 3  is associated with and corresponds to data carried by subcarrier SC 1 ; and the frequency domain data stored in locations L- 1  is associated with and corresponds to data carried by subcarrier SC 4 . 
     Similarly, as shown in  FIG.  5 B , the filters  570 - 1  to  570 - 4  output frequency domain data to location groupings L 2 - 1  to L 2 - 4 , respectively in the memory  480 . In addition, zero-bit-insertion-block circuitry components  475  may provide zero bits to location groupings Z 2 - 1  to Z 2 - 4 , respectively, in the memory  480 . When the resulting combination of numbers stored in location groupings L 1  to L 4  and the zero bits stored in locations Z 1  to Z 5  of memory  480  are output to the IFFT component  490 , the IFFT component  490  outputs time domain signals, in digital form, that result in optical subcarriers SC 1  to SC 4  having frequencies f 1 ′ to f 4 ′, respectively in  FIG.  5 B , and associated frequency gaps G 2 - 1  to G 2 - 3 , as further shown in  FIG.  5 B . Frequencies f 1 ′ to f 4 ′ may differ from frequencies f 1  to f 4 , and frequency gaps G 2 - 1  to G 2 - 3  may differ from frequency gaps G 1  to G 3 . Thus, based on the locations frequency domain data and the zero bit data the gaps and frequencies of the subcarriers can be controlled or adjusted, such that different locations in which the frequency domain and zero bit data are stored can result in different subcarrier frequencies and gaps. 
       FIGS.  5 C and  5 D  illustrate further examples of subcarriers having variable spacing between the subcarriers and varying combinations of spacing between groups of subcarriers. For example, in  FIG.  5 C , a first group of subcarriers SC 1 -SC 4  in a first carrier C 1  (such as from a first transmitter  212 ) are routed together, with a gap G 2  between the first carrier C 1  and a second carrier C 2  (such as from a second transmitter  212 ) having a second group of subcarriers SC 1 -SC 4 . Additionally,  FIG.  5 C  illustrates another pattern of carriers Cn−1, Cn, Cn+1, in which a pair of subcarriers SC 2 , SC 4  from a first carrier Cn- 1  are routed with a pair of subcarriers SC 3 , SC 1  from a second subcarrier Cn; while a pair of subcarriers SC 2 , SC 4  from the second carrier Cn are routed with a pair of subcarriers SC 3 , SC 1  from a third subcarrier Cn+1; with a gap G 2  between the first subcarrier SC 1  in Cn−1 and the second subcarrier SC 2  in Cn−1, and also with a gap G 2  between the first subcarrier SC 1  in Cn and the second subcarrier SC 2  in Cn, and so on. The pattern of grouping of and spacing between subcarriers may repeat for multiple carriers Cn, or may vary. Each of carriers Cn may be supplied from a corresponding one of transmitters  370 . 
     In another example,  FIG.  5 D  illustrates a variety of combinations of routing of subcarriers with and without gaps between exemplary carriers C 1 , C 2 , C 3 , and C 4  and/or subcarriers within the carriers Cn. In this example, an Intra-Carrier Gap (G) may be allocated between 0, 1, 2 or N of the subcarriers. The Intra-Carrier Gap (G) may be the total gap budgeted for the channel. The size of the gaps G 1 , G 2 , . . . Gn, between the subcarriers may range from zero GHz to a maximum of the total Intra-Carrier Gap G. In the example illustrated in  FIG.  5 D , G 1 =6.25 GHz. The frequency width of the subcarriers SC 1 , SC 2 , SC 3 , SC 4  in a carrier Cn may vary. In the example of  FIG.  5 D , a combination of gaps G 1  is used with the illustrated carriers C 1 -C 4 . For example, in carrier C 1 , a gap G 1  is shown between each of the subcarriers SC 1 -SC 4 , and between the carrier C 1  and the carrier C 2 . Further in this example, in carriers C 2  and C 3 , no gap is shown between the respective subcarriers SC 3  and SC 1  or SC 2  and SC 4 , while a gap G 1  is shown between the respective subcarriers SC 1  and SC 2  and between the carrier C 2  and the carrier C 3 . In the example, carrier  4  does not have gaps between the subcarriers or between carrier  4  and carrier  3 , but does have a gap between carrier  4  and any additional carriers. 
     In one implementation, the subcarriers may not occupy the center frequency ω_c (that is, the laser wavelength). The frequency width of a carrier (Cn) may equal the sum of the frequency width of the subcarriers plus the sum of the frequency width of the gaps Gn (that is, the total Intra-Carrier Gap G). In another implementation, up to a maximum of half of the total Intra-Carrier gaps G may be allocated on either side of the center frequency ω_c, that is, the laser frequency. It will be understood that these combinations of spacing are exemplary, and that any combination of spacing between subcarriers and/or carriers may be used. 
     The variable spacing of the subcarriers may be based at least in part on the routing and/or destination of the subcarriers. For example, the types of filters in the OADMs  229  and/or their widths and/or the number of filters in the route through the optical network  200  that the subcarrier will take, as well as the number and locations of receivers  226 , and the capacity of such receivers  226 , may determine the number of subcarriers, the width of the subcarriers, the frequencies of the subcarriers, and/or the spacing between the subcarriers that are transmitted in a particular optical network  200 . The spacing of the subcarriers may be based at least in part on one or more data rate of one or more of the subcarriers. 
     An example of subcarrier frequency selection based on filter bandwidth will next be described with reference to  FIG.  5   e   . As noted above, optical signals including subcarriers may be output from transmitters  212  onto optical fiber link  230 . The optical signals may be transmitted through OADMs  229 - 1  and  229 - 2  coupled along fiber link  230 . Each of OADMs  229 - 1  and  229 - 2  may include wavelength selective switches, each of which may further including one or more optical filters. One of these optical filters provided in OADM  229 - 1 , for example, may have a bandwidth or transmission characteristic  529 - 1  defined by edge frequencies f 2  and f 4 , and at least one of the optical filters in OADM  229 - 2  may have a bandwidth or transmission characteristic  529 - 2  defined by frequencies f 5  and f 6 . In order to transmit a subcarrier, such as subcarrier SC 1  through filters in both OADM  229 - 1  and  229 - 2 , the frequency of the subcarrier is preferably selected to be within a high transmission frequency range R that is common to both filter bandwidths  529 - 1  and  529 - 2 . As further shown in  FIG.  5 E , range R is defined by edge frequency F 4  of bandwidth  529 - 1  and edge frequency f 5  of bandwidth  529 - 2 . Accordingly, the frequency f 1  of subcarrier SC 1  is controlled or selected by controlling the subcarrier gap in a manner similar to that described above so that SC 1  falls within the high transmission frequency range R that is common to or overlaps between filter bandwidths  529 - 1  and  529 - 2 . 
     Returning now to  FIG.  1   , in one example, subcarriers that are output from the transmitter  212  may be supplied to the multiplexer  216  and sent via the link  230  to one or more receiver module, such as receiver module  220 , which may select data carried by one of such subcarriers, as described in greater detail below with reference to  FIGS.  6  and  7   . 
     At the receiver module  220 , the subcarriers may be supplied to one or more of the receivers  226 .  FIG.  6    illustrates an exemplary one of the optical receivers  226  of the receiver module  220 . The optical receiver  226  may include a polarization splitter  605  (having a first output  606 - 1  and a second output  606 - 2 ), a local oscillator laser  610 , two ninety-degree optical hybrids or mixers  620 - 1  and  620 - 2  (referred to generally as hybrid mixers  620  and individually as hybrid mixer  620 ), two detectors  630 - 1  and  630 - 2  (referred to generally as detectors  630  and individually as detector  630 , each including either a single photodiode or balanced photodiode), two analog-to-digital converters (ADCs)  640 - 1  and  640 - 2  (referred to generally as ADCs  640  and individually as ADC  640 ), and a receiver digital signal processor (RX DSP)  650 . 
     The polarization beam splitter (PBS)  605  may include a polarization splitter that splits an input optical signal  607 , having subcarriers, as noted above, into two orthogonal polarizations, such as the first polarization and the second polarization. The hybrid mixers  620  may combine the polarization signals with light from the local oscillator laser  610 . For example, the hybrid mixer  620 - 1  may combine a first polarization signal (e.g., the component of the incoming optical signal having a first or TE polarization output from the first output  606 - 1 ) with the optical signal from the local oscillator  610 , and the hybrid mixer  620 - 2  may combine a second polarization signal (e.g., the component of the incoming optical signal having a second or TM polarization output from the second output  606 - 2 ) with the optical signal from the local oscillator  610 . In one example, a polarization rotator may be provided at the second output  606 - 2  to rotate the second polarization to be the first polarization. 
     The detectors  630  may detect mixing products output from the optical hybrid mixers  620 , to form corresponding voltage signals. The ADCs  640  may convert the voltage signals to digital samples. For example, two detectors  630 - 1  (or photodiodes) may detect the first polarization signals to form the corresponding voltage signals, and a corresponding two ADCs  640 - 1  may convert the voltage signals to digital samples for the first polarization signals after amplification, gain control and AC coupling. Similarly, two detectors  630 - 2  may detect the second polarization signals to form the corresponding voltage signals, and a corresponding two ADCs  640 - 2  may convert the voltage signals to digital samples for the second polarization signals after amplification, gain control, and AC coupling. 
     The RX DSP  650  may process the digital samples for the first and second polarization signals to generate resultant data, which may be outputted as output data  652 , such as Client Data Streams  352 . 
     While  FIG.  6    shows the optical receiver  226  as including a particular quantity and arrangement of components, in some implementations, the optical receiver  226  may include additional components, fewer components, different components, or differently arranged components. The quantity of detectors  630  and/or ADCs  640  may be selected to implement an optical receiver  226  that is capable of receiving a polarization diverse signal. In some instances, one of the components illustrated in  FIG.  6    may perform a function described herein as being performed by another one of the components illustrated in  FIG.  6   . 
     Consistent with the present disclosure, in order to select one or more subcarriers at a remote node, the local oscillator laser  610  may be tuned to output light having a wavelength relatively close to the selected subcarrier(s) wavelength(s) to thereby cause a beating between the local oscillator light and the selected subcarrier(s). Such beating will either not occur or will be significantly attenuated for the other non-selected subcarriers so that data from the Client Data Stream(s)  352  carried by the selected subcarrier is detected and processed by the Rx DSP  650 . 
     In the example shown in  FIG.  6   , appropriate tuning of the wavelength of the local oscillator laser  610  enables selection of one of the subcarriers, e.g., SC 1 , carrying signals or data indicative of Client Data Stream  352 - 1 . Accordingly, subcarriers may be effectively routed through the optical network  200  to a desired receiver  226  in a particular node of the optical network  200 . 
     Accordingly, at each receiver  226 , the local oscillator laser  610  may be tuned to have a wavelength close to that of one of the subcarriers carrying signals and data indicative of the desired client data from the Client Data Stream  352  to be output from the Rx DSP  650 . Such tuning may be achieved by adjusting a temperature or current flowing through local oscillator laser  610 , which may include a semiconductor laser, such as a distributed feedback (DFB) laser or distributed Bragg reflector (DBR) laser (not shown). Thus, different optical components in each receiver are not required to select optical signals carrying a desired data stream. Rather, as noted above, the same or substantially the same circuitry may be proved in the receiver module  220  of each node, in the optical network  200 , and signal or data selection may be achieved by tuning the local oscillator laser  610  to the desired beating wavelength. 
     As further shown in  FIG.  6   , the Rx DSP  650  may have output data  652 , such that based on such output, the temperature of, or the current supplied to, local oscillator laser  610  may be controlled. In the case of temperature control, a thin film heater may be provided adjacent local oscillator laser  610 , and an appropriate current may be supplied to such heater, based on output  652 , to heat laser  610  to the desired temperature. Control circuitry in the Rx DSP  650  may generate output or control the output signal  652 . Additionally or alternatively, such circuitry may be provided outside the Rx DSP  650 . 
       FIG.  7    illustrates exemplary components of an example of the receiver digital signal processor (Rx DSP)  650  shown in  FIG.  6   . The RX DSP  650  may include an overlap and save buffer  805 , a FFT component  810 , a de-mux component  815 , four fixed filters  820 - 1  to  820 - 4  (referred to generally as fixed filters  820  and individually as fixed filter  820 ), four polarization mode dispersion (PMD) components  825 - 1  to  825 - 4  (referred to generally as PMD components  825  and individually as PMD component  825 ), four IFFT components  830 - 1  to  830 - 4  (referred to generally as IFFT components  830  and individually as IFFT component  830 ), four take last 128 components  835 - 1  to  835 - 4  (referred to generally as take last 128 components  835  and individually as take last 128 component  835 ), four carrier recovery components  840 - 1  to  840 - 4  (referred to generally as carrier recovery components  840  and individually as carrier recovery component  840 ), four symbols to bits components  845 - 1  to  845 - 4  (referred to generally as symbols to bits components  845  and individually as symbols to bits component  845 ), four output bits components  850 - 1  to  850 - 4  (referred to generally as output bits components  850  and individually as output bits component  850 ), and four FEC decoders  860 - 1  to  860 - 4  (referred to generally as FEC decoders  860  and individually as FEC decoder  860 ). In one implementation, the receiver digital signal processor  650  may optionally include a clock recovery circuit  817 . 
     In greater detail, the overlap and save buffer  805  may receive samples from the ADCs  640 - 1  and  640 - 2 . In one implementation, the ADC  640  may operate to output samples at 64 GSample/s. The overlap and save buffer  805  may receive 1024 samples and combine the current 1024 samples with the previous 1024 samples, received from the ADC  640 , to form a vector of 2048 elements. The FFT component  810  may receive the 2048 vector elements, for example, from the overlap and save buffer  805  and convert the vector elements to the frequency domain using, for example, a fast Fourier transform (FFT). The FFT component  810  may convert the 2048 vector elements to 2048 frequency bins as a result of performing the FFT. 
     The de-mux component  815  may receive the 2048 frequency bins or outputs from FFT component  810 . The de-mux component  815  may demultiplex the 2048 frequency bins to element vectors for each of the subcarriers, for example, 512 vectors, which may have, in one example an associated baud rate of 8 Gbaud. 
     In some implementations, clock and/or phase recovery circuitry  817  may be connected or coupled between the de-mux component  815  and the filter  820 . In cases where one of the subcarriers (such as SC 2  in  FIGS.  4 H- 4 I ) has a wider bandwidth, due to a corresponding roll-off in the associated transmitter filter  470  discussed above, than the other subcarriers, the wider subcarrier SC 2  may be selected from the output of the de-mux component  815  for clock recovery and the recovered or detected clock or phase related signal may be provided to the ADCs  640  in the receiver  226  (see  FIG.  6   ). The clock may be used to set and/or adjust the timing of sampling of the ADCs  640  for the plurality of the subcarriers. 
     The clock may be recovered using information from all subcarriers, or from fewer than all the subcarriers, or just from one subcarrier. In some implementations, clock recovery with the clock recovery circuit  817  in the RX DSP  650  of the receiver  226  is based on the subcarrier with the widest bandwidth and associated filter  470  having a corresponding roll-off (such as subcarrier SC 2  in  FIGS.  4 D and  4 E ). The subcarrier with the widest bandwidth may be used to recover the clock signal and such clock signal may be used for the other ADCs  640 . 
     In one example, where the data associated with more than one of subcarriers SC 1 -SC 4 , such as subcarriers SC 2  and SC 3 , is to be output from the receiver  226 , the clock recovered from the widest subcarrier SC 2  may be used as the clock for the other subcarriers SC 1 , SC 3 , and SC 4 . As noted above, by reducing the frequency bandwidth of the other subcarriers SC 1 , SC 3 , and SC 4 , more subcarriers fit in a given spectrum or bandwidth to thereby increase overall capacity (as shown in  FIGS.  4 H and  4 I ). In one example, where each node outputs the data of only one subcarrier SC 1 , clock recovery may be performed based on the corresponding subcarrier SC 1  to be detected at that node  202 . 
     Fixed filters  820  may apply a filtering operation for, for example, dispersion compensation or other relatively slow varying impairment of the transmitted optical signals and subcarriers. The fixed filters  820  may also compensate for skew across subcarriers introduced in link  230 , or skew introduced intentionally in optical transmitter  212 . 
     The PMD component  825  may apply polarization mode dispersion (PMD) equalization to compensate for PMD and polarization rotations. The PMD component  825  may also receive and operate based upon feedback signals from the take last 128 component  835  and/or the carrier recovery component  840 . 
     The IFFT component  830  may covert the 512 element vector, in this example, (after processing by the fixed filter component  820  and the PMD component  825 ) back to the time domain as 512 samples. The IFFT component  830  may convert the 512 element vector to the time domain using, for example, an inverse fast Fourier transform (IFFT). The take last 128 component  835  may select the last 128 samples from the IFFT component  830  and output the 128 samples to the carrier recovery component  840 . 
     The carrier recovery component  840  may apply carrier recovery to compensate for transmitter and receiver laser linewidths. In some implementations, the carrier recovery component  840  may perform carrier recovery to compensate for frequency and/or phase differences between the transmit signal and the signal from the local oscillator  610 . After carrier recovery, the data may be represented as symbols in the QPSK constellation or other modulation formats. In some implementations, the output of the take last 128 component  835  and/or the carrier recovery component  840  could be used to update the PMD component  825 . 
     The symbols to bits component  845  may receive the symbols output from the carrier recovery component  840  and map the symbols back to bits. For example, the symbol to bits component  845  may map one symbol, in the QPSK constellation, to X bits, where X is an integer. For dual-polarization QPSK, X is four. In some implementations, the bits could be decoded for error correction using, for example, FEC. The output bits component  850  may output 128*X bits at a time, for example. For dual-polarization QPSK, the output bits component  850  may output 512 bits at a time, for example. 
     The FEC decoder  860  may process the output of the output bits component  850  to remove errors using forward error correction. As further shown in  FIG.  7   , a switch, blocking, or terminating circuit  865  may be provided to terminate one or more client data streams  352  that are not intended for output from receiver  226 . 
     While  FIG.  7    shows the RX DSP  650  as including a particular quantity and arrangement of functional components, in some implementations, the RX DSP  650  may include additional functional components, fewer functional components, different functional components, or differently arranged functional components. 
     In some implementations, the subcarriers may have variable and flexible capacity per subcarrier, but fixed subcarrier width such as a fixed baud rate per subcarrier. Such parameters may be selected or set in a manner similar to that described above. An exemplary optical network  200  having subcarriers with flexible capacity and fixed width may include independent clock and carrier recovery for each subcarrier or the clock recovery for one subcarrier may be used for other subcarriers, as described previously.  FIGS.  8 A- 8 G  illustrates some such examples in use. For explanatory purposes,  FIGS.  8 A- 8 G  illustrate use case examples with a constant baud rate of 16 GHz per subcarrier. The subcarriers for each example may be transmitted from one transmitter  212  or from a combination of two or more transmitters  212 . 
     In the example of  FIG.  8 A , the subcarriers SC 1 -SC 6  are modulated at 128 QAM to be transferred 100 km, with a constant baud rate of 16 GHz per subcarrier, and 175 G bit rate per subcarrier. In the example of  FIG.  8 B , the subcarriers SC 1 -SC 6  are modulated at 64QAM to be transferred 500 km, with a constant baud rate of 16 GHz per subcarrier, and 150 G bit rate per subcarrier. In the example of  FIG.  8 C , the subcarriers SC 1 -SC 6  are modulated at 32QAM to be transferred 1000 km, with a constant baud rate of 16 GHz per subcarrier, and 125 G bit rate per subcarrier. In the example of  FIG.  8 D , the subcarriers SC 1 -SC 6  are modulated at 16QAM to be transferred 2000 km, with a constant baud rate of 16 GHz per subcarrier, and 100 G bit rate per subcarrier. 
     In the example of  FIG.  8 E , the subcarriers SC 1 -SC 4  are modulated at 128QAM to be transferred a variety of distances, with a constant baud rate of 16 GHz per subcarrier, with a variety of bit rates per subcarrier, and illustrating frequency spacing between subcarrier SC 1  and subcarrier SC 2 , as well as between subcarrier SC 3  and subcarrier SC 4 , such as described in relation to  FIGS.  5 A- 5 D . In the example of  FIG.  8 F , the subcarriers SC 1 -SC 5  are modulated at 64 QAM to be transferred a variety of distances, with a constant baud rate of 16 GHz per subcarrier, with a variety of bit rates per subcarrier, and illustrating frequency spacing between subcarrier SC 3  and subcarrier SC 4 , such as described in relation to  FIGS.  5 A- 5 D . In the example of  FIG.  8 G , the subcarriers SC 1 -SC 6  are modulated at 16 QAM to be transferred a variety of distances, with a constant baud rate of 16 GHz per subcarrier, and with a variety of bit rates per subcarrier. 
     Though the examples of  FIGS.  8 A- 8 G  show particular exemplary configurations of subcarriers having variable and flexible capacity per subcarrier, and having fixed subcarrier width such as a fixed baud rate per subcarrier, the subcarriers may have any combination of data rates, modulations, spacing, and/or number of subcarriers, and/or other configuration factors. Additionally, two or more of the subcarriers may be provided from one transmitter  212  or from two or more transmitters  212  and from one transmitter module  210  or from two or more transmitter modules  210 . The particular combination and/or configuration of subcarriers used may be based on requirements for transmission distance, data rates, error rates, and/or filter configurations, for example. 
     In some implementations, the subcarriers may have flexible width, but the capacity of each subcarrier may be fixed. Such parameters may be set in a manner similar to that described above. To maintain a constant bit rate, the width of the subcarriers may vary as discussed above, but the data rate may be controlled to be the same as further noted above. An exemplary optical network  200  having subcarriers with flexible width and fixed capacity may include independent clock and carrier recovery for each subcarrier (though the subcarriers may optionally be tied together). The position of the subcarriers may be arbitrary within the analog bandwidth, as described above.  FIGS.  9 A- 9 D  illustrates some such examples in use. For explanatory purposes, the capacity of each subcarrier in  FIGS.  9 A- 9 D  may be fixed at  100 G per subcarrier. 
     In the example of  FIG.  9 A , the subcarriers SC 1 -SC 6  are modulated at greater than 64 QAM to be transferred a variety of distances with a constant bit rate of 100 G and illustrating frequency spacing between subcarrier SC 2  and subcarrier SC 3 , as well as between subcarrier SC 5  and subcarrier SC 6 , such as described in relation to  FIGS.  5 A- 5 D . In the example of  FIG.  9 B , the subcarriers SC 1 -SC 7  are modulated at approximately 64 QAM to be transferred a variety of distances with a constant bit rate of 100 G and illustrating frequency spacing between subcarrier SC 5  and subcarrier SC 6 , such as described in relation to  FIGS.  5 A- 5 D . 
     In the example of  FIG.  9 C , the subcarriers SC 1 -SC 4  are modulated at 32 QAM to be transferred a variety of distances with a constant bit rate of 100 G and illustrating frequency spacing between subcarrier SC 1  and subcarrier SC 2  and subcarrier SC 3  and subcarrier SC 4 , such as described in relation to  FIGS.  5 A- 5 D . In the example of  FIG.  9 D , the subcarriers SC 1 -SC 7  are modulated at 16 QAM to be transferred a variety of distances with a constant bit rate of 100 G. 
     Though the examples of  FIGS.  9 A- 9 D  show particular exemplary configurations of subcarriers having flexible width, but the capacity of each subcarrier may be fixed, the subcarriers may have any combination of widths, modulations, spacing, and/or number of subcarriers and/or other configuration factors. Additionally, two or more of the subcarriers may be provided from one transmitter  212  or from two or more transmitters  212  and from one transmitter module  210  or from two or more transmitter modules  210 . The particular combination and/or configuration of subcarriers used may be based on requirements for transmission distance, data rates, error rates, and/or filter configurations, for example. 
       FIG.  1   , discussed above, shows an example of an optical network  200  having a point-to-point configuration. It is understood, that other network or system configurations or architectures are contemplated herein. Examples of such architectures are discussed in greater detail below. The subcarriers may be transmitted and received in a variety of types of optical networks  200 . For example,  FIG.  10    illustrates an exemplary optical network  200   a  having a mesh network configuration consistent with a further aspect of the present disclosure. The mesh network configuration may include three or more nodes  202 - 1  to  202 - n  (referred to as nodes  202  and individually as node  202 ), each node  202  having at least one of the transmitter module  210  and the receiver module  220  such as previously described, but not shown in  FIG.  10    for purposes of clarity. The nodes  202  may be interconnected by one or more of the links  230 , thereby forming a mesh configuration. For purposes of clarity, not all links  230  are numbered in  FIG.  10   . 
     In the optical network  200   a , one or more subcarriers, such as subcarriers SC 1 -SC 4 , may be routed to different nodes  202  in the optical network  200   a . For example, a first subcarrier SC 1  may be routed from node  202 - 1  to node  202 - 2 , while a second subcarrier SC 2  may be routed from node  202 - 1  to node  202 - 8 . In the optical network  200   a , one or more subcarriers, such as subcarriers SC 1 -SC 4 , may be directed to the same node  202 . For example, four subcarriers SC 1 -SC 4  may be routed from node  202 - 1  to node  202 - 4 . 
     In the optical network  200   a , a particular node  202  may detect multiple subcarriers or may be configured to detect only particular subcarriers or one subcarrier. For example, node  202 - 4  may receive four subcarriers SC 1 -SC 4  and detect two subcarriers SC 1  and SC 4 . 
     In some implementations, a particular node  202  may receive a plurality of subcarriers, and transfer on to another node  202  different data in one or more of the subcarriers. For example, node  202 - 2  may receive subcarriers SC 1  and SC 3 , but may place new data into subcarrier SC 1 - 1  and transmit the subcarriers SC 1 - 1  and SC 3  to node  202 - 3 . 
       FIG.  11 A  illustrates an exemplary optical network  200   b  having a ring network configuration consistent with a further aspect of the present disclosure. The ring network configuration may include three or more of the nodes  202  interconnected by two or more of the links  230  to form a ring. The links  230  may be bi-directional between the nodes  202 . In the example illustrated in  FIG.  11 A , a simple ring configuration is shown having five nodes  202 , though it will be understood that a different number of nodes  202  in a ring configuration may be included. Such a configuration reduces the number of optics assemblies (transmitter and receiver) from two sets per node  202  to one set per node  202 . However, one of the nodes  202  in the optical network  200   b  (here, illustrated as Node  1 ) may still utilizing two sets of optics assemblies, such as two sets of transmitters  212  and receivers  226 . 
     One or more subcarriers may be transmitted within the optical network  200   b . In this example of the optical network  200   b  in the ring network configuration, subcarriers SC 1 -SC 5  may be transmitted within the optical network  200   b . For example, a first subcarrier SC 1  may be transmitted bi-directionally on bi-directional fibers between Node  1  and Node  5 . Similarly, a second subcarrier SC 2  may be transmitted bi-directionally on bi-directional fibers between Node  5  and Node  4 ; a third subcarrier SC 3  may be transmitted bi-directionally on bi-directional fibers between Node  4  and Node  3 ; a fourth subcarrier SC 4  may be transmitted bi-directionally on bi-directional fibers between Node  3  and Node  2 ; and a fifth subcarrier SC 5  may be transmitted bi-directionally on bi-directional fibers between Node  2  and Node  1 . 
       FIG.  11 B  illustrates exemplary components of Node  5 , comprising a transmitter  212 , a receiver  226 , a laser (LO), a de-mux component  902 , and a combiner component  904 . The de-mux component  902  may be configured to split the subcarriers from the transmitter to direct the subcarriers to particular other nodes. In this example, the de-mux component  902  may split the subcarriers SC 1  and SC 2  from the transmitter  212  to direct subcarrier SC 1  to Node  1  and subcarrier SC 2  to Node  4 . The combiner component  904  may be configured to combine the subcarriers entering Node  5  to the receiver  226 . In this example, the de-mux component  902  may combine the subcarriers SC 1  and SC 2 . 
       FIG.  12 A  illustrates an exemplary optical network  200   c  having a hub configuration consistent with a further aspect of the present disclosure. The optical network  200   c  may comprise a hub  920 , a power splitter  922 , and two or more leaf nodes. 
     The hub  920  may have a transmitter  212  and a receiver  226 . The hub  920  may output a plurality of subcarriers, such as, for example, SC 1 -SC 4 , to the power splitter  922 . The power splitter  922  may supply a power split portion of the plurality of subcarriers to one or more leaf node, such as, for example, Leaf  1 - 4 . Each Leaf Node may comprise a receiver  226  that may receive all the subcarriers SC 1 -SC 4  and that may output less than all of the data from the client data streams of all of the subcarriers. For example, Leaf  1  may detect all of the subcarriers SC 1 -SC 4 , but may output the data from the data stream from one of the subcarriers SC 1 . As described above regarding  FIG.  7   , the switch, blocking, or terminating circuit  865  in the receiver  226 , may select one of the subcarriers (or less than all of the subcarriers) and may output data from one of the client data streams  352  (or less than all of the data streams). 
       FIG.  12 B  illustrates an exemplary optical network  200   d  having a hub configuration consistent with a further aspect of the present disclosure. The optical network  200   c  may comprise a hub  920 , wavelength selective switch (WSS) or de-mux component  930 , and two or more leaf nodes (Leaf  1 -Leaf  4 ). The WSS or de-mux component  930  may output less than all of the subcarriers received from the hub  920  to a particular one of the leaf nodes. For example, the hub  920  may output a plurality of subcarriers, for example, SC 1 -SC to the WSS or de-mux component  930 , and the WSS or de-mux component  930  may output less than all of the plurality of subcarriers to the leaf nodes, such as, for example, outputting subcarrier SC 1  to Leaf  1 , while not outputting subcarriers SC 2 -SC 4  to Leaf  1 . Additionally, the leaf nodes may each, on a separate fiber, transmit a corresponding subcarrier back to the WSS or de-mux component  930 , which may detect all of the subcarriers SC-SC 4  and output them to the hub  920 . 
       FIG.  13    illustrates an exemplary optical network  200   e  having a ring and hub network configuration consistent with a further aspect of the present disclosure. The optical network  200   d  may include two or more nodes  202  interconnected with one another, such as exemplary nodes  202 - 1  to  202 - 4 , and further interconnected with at least one hub  206 . The hub  206  may comprise a transmitter  212  and a receiver  226  and may send and receive a plurality of subcarriers, such as, for example, SC 1 -SC 4 . Each of the nodes  202 - 1  to  202 - 4  on the ring may detect and output the data associated with a particular subcarrier of the plurality of subcarriers. A particular node may also transmit new data on the particular subcarrier. The optical network  200   e  may have bi-directional fibers between nodes  202  for bi-directional transmission. In some implementations, a plurality of subcarriers may all be transmitted to all of the nodes  202 , and each particular node  202  may extract and add a particular subcarrier from the plurality of subcarriers. For example, subcarriers SC 1 -SC 4  may be sent to node  202 - 1 , which may extract data from subcarrier SC 1  and add data to subcarrier SC 1  and transmit all of the subcarriers SC 1 -SC 4  on to node  202 - 2 . Node  202 - 2  may receive all of the subcarriers SC 1 -SC 4 , and may extract data from subcarrier SC 2  and add data to subcarrier SC 2  and transmit all of the subcarriers on to node  202 - 3 , and so on. 
     The below table illustrates a list of exemplary spectral efficiencies (that is, bits per unit spectrum) consistent with the present disclosure: 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
               
               
                 Spectral Efficiency 
                 Format 
                 RSNR 
                 RSNR-PS 
                 # Bins 
                 Fbaud 
                 Interpolation 
                 Max Cap 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 11.64 
                 64QAM:9; 
                 17.8 
                 17 
                 88 
                 11.3 
                 11:32 
                 800 
               
               
                   
                 32QAM:2 
               
               
                 10.67 
                 64QAM:1; 
                 16.6 
                 15.4 
                 96 
                 12.3 
                 3:8 
                 800 
               
               
                   
                 32QAM:2 
               
               
                 9.85 
                 32QAM:12; 
                 15.3 
                 14.3 
                 104 
                 13.3 
                 13:32 
                 700 
               
               
                   
                 16QAM:1 
               
               
                 9.14 
                 32QAM:4; 
                 14.4 
                 13.3 
                 112 
                 14.3 
                  7:16 
                 600 
               
               
                   
                 16QAM:3 
               
               
                 8.53 
                 32QAM:4; 
                 13.5 
                 12.5 
                 120 
                 15.4 
                 15:32 
                 600 
               
               
                   
                 16QAM:11 
               
               
                 8 
                 16QAM 
                 12.5 
                 11.8 
                 128 
                 16.4 
                 1:2 
                 600 
               
               
                 7.11 
                 16QAM:5; 
                 11.4 
                 10.6 
                 144 
                 18.4 
                  9:16 
                 500 
               
               
                   
                 8QAM:4 
               
               
                 6.4 
                 16QAM:1; 
                 10.2 
                 9.7 
                 160 
                 20.5 
                 5:8 
                 400 
               
               
                   
                 8QAM:4 
               
               
                 5.82 
                 8QAM:10; 
                 9.2 
                 8.8 
                 176 
                 22.5 
                 11:16 
                 400 
               
               
                   
                 QPSK:1 
               
               
                 5.33 
                 8QAM:2; 
                 8.6 
                 8.1 
                 192 
                 24.6 
                 3:4 
                 400 
               
               
                   
                 QPSK:1 
               
               
                 4.92 
                 8QAM:6; 
                 7.9 
                 7.5 
                 208 
                 26.6 
                 13:16 
                 300 
               
               
                   
                 QPSK:7 
               
               
                 4.57 
                 8QAM:2; 
                 7.3 
                 7.0 
                 224 
                 28.7 
                 7:8 
                 300 
               
               
                   
                 QPSK:5 
               
               
                 4 
                 QPSK 
                 6 
                 6 
                 256 
                 32.8 
                 1:1 
                 300 
               
               
                   
               
            
           
         
       
     
     Accordingly, as noted above, a simplified and less expensive transmitter may be realized consistent with the present disclosure in which a laser and modulator may be employed to generate multiple subcarriers, whereby each of which may be detected and the client data associated therewith may be output from receivers provided at different locations in an optical network, for example. Improved network flexibility can therefore be achieved. 
     Other embodiments will be apparent to those skilled in the art from consideration of the specification. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 
     CONCLUSION 
     Conventionally, a plurality of lasers and modulators were necessary to create optical signals to carry a plurality of data streams. In accordance with the present disclosure, a plurality of subcarriers is generated from a single laser to carry a plurality of data streams, such that a lesser number of lasers and modulators are needed across an optical network. 
     The foregoing description provides illustration and description, but is not intended to be exhaustive or to limit the inventive concepts to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the methodologies set forth in the present disclosure. 
     Even though particular combinations of features are recited in the claims and/or disclosed in the specification, these combinations are not intended to limit the disclosure. In fact, many of these features may be combined in ways not specifically recited in the claims and/or disclosed in the specification. Although each dependent claim listed below may directly depend on only one other claim, the disclosure includes each dependent claim in combination with every other claim in the claim set. 
     No element, act, or instruction used in the present application should be construed as critical or essential to the invention unless explicitly described as such outside of the preferred embodiment. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise.