Patent Publication Number: US-2021167857-A1

Title: Systems and methods for full duplex coherent optics

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/676,295, filed Nov. 6, 2019. U.S. application Ser. No. 16/676,295 is a continuation in part of U.S. application Ser. No. 16/543,509, filed Aug. 16, 2019. U.S. application Ser. No. 16/543,509 is a continuation in part of U.S. application Ser. No. 16/274,152, filed Feb. 12, 2019. U.S. application Ser. No. 16/274,152 is a continuation in part of U.S. application Ser. No. 16/198,396, filed Nov. 21, 2018. U.S. application Ser. No. 16/198,396 claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/589,121, filed Nov. 21, 2017, and to U.S. Provisional Patent Application Ser. No. 62/636,249, filed Feb. 28, 2018. U.S. application Ser. No. 16/274,152 also claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/629,555, filed Feb. 12, 2018. U.S. application Ser. No. 16/543,509 also claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/719,072, filed Aug. 16, 2018. U.S. application Ser. No. 16/676,295 also claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/756,378, filed Nov. 6, 2018. All of these applications are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     The field of the disclosure relates generally to communication networks, and more particularly, to bidirectional networks employing coherent optics technologies. 
     Most network operators have very limited fiber available between the headend (HE)/hub and the fiber node to use for data and video services, often only just 1-2 fiber strands. With end users demanding more bandwidth to the home, operators need a strategy on how to increase capacity in the access network. One way is to add more fiber between the HE/hub and the fiber node, but retrenching is costly and time consuming, so return on investment (RoI) makes this option unattractive. A solution that re-uses the existing infrastructure is therefore considerably preferable. 
     Coherent optics technology is becoming common in the subsea, long-haul, and metro networks, but has not yet been fully integrated into the access networks. However, it is desirable to utilize coherent optics technology in the access network because the distances from the HE/hub to the fiber node are much shorter using coherent optics technologies in comparison with conventional system technologies. One proposed technique for expanding the capacity of existing fiber infrastructures implements coherent optics bidirectional transmission on a single fiber. Bidirectional transmission effectively doubles (or more) the amount of transmission capability available to cable operators. 
     Bidirectional transmission is attractive to network operators that have limited availability of leased or owned fibers, and who desire separation of different services (residential, business, and cellular connections) to end users/endpoints of the network. However, existing coherent transceiver designs have been unable to fully leverage the capabilities of bidirectional transmission. In particular, conventional implementations of single laser sources for both the transmitter and the local oscillator (LO) result in significant crosstalk that has prevented bidirectional transmission. Accordingly, it is desirable to develop systems and methods that successfully implement coherent optics technology in bidirectional transmission between the hub and the fiber node. 
     SUMMARY 
     In an embodiment, a communication network, includes an optical hub having a first coherent optics transceiver, a fiber node having a second coherent optics transceiver, an optical transport medium operably coupling the first coherent optics transceiver to the second coherent optics transceiver, a first optical circulator disposed at the optical hub, and a second optical circulator disposed at the fiber node. The first coherent optics transceiver is configured to (i) transmit a downstream optical signal at a first wavelength, and (ii) receive an upstream optical signal at the first wavelength. The second coherent optics transceiver is configured to (i) receive the downstream optical signal from the first coherent optics transceiver at the first wavelength, and (ii) transmit the upstream optical signal at the first wavelength. The first and second optical circulators are configured to separate the downstream optical signal from the upstream optical signal. 
     In an embodiment, a full duplex communication network includes an optical transmitter end having a first coherent optics transceiver, an optical receiver end having a second coherent optics transceiver, and an optical transport medium operably coupling the first coherent optics transceiver to the second coherent optics transceiver. The first coherent optics transceiver is configured to (i) transmit a downstream optical signal at a first wavelength, and (ii) simultaneously receive an upstream optical signal at a second wavelength. The second coherent optics transceiver is configured to (i) receive the downstream optical signal, and (ii) simultaneously transmit the upstream optical signal. The first wavelength has a first center frequency separated from a second center frequency of the second wavelength. 
     In an embodiment, a full duplex communication network includes an optical transmitter end having a first coherent optics transceiver, an optical receiver end having a second coherent optics transceiver, and an optical transport medium operably coupling the first coherent optics transceiver to the second coherent optics transceiver. The first coherent optics transceiver is configured to simultaneously transmit a downstream optical signal and receive an upstream optical signal. The second coherent optics transceiver is configured to simultaneously receive the downstream optical signal from the first coherent optics transceiver and transmit the upstream optical signal first coherent optics transceiver. At least one of the downstream optical signal and the upstream optical signal includes at least one coherent optical carrier and at least one non-coherent optical carrier. 
     In an embodiment, a full duplex communication network includes a first coherent optics transceiver having a first transmitting portion and a first receiving portion, a second coherent optics transceiver having a second transmitting portion and a second receiving portion, an optical transport medium operably coupling the first coherent optics transceiver to the second coherent optics transceiver, and a coherent optics interface disposed within the first coherent optics transceiver. The coherent optics interface includes a lineside interface portion, a clientside interface portion, and a control interface portion. The clientside interface portion is configured for electrical communication within at least one of the first optics transmitting portion and the first receiving portion. The lineside interface portion is configured for optical communication with the optical transport medium. 
     In an embodiment, an interface subsystem for a coherent optical transceiver includes an optical transmitter, an optical receiver, a control layer, an electrical interface portion in operable communication with the optical transmitter and the optical receiver on a host side of the interface subsystem, a management interface portion in operable communication with the control layer, and an optical interface portion in operable communication with the optical transmitter and the optical receiver on a host side of the interface subsystem. 
     BRIEF DESCRIPTION 
     These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
       FIG. 1  is a schematic illustration of a coherent optics network architecture. 
       FIG. 2  is a schematic illustration of a coherent optics network architecture. 
       FIG. 3  is a schematic illustration of a coherent optics network architecture. 
       FIG. 4  is a schematic illustration of a coherent optics network architecture. 
       FIG. 5  is a schematic illustration of a coherent optics network architecture. 
       FIG. 6  is graphical illustration of a comparative plot of bit error rate against received optical power. 
       FIG. 7  is graphical illustration of a comparative plot of bit error rate against received optical power. 
       FIG. 8  is graphical illustration of a superposition of the plots depicted in  FIGS. 6 and 7 . 
       FIG. 9  depicts a stimulated Brillouin scattering effect. 
       FIG. 10  is graphical illustration of a comparative plot of stimulated Brillouin scattering threshold against fiber effective area. 
       FIG. 11  is graphical illustration of a comparative plot of stimulated Brillouin scattering threshold against fiber effective area. 
       FIG. 12  is a schematic illustration of a coherent optics network test system. 
       FIG. 13  is graphical illustration of a comparative plot of measured power against input power utilizing the test system depicted in  FIG. 12 . 
       FIG. 14  is graphical illustration of an alternative comparative plot of measured power against input power. 
       FIG. 15  is a zigzag reflection diagram of Rayleigh scattering. 
       FIG. 16  is a graphical illustration depicting a histogram of frequency over optical return loss for a single mode fiber. 
       FIG. 17  is a graphical illustration depicting a comparative plot of reflected power against input power. 
       FIG. 18  is a schematic illustration of a multipath interference source system. 
       FIG. 19  is graphical illustration depicting a comparative plot of optical signal-to-noise ratio penalty as a function of multipath interference. 
       FIG. 20  depicts alternative fiber connector structures. 
       FIG. 21  is a schematic illustration of a coherent optics network architecture. 
       FIG. 22  is a schematic illustration of a coherent optics network architecture. 
       FIG. 23  is a schematic illustration of a coherent optics network architecture. 
       FIG. 24  is a schematic illustration of a coherent optics network architecture. 
       FIG. 25  is a schematic illustration of a coherent optics network architecture. 
       FIG. 26  is a schematic illustration of a coherent optics network architecture. 
       FIG. 27  is a schematic illustration of a coherent optics network architecture. 
       FIG. 28  is a graphical illustration of a comparative optical spectrum plot for a single channel. 
       FIG. 29  is a graphical illustration of a comparative optical spectrum plot for a wavelength division multiplexing channel. 
       FIG. 30  is a graphical illustration of a comparative optical spectrum plot for a C-Band channel. 
       FIG. 31  is a graphical illustration depicting a comparative plot of reflected power against input power. 
       FIG. 32  is a graphical illustration depicting a comparative plot of reflected power against input power. 
       FIG. 33  is a graphical illustration depicting a comparative plot of reflected power against input power. 
       FIG. 34  is a graphical illustration depicting a comparative plot of reflected power against input power. 
       FIG. 35  is a graphical illustration depicting a comparative plot of reflected power against input power. 
       FIG. 36  is a graphical illustration depicting a comparative plot of reflected power against input power. 
       FIG. 37  is a schematic illustration of a coherent optics network architecture. 
       FIG. 38  is a schematic illustration of a coherent optics network architecture. 
       FIG. 39  is a schematic illustration of an exemplary optical network unit. 
       FIG. 40  is a schematic illustration of an exemplary optical network unit. 
       FIG. 41  is a schematic illustration of an exemplary optical network unit. 
       FIG. 42  is a schematic illustration of an exemplary optical conversion architecture. 
       FIG. 43  is a graphical illustration depicting relative signal distributions for the architecture depicted in  FIG. 37 . 
       FIG. 44  is a graphical illustration depicting relative signal distributions for the architecture depicted in  FIG. 38 . 
       FIG. 45  is a schematic illustration of a coherent optics network architecture. 
       FIG. 46  is a schematic illustration of a coherent optics network architecture. 
       FIG. 47  is a schematic illustration of a coherent optics network architecture. 
       FIG. 48  is a schematic illustration of a coherent optics network architecture. 
       FIGS. 49A-B  are graphical illustrations of comparative plots of bit error rate against received power for the architecture depicted in  FIG. 48 . 
       FIG. 50  is a schematic illustration of a coherent optics network architecture. 
       FIG. 51A  is graphical illustration of a comparative plot of bit error rate against received power for the architecture depicted in  FIG. 50 . 
       FIG. 51B  is graphical illustration of a comparative plot of required received power against reflected power for the architecture depicted in  FIG. 50 . 
       FIG. 52  is a schematic illustration of a coherent optics network architecture. 
       FIGS. 53A-C  are graphical illustrations of comparative plots of bit error rate against received power for the architecture depicted in  FIG. 52 . 
       FIG. 54  is a schematic illustration of a coherent optics network architecture test subsystem. 
       FIG. 55  is graphical illustration of a comparative plot of bit error rate against received power for the architecture depicted in  FIG. 54 . 
       FIG. 56  is graphical illustration of a comparative plot of required power against reflected power for the architecture depicted in  FIG. 54 . 
       FIG. 57  is a schematic illustration of a coherent optics network architecture. 
       FIG. 58  is a schematic illustration of a coherent optics network architecture. 
       FIG. 59  is a schematic illustration of a coherent optics network architecture. 
       FIG. 60  is a schematic illustration of a coherent optics network architecture. 
       FIG. 61  is a schematic illustration of a coherent optics network architecture. 
       FIG. 62  is a schematic illustration of a coherent optics network architecture. 
       FIG. 63  is a schematic illustration of a coherent optics network architecture. 
       FIG. 64  is a schematic illustration of an optical communications network system. 
       FIGS. 65A-B  are graphical illustrations of comparative optical spectrum plots. 
       FIG. 66  is a schematic illustration of a coherent optics network architecture test subsystem. 
       FIGS. 67A-B  are graphical illustrations of comparative optical spectrum plots. 
       FIG. 68A  is a graphical illustration of a comparative plot  7000  of modulation error ratio against carrier frequency for the analog channels transmitted by the subsystem depicted in  FIG. 66 . 
       FIG. 68B  is a graphical illustration of a comparative plot  7002  of bit error rate against modulation format for the coherent channels transmitted by the subsystem depicted in  FIG. 66 . 
       FIG. 69  is a graphical illustration of a comparative optical spectrum plot. 
       FIG. 70  is a graphical illustration of a comparative plot of normalized power penalty against data rate. 
       FIG. 71  is a schematic illustration of a transceiver having a dual optical interface structure. 
       FIG. 72  is a schematic illustration of a transceiver having a single optical interface structure. 
       FIG. 73  is a functional schematic illustration of a transmitter. 
       FIG. 74  is a functional schematic illustration of a receiver. 
    
    
     Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of this disclosure. These features are believed to be applicable in a wide variety of systems including one or more embodiments of this disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein. 
     DETAILED DESCRIPTION 
     In the following specification and claims, reference will be made to a number of terms, which shall be defined to have the following meanings. 
     The singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     As used herein, unless specified to the contrary, “modem termination system,” or “MTS” may refer to one or more of a cable modem termination system (CMTS), an optical network terminal (ONT), an optical line terminal (OLT), a network termination unit, a satellite termination unit, and/or other termination devices and systems. Similarly, “modem” may refer to one or more of a cable modem (CM), an optical network unit (ONU), a digital subscriber line (DSL) unit/modem, a satellite modem, etc. 
     As used herein, the term “database” may refer to either a body of data, a relational database management system (RDBMS), or to both, and may include a collection of data including hierarchical databases, relational databases, flat file databases, object-relational databases, object oriented databases, and/or another structured collection of records or data that is stored in a computer system. 
     Furthermore, as used herein, the term “real-time” refers to at least one of the time of occurrence of the associated events, the time of measurement and collection of predetermined data, the time for a computing device (e.g., a processor) to process the data, and the time of a system response to the events and the environment. In the embodiments described herein, these activities and events occur substantially instantaneously. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” “approximately,” and “substantially,” are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged; such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. 
     The embodiments described herein provide innovative systems and methods for full-duplex coherent optics, that is, bidirectional (BiDi) coherent optics networks. The present techniques may further advantageously implement passive optical circulators and a variety of versatile architectural configurations to separate the upstream and downstream signal flows of the BiDi network. According to these embodiments, spectral efficiency is significantly improved in both the downstream and upstream directions. As described further herein, both the downstream and upstream transmissions may utilize the same wavelength and simultaneous transmission over the same fiber, thereby doubling the spectral efficiency of existing coherent transmission systems or networks. 
       FIG. 1  is a schematic illustration of a coherent optics network architecture  100 . In the example depicted in  FIG. 1 , architecture  100  represents an aggregation use case for a distributed coherent optics network. Architecture  100  includes a hub  102 , a node  104 , and a transport medium  106  (e.g., an optical fiber) communicatively coupled therebetween. In an exemplary embodiment, transport medium  106  is a single strand fiber for a coherent optic link. Architecture  100  further includes a hub coherent transceiver  108  and a hub optical circulator  110  (e.g., a three-port optical circulator in the illustrated example) at hub  102 . Similarly, architecture  100  includes a node coherent transceiver  112  and a node optical circulator  114 . 
     In an exemplary embodiment, coherent transceivers  108 ,  112  include a single laser source, a transmitting portion, and a receiving portion, respectively (not separately numbered). In operation, architecture  100  is configured for bidirectional transmission of a wavelength X in both the downstream (DS) and upstream (US) directions. In particular, architecture  100  represents transmission over a single channel (e.g., 100G, 200G, etc.), where both coherent transceivers  108 ,  112  use their respective single laser sources for both transmitter LO and receiver LO. That is, the wavelength λ is the same for both the downstream and upstream transmission. 
     Exemplary architectures of coherent hub-to-node networks and systems are described in greater detail in co-pending U.S. Pat. No. 9,912,409, and in U.S. Pat. No. 10,200,123, the disclosures of both which are incorporated by reference herein. Additionally, the person of ordinary skill in the art will understand that architecture  100  is simplified for ease of explanation, does not necessarily illustrate all components that may be implemented within a hub and/or fiber node. 
       FIG. 2  is a schematic illustration of a coherent optics network architecture  200 . Architecture  200  is similar to architecture  100 ,  FIG. 1 , and also represents an example of an aggregation use case. Accordingly, architecture  200  includes a hub  202 , a fiber node  204 , a transport medium/fiber  206 , a hub coherent transceiver  208 , a hub optical circulator  210 , a node coherent transceiver  212 , and a node optical circulator  214 . In the example depicted in  FIG. 2 , hub coherent transceiver  208  includes a hub transmitter  216  and a separate hub receiver  218 . Similarly, node coherent transceiver  212  includes a node receiver  220  and a node transmitter  222 . 
     In an exemplary embodiment, architecture  200  is configured to implement transmission over a dense wavelength division multiplexing (DWDM) channel, and further includes a first optical splitter  228  at hub  202  and a second optical splitter  230  at node  204 . In an embodiment, architecture  200  further includes a first optical multiplexer  224  at hub  202  and a second optical multiplexer  226  at node  204 . In this example, architecture  200  is configured to transmit multiple wavelengths λ 1 , λ 2 , λ N  in both directions. In the example depicted in  FIG. 2 , a demultiplexer is optionally the necessary at coherent receivers  218 ,  220  where the respective LO serves for signal selectivity. This embodiment may, for example, be particularly advantageous in the case of a limited number of DWDM channels. 
       FIG. 3  is a schematic illustration of a coherent optics network architecture  300 . Architecture  300  is similar to architecture  200 ,  FIG. 2 , and also represents an example of an aggregation use case and is configured to implement DWDM transmission. Accordingly, architecture  300  includes a hub  302 , a fiber node  304 , a transport medium/fiber  306 , a hub coherent transceiver  308 , a hub optical circulator  310 , a node coherent transceiver  312 , a node optical circulator  314 , a hub transmitter  316 , a hub receiver  318 , a node receiver  320 , a node transmitter  322 , a first optical multiplexer  324 , and a second optical multiplexer  326 . Architecture  300  differs though, from architecture  200  in that architecture  300  further includes a first optical demultiplexer  328  at hub  302  and a second optical demultiplexer  330  at node  304 . That is, architecture  300  effectively replaces first optical splitter  228  and second optical splitter  230  with first optical demultiplexer  328  and second optical demultiplexer  330 , respectively. 
       FIG. 4  is a schematic illustration of a coherent optics network architecture  400 . Architecture  400  is similar to architecture  300 ,  FIG. 3 , and also represents an example of an aggregation use case for DWDM channels. Accordingly, architecture  400  includes a hub  402 , a fiber node  404 , a transport medium/fiber  406 , a hub coherent transceiver  408 , a hub optical circulator  410 , a node coherent transceiver  412 , a node optical circulator  414 , a hub transmitter  416 , a hub receiver  418 , a node receiver  420 , a node transmitter  422 , a first optical multiplexer  424 , a second optical multiplexer  426 , a first optical demultiplexer  428 , and a second optical demultiplexer  430 . Architecture  400  differs though, from architecture  300  in that architecture  400  further includes a boost amplifier  432  between first optical multiplexer  424  and hub optical circulator  410 , and a pre-amplifier  434  between hub optical circulator  410  and first optical demultiplexer  428 . In an exemplary embodiment, boost amplifier  432  and pre-amplifier  434  are erbium-doped fiber amplifiers (EDFAs) functioning as optical repeater devices that boost the intensity of optical signals being carried through the fiber optic communications system of architecture  400 . 
       FIG. 5  is a schematic illustration of a coherent optics network architecture  500 . Architecture  500  is similar to architecture  400 ,  FIG. 4 , and also represents an example of an aggregation use case for DWDM channels. Accordingly, architecture  500  includes a hub  502 , a fiber node  504 , a transport medium/fiber  506 , a hub coherent transceiver  508 , a hub optical circulator  510 , a node coherent transceiver  512 , a node optical circulator  514 , a hub transmitter  516 , a hub receiver  518 , a node receiver  520 , a node transmitter  522 , a first optical multiplexer  524 , a second optical multiplexer  526 , a first optical demultiplexer  528 , a second optical demultiplexer  530 , a hub boost amplifier  532 , and a hub pre-amplifier  534 . Architecture  500  differs though, from architecture  400  in that architecture  500  further includes a node pre-amplifier  538  between node optical circulator  514  and second optical demultiplexer  530 , and a node boost amplifier  538  between second optical multiplexer  526  and node optical circulator  514 . In an exemplary embodiment, node pre-amplifier  536  and node boost amplifier  432  are also EDFAs. 
     The several architectures described herein were subject to proof of concept, which produced significant preliminary experimental results. In exemplary experimentation, forward error correction (FEC) encoding was employed (e.g., staircase FEC). Some of the FEC results reflect the use of hard decision (HD) FEC (HD-FEC) for case of 100 G with 7% overhead, staircase FEC. In one particular embodiment, approximately a 1 dB power penalty was had for a 7% staircase FEC at 4.5e-3 for both directions in single channel 100 G testing (single channel case). 
     A difference may then be seen between the upstream and downstream transmissions due to the sensitivity differences of the respective coherent receivers. However, after correction by HD-FEC techniques, no error was found over a 80-km transmission. Nevertheless, the different output powers from the respective coherent transmitters exhibits a noticeable impact on the link receiver sensitivity. Accordingly, for a particular transmission link, it is further desirable to utilize the present techniques to optimize output power to minimize the power penalty, as described further below. The experimental results described herein also consider various parameters of the respective optical circulators as featured below in Table 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Wavelength Range 
                 nm 
                 1525-1610 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Insertion Loss 
                 Port 1 → 2 
                 dB 
                 0.73 
               
               
                   
                   
                 Port 2 → 3 
                   
                 0.66 
               
               
                   
                 PDL 
                 Port 1 → 2 
                 dB 
                 0.05 
               
               
                   
                   
                 Port 2 → 3 
                   
                 0.04 
               
               
                   
                 Return Loss 
                 Port 1 
                 dB 
                 60 
               
               
                   
                   
                 Port 2 
                   
                 60 
               
               
                   
                   
                 Port 3 
                   
                 60 
               
               
                   
                 Isolation at  
                 Port 2 → 1 
                   
                 52 
               
               
                   
                 1570 nm 
                 Port 3 → 2 
                   
                 57 
               
               
                   
                 Directivity 
                 Port 1 → 3 
                   
                 60 
               
               
                   
                   
                 Port 3 → 1 
                   
                 55 
               
               
                   
                 PMD 
                   
                 ps 
                 &lt;0.05 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 6  is graphical illustration of a comparative plot  600  of bit error rate (BER) against received optical power. In an exemplary embodiment, plot  600  represents the received optical power over an 80-km single mode fiber (SMF), such as an SMF-28, and for a transmitter output power of −8 dBm (without EDFAs, in this example) and a 4.5e-3 staircase FEC threshold. Plot  600  includes a first sub-plot  602  representing a downstream transmission in a bidirectional use case, a second sub-plot  604  representing a downstream transmission in a single direction use case, a third sub-plot  606  representing an upstream transmission in the bidirectional use case, and a fourth sub-plot  608  representing an upstream transmission in the single direction use case. As can be seen from the example depicted in  FIG. 6 , the received optical power is consistently greater in the bidirectional case, in both the downstream and upstream directions. 
       FIG. 7  is graphical illustration of a comparative plot  700  of BER against received optical power. Plot  700  is similar to plot  600 ,  FIG. 6 , in that plot  700  represents the received optical power over an 80-km SMF-28, a 4.5e-3 staircase FEC threshold, and without EDFA. Plot  700  differs from plot  600  though, in that plot  700  represents the experimental results for a transmitter output power of 0 dBm. Plot  700  includes a first sub-plot  702  representing a downstream transmission in the bidirectional use case, a second sub-plot  704  representing a downstream transmission in the single direction use case, a third sub-plot  706  representing an upstream transmission in the bidirectional use case, and a fourth sub-plot  708  representing an upstream transmission in the single direction use case. As can be seen from the example depicted in  FIG. 7 , the received optical power is again consistently greater in the bidirectional case, in both the downstream and upstream directions. 
       FIG. 8  is graphical illustration of a superposition  800  of plot  600 ,  FIG. 6 , and plot  700 ,  FIG. 7 . Superposition  800  illustrates how the received optical power generally tracks with the results of different transmitter output powers, and is generally higher as the transmitter output power increases, except in the bidirectional downstream transmission. 
     From the preliminary results of the embodiments described immediately above, additional analysis of testing results were obtained several implementations of full duplex coherent optics architectures and systems. Conventional full duplex coherent optics systems are subject to significant impairments, including: (i) Stimulated Brillouin Scattering (SBS); (ii) Rayleigh scattering (continuous reflection); (iii) Multiple-Path/Multipath Interference (MPI), for coherent or incoherent interference, and including double-Rayleigh scattering, local reflections (mechanical splices, fusion, jumper cables, etc.), and/or optical amplifiers; and (iv) Fresnel reflection (discrete reflections), including jumper cables, optical distribution panels, fusion, mechanical splices, etc. 
       FIG. 9  depicts a SBS effect  900 . SBS effect  900  occurs, for example, where variations in the electric field of an incident beam of light (e.g., from a laser) traveling through a transport medium (e.g., an optical fiber), induce acoustic vibrations (i.e., an acoustic wave) in the fiber by electrostriction or radiation pressure. Brillouin scattering (e.g., scattered light) thus occurs, in the direction opposite the incident light beam as a result of the acoustic wave vibrations, as illustrated in  FIG. 9 . More particularly, SBS effect  900  occurs from the interaction between the light and acoustic waves. The propagating light beam in the fiber generates a propagating acoustic wave that creates a periodic variation of the fiber refractive index. The back-scattered wave, also referred to as a Stokes wave, is downshifted by approximately 11 GHz with respect to the incident light wave frequency. When increasing the launched power of the optical beam, the reflected power will increase linearly as a result of the Rayleigh back-scattering effect in the fiber. Above a given threshold, the reflected power will then increase exponentially due to SBS effect  900 . 
       FIG. 10  is graphical illustration of a comparative plot  1000  of SBS threshold against fiber effective area. Comparative plot  1000  includes a first sub-plot  1002  representing a case of a 200 MHz linewidth, and a second sub-plot  1004  representing a case of a 20 MHz linewidth. As can be seen from the example depicted in  FIG. 10 , the SBS threshold is considerably greater as the linewidth increases. 
     In an exemplary embodiment, SBS threshold (Power_th) for an unmodulated continuous wave (CW) may be represented as: 
     
       
         
           
             
               Power_th 
                
               
                 ( 
                 
                   B 
                   , 
                   
                     g 
                     b 
                   
                   , 
                   
                     α 
                     dB 
                   
                   , 
                   
                     A 
                     eff 
                   
                   , 
                   Length 
                   , 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       v 
                       s 
                     
                   
                   , 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       v 
                       B 
                     
                   
                 
                 ) 
               
             
             := 
             
               
                 
                   21 
                   · 
                   B 
                   · 
                   
                     A 
                     eff 
                   
                 
                 
                   
                     g 
                     b 
                   
                   · 
                   
                     
                       
                         - 
                         
                           α 
                           dB 
                         
                       
                       · 
                       
                         
                           ln 
                            
                           
                             ( 
                             10 
                             ) 
                           
                         
                         10 
                       
                       · 
                       Length 
                     
                     
                       
                         α 
                         dB 
                       
                       · 
                       
                         
                           ln 
                            
                           
                             ( 
                             10 
                             ) 
                           
                         
                         10 
                       
                     
                   
                 
               
               · 
               
                 ( 
                 
                   1 
                   + 
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         v 
                         s 
                       
                     
                     
                       Δ 
                        
                       
                           
                       
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                         v 
                         B 
                       
                     
                   
                 
                 ) 
               
             
           
         
       
     
     Where B is a number between 1 and 2 of a polarization state, A eff  is the effective area of fiber, g b  is an SBS gain coefficient, Length is the fiber distance, α dB  is a fiber attenuation coefficient, Δv S  is a linewidth of signal source, and Δv B  is an SBS interaction bandwidth. In the example depicted in  FIG. 10 , the SBS threshold for the unmodulated CW was Power_th(1, 4*10 −11 , 0.0002, A eff , 50*10 3 , 20*10 6 , 20*10 6 ). 
       FIG. 11  is graphical illustration of a comparative plot  1100  of SBS threshold against fiber effective area. Comparative plot  1100  is similar to comparative plot  1000 ,  FIG. 10 , except that comparative plot  1100  depicts a comparison of different baud levels, as opposed to different linewidths. More specifically, comparative plot  1100  includes a first sub-plot  1102  representing a 32 GBaud case, and a second sub-plot  1004  representing a 28 GBaud case. As can be seen from comparative plot  1100 , the SBS threshold is greater as the baud increases. In the example depicted in  FIG. 11 , the experimental results were gathered using PM-QPSK signals over a 50-km (19.54-km effective length) transmission, the SBS threshold for the PM-QPSK signals was Power_th(1, 4*10-11, 0.0002, A eff , 50*10 3 , 28*10 9 , 20*10 6 ). 
       FIG. 12  is a schematic illustration of a coherent optics network test system  1200 . Test system  1200  was used to obtain, from an input power source  1202 , measured power results  1204 , as described further below with respect to  FIGS. 13 and 14 , for a CW source (e.g., comparative plot  1000 ,  FIG. 10 ) and a QPSK source (e.g., comparative plot  1100 ,  FIG. 11 ), respectively. 
       FIG. 13  is graphical illustration of a comparative plot  1300  of measured power against input power utilizing test system  1200 ,  FIG. 12 , in a simulation. In an exemplary embodiment, the input power is representative of a CW source, and the measured power is for a 20 MHz linewidth over a 20-km SMF-28. In this example, the measured power of comparative plot  1300  includes a first sub-plot  1302  representing the transmitted signal power, and a second sub-plot  1304  representing the Stokes power. As can be seen from the example depicted in  FIG. 13 , the measured transmitted signal power  1302  is significantly greater than the Stokes power  1304  until the input power reaches approximately 7 dBm, above which the measured Stokes power  1304  exceeds the measured transmitted signal power. 
       FIG. 14  is graphical illustration of an alternative comparative plot  1400  of measured power against input power. Comparative plot  1400  is similar to comparative plot  1300 ,  FIG. 13 , except that comparative plot  1400  demonstrates the result of the simulation for a QPSK (32 GBaud) source utilizing test system  1200 ,  FIG. 12 , in in alternative simulation. In this example, the measured power of comparative plot  1400  includes a first sub-plot  1402  representing the transmitted signal power, and a second sub-plot  1404  representing the Stokes power. As can be seen from the example depicted in  FIG. 14 , the measured transmitted signal power  1402  is consistently greater than the Stokes power  1404  across the entire range of input power levels. 
     Accordingly, in the case of SBS in coherent optic systems, because of the effect of phase-modulated signals on the reduction of optical carrier power, the effective linewidth is proportional to the signal baud rate. Accordingly, the SBS threshold power will significantly increase in a similar manner. However, the SBS was found to be negligible for a launch power less than 7 dBm/channel in the coherent optical systems described above with respect to  FIG. 13  (CW source). 
     Simulations in consideration of Rayleigh scattering impairments are described further below with respect to  FIGS. 15-17 . 
       FIG. 15  is a zigzag reflection diagram  1500  of Rayleigh scattering. Reflection diagram  1500  demonstrates the significant of time and distance with respect to the scattering effect.  FIG. 16  is a graphical illustration depicting a histogram  1600  of frequency against optical return loss (ORL) for an SMF-28. The example depicted in  FIG. 16  illustrates a case of a non-dispersion-shifted fiber (NDSF) SMF-28. 
       FIG. 17  is a graphical illustration depicting a comparative plot  1700  of reflected power against input power. Comparative plot  1700  depicts simulated results at a 1548.52 nm wavelength, and includes sub-plots  1702  representative of a 35 dB reflection power measured over different transmission distances of 26-km, 52-km, and 78-km, respectively. As can be seen from the example depicted in  FIG. 17 , the reflected power tracks fairly linearly with the input power, and is substantially agnostic of the various changes to the transmission distances. 
       FIG. 18  is a schematic illustration of an MPI source system  1800 . In the embodiment depicted in  FIG. 18 , system  1800  is utilized to produce MPI interference  1802  over eight separate paths (i.e., 5-km, 10-km, 15-km, 20-km, 25-km, 30-km, 35-km, and 40-km delays, in this example). In an exemplary embodiment, system  1800  implements a plurality of variable optical attenuators (VOAs). 
       FIG. 19  is graphical illustration depicting a comparative plot  1900  of optical signal-to-noise ratio (OSNR) penalty as a function of MPI (e.g.,  FIG. 18 ). In this example, comparative plot  1900  includes a first sub-plot  1902  representative of a 3.8e-3 BER, and a second sub-plot  1904  representative of a 1.9e-3 BER. As can be seen from the example depicted in  FIG. 19 , the OSNR penalty increases exponentially as a function of MPI, and that this effect is significantly greater as the BER increases. Nevertheless, sub-plot  1902  demonstrates that, at 3.8e-3 BER, approximately 1 dB of OSNR penalty can be observed for −16 dBc of MPI, which indicates a significantly high tolerance to MPI. In the example depicted in  FIG. 19 , the results were obtained in consideration of the post-FEC error count against the pre-FEC BER, which included a substantial range having zero post-FEC errors. 
       FIG. 20  depicts alternative fiber connector structures  2000 ,  2002 ,  2004 ,  2006  having different respective ferrule end finishes to reduce reflectance/loss. More specifically, structure  2000  represents a flat fiber optic end finish (e.g., less than −30 dB back reflection), structure  2002  represents a physical contact (PC) fiber optic end finish (e.g., less than −35 dB back reflection), structure  2004  represents an ultra physical contact (UPC) fiber optic end finish (e.g., less than −55 dB back reflection), and structure  2006  represents an angled physical contact (APC) optical end finish (e.g., less than −65 dB back reflection). APC structure  2006  is often found in existing hybrid fiber coaxial (HFC) networks, whereas UPC structure  2004  is often considered for networks having a relatively small number of digital links. 
     The following embodiments describe additional systems and methods for experimental analysis and lab testing for further proof of concept from the experimental results obtained thereby. More particularly, the embodiments depicted in  FIGS. 21-27  generally correspond with the several hub-to-fiber node architectures depicted in  FIGS. 1-5 , described above, but are addressed more generally to the full duplex paradigm of bidirectionality, which may be more significantly agnostic of which direction is considered “downstream” versus “upstream.” 
       FIG. 21  is a schematic illustration of a coherent optics network architecture  2100 . In an exemplary embodiment, architecture  2100  includes a first coherent transceiver  2102  in operable communication with a second coherent transceiver  2104  over an SMF  2106 . First coherent transceiver  2102  includes a first transmitter portion  2108  and second coherent transceiver  2104  includes a second transmitter portion  2110 . Similarly, second coherent transceiver  2104  includes a first receiver portion  2112 , and first coherent transceiver  2102  includes a second receiver portion  2114 . First transmitter portion  2110  and second receiver portion  2114  communicate with fiber  2106  through a first optical circulator  2116 , and first receiver portion  2112  and second transmitter portion  2110  communicate with fiber  2106  through a second optical circulator  2118 . 
     In exemplary operation of architecture  2100 , first and second transmitter portions  2108 ,  2110  both transmit at X-dBm of transmit power, and fiber  2106  is subject to Y-dB loss. Accordingly, architecture  2100  should function such that values for X−Y≥−30 dBm (e.g., the receiver sensitivity), and that values for [(X−Y)−(X−35)]≥15.4 dB (e.g., the OSNR, however, larger OSNR values are contemplated due to only 0.1 nm noise power included in this example). Further to this example, the loss Y should be such that Y(loss)≤19.6 dB. 
       FIG. 22  is a schematic illustration of a coherent optics network architecture  2200 . Architecture  2200  is similar to architecture  2100 ,  FIG. 21 , and similarly includes a first coherent transceiver  2202 , a second coherent transceiver  2204 , an SMF  2206 , a first transmitter portion  2208 , a second transmitter portion  2210 , a first receiver portion  2212 , a second receiver portion  2214 , a first optical circulator  2216 , and a second optical circulator  2218 . Architecture  2200  differs from architecture  2100  though, in that architecture  2200  further includes a first optical multiplexer  2220  at first transceiver  2202  and a second optical multiplexer  2222  at second transceiver  2204 , and also a first optical demultiplexer  2224  at second transceiver  2204  and a second optical demultiplexer  2226  at first transceiver  2202 . This dual-multiplexer/demultiplexer configuration operates similarly to architecture  300 ,  FIG. 3 . 
     In the example depicted in  FIG. 22 , first and second transmitter portions  2208 ,  2210  operate at 3 (e.g., X) dBm of transmit power, and the sensitivity of first receiver portion  2212  is −30 dBm received power. According to the calculations described above, the total loss Y will be (X−receiver sensitivity), which is [3−(−30)], or 33 dB. Further in this example, loss at each optical circulator  2216 ,  2218  is 2 dB, and loss at the multiplexers/demultiplexers is 5 dB each. Accordingly, the fiber loss may then be calculated as [33−(5+2)*2], or 19 dB. The reflected power before second optical circulator  2218  is [−33−(5+2)], or −39 dB, and the reflected power at receiver portion  2212  will be [−39−(2+5)], or −46 dBm. From these values, the OSNR is found from [−30−(−46)], or 16 dB. 
       FIG. 23  is a schematic illustration of a coherent optics network architecture  2300 . Architecture  2300  is also similar to architecture  2100 ,  FIG. 21 , and similarly includes a first coherent transceiver  2302 , a second coherent transceiver  2304 , an SMF  2306 , a first transmitter portion  2308 , a second transmitter portion  2310 , a first receiver portion  2312 , a second receiver portion  2314 , a first optical circulator  2316 , and a second optical circulator  2318 , with both first and second transmitter portions  2308 ,  2310  operating at 3 dBm of transmit power. In the example depicted in  FIG. 23  fiber  2306  includes a 51.8-km portion and a 26.3-km portion, and exhibits a 17.8 dB fiber loss (i.e., approximately 18 dB total common link loss). Accordingly, in this embodiment, first receiver portion  2312  has a BER of 1.3e-6, and second receiver portion  2314  has a BER of 5.14e-7. 
       FIG. 24  is a schematic illustration of a coherent optics network architecture  2400 . Architecture  2400  is also similar to architecture  2300 ,  FIG. 23 , and similarly includes a first coherent transceiver  2402 , a second coherent transceiver  2404 , an SMF  2406  (17.8 dB fiber loss, in this example also), a first transmitter portion  2408 , a second transmitter portion  2410 , a first receiver portion  2412 , a second receiver portion  2414 , a first optical circulator  2416 , and a second optical circulator  2418 , with both first and second transmitter portions  2408 ,  2410  operating at 3 dBm of transmit power and at approximately 18 dB total common link loss. Architecture  2400  differs from architecture  2300  though, in that architecture  2400  further includes a first attenuator  2420  between first transmitter portion  2408  and first optical circulator  2416 , and a second attenuator  2422  between second optical circulator  2418  and first receiver portion  2412 , with each attenuator  2420 ,  2422  having 3 dB of attenuation. Accordingly, in this embodiment, first receiver portion  2412  has a BER of 2.48e-6, and second receiver portion  2414  has a BER of 1.19e-6. 
       FIG. 25  is a schematic illustration of a coherent optics network architecture  2500 . Architecture  2500  is also similar to architecture  2300 ,  FIG. 23 , and similarly includes a first coherent transceiver  2502 , a second coherent transceiver  2504 , an SMF  2506  (17.8 dB fiber loss, in this example also), a first transmitter portion  2508 , a second transmitter portion  2510 , a first receiver portion  2512 , a second receiver portion  2514 , a first optical circulator  2516 , and a second optical circulator  2518 . Different from architecture  2400  though, in the example depicted in  FIG. 25 , first and second transmitter portions  2508 ,  2510  are configured to operate at various transmit power levels. 
     More particularly, architecture  2500  operates according to a first case, where the transmit power of first transmitting portion  2508  varies from −5 dBm through −10 dBm, while the transmit power of second transmitting portion  2510  remains constant at −5 dBm. Accordingly, the BER values at first receiver portion  2512  correspondingly change, as reflected in table  2520 . Similarly, architecture  2500  operates according to a second case, where the transmit power of second transmitting portion  2510  varies from 0 dBm through −5 dBm, while the transmit power of first transmitting portion  2508  remains constant at −5 dBm. Accordingly, the BER values at second receiver portion  2514  correspondingly change, as reflected in table  2522 . 
       FIG. 26  is a schematic illustration of a coherent optics network architecture  2600 . Architecture  2600  is similar to architecture  2400 ,  FIG. 24 , and similarly includes a first coherent transceiver  2602 , a second coherent transceiver  2604 , an SMF  2606  (17.8 dB fiber loss, in this example also), a first transmitter portion  2608 , a second transmitter portion  2610 , a first receiver portion  2612 , a second receiver portion  2614 , a first optical circulator  2616 , a second optical circulator  2618 , a first attenuator  2620  (3 dB), and a second attenuator  2622  (3 dB), with both first and second transmitter portions  2608 ,  2610  operating at 3 dBm of transmit power. Architecture  2600  differs from architecture  2400  though, in that architecture  2600  further includes, between first and second optical circulators  2416 ,  2418 , third and fourth attenuators  2624 ,  2422 , each having 5 dB of attenuation, thereby resulting in approximately 28 dB of total common link loss (i.e., 18 dB+(5 dB)*2), and a BER of 2.14e-4 at first receiver portion  2612  and a BER of 1.549e-4 at second receiver portion  2614 . 
       FIG. 27  is a schematic illustration of a coherent optics network architecture  2700 . Architecture  2700  is also similar to architecture  2400 ,  FIG. 24 , and similarly includes a first coherent transceiver  2702 , a second coherent transceiver  2704 , an SMF  2706  (50-km single fiber, in this example), a first transmitter portion  2708 , a second transmitter portion  2710 , a first receiver portion  2712 , a second receiver portion  2714 , a first optical circulator  2716 , a second optical circulator  2718 , and a first attenuator  2720  between first transmitter portion  2708  and first optical circulator  2716 . Architecture  2700  differs from architecture  2400  though, in that architecture  2700  further includes a second attenuator  2722  between second optical circulator  2718  and second transmitter portion  2710 . Accordingly, in this example, each attenuator  2720 ,  2722  results in −20 dBm transmit power seen at the respective optical circulator. 
     The architectural embodiments described above are depicted with respect to single channel operation, for ease of explanation. In an exemplary spectrum plot of single channel operation is described further below with respect to  FIG. 28 . The person of ordinary skill in the art, however, will understand how the present systems and methods may be applied with respect to WDM operations as well. Some exemplary results of WDM operation, in accordance with the present embodiments, are described further below with respect to  FIGS. 29-35 . 
       FIG. 28  is a graphical illustration depicting a comparative optical spectrum plot  2800  for a single channel. Comparative optical spectrum plot  2800  is representative of power over wavelength for a single channel operation (at 0.1-nm resolution, in this example), and includes a first sub-plot  2802  illustrating the downstream optical spectrum of the single channel, and a second sub-plot  2804  illustrating the upstream optical spectrum of the single channel. As can be seen from the example depicted in  FIG. 28 , upstream optical spectrum  2804  tracks fairly closely with downstream optical spectrum  2802 , with downstream optical spectrum  2802  being slightly greater about a central wavelength (1548.52 nm, in the illustrated example). 
       FIG. 29  is a graphical illustration depicting a comparative optical spectrum plot  2900  for a WDM channel. Comparative optical spectrum plot  2900  is representative of power over wavelength for a two-wavelength WDM channel operation (e.g., again at 0.1-nm resolution), and includes a first sub-plot  2902  illustrating the downstream optical spectrum of the WDM channel, and a second sub-plot  2904  illustrating the upstream optical spectrum of the WDM channel. As can be seen from the example depicted in  FIG. 29 , upstream optical spectrum  2904  tracks fairly closely with downstream optical spectrum  2902 , however, in this case, downstream optical spectrum  2902  has slightly lower power about the central peak wavelengths of the WDM channel (1547.57 nm and 1548.52 nm, in the illustrated example). 
       FIG. 30  is a graphical illustration depicting a comparative optical spectrum plot  3000  for a C-Band channel. Comparative optical spectrum plot  3000  is representative of power over wavelength for a C-Band channel operation (e.g., again at 0.1-nm resolution), and includes a first sub-plot  3002  illustrating the downstream optical spectrum of the C-Band channel, and a second sub-plot  3004  illustrating the upstream optical spectrum of the C-Band channel. As can be seen from the example depicted in  FIG. 30 , upstream optical spectrum  3004  tracks fairly closely with downstream optical spectrum  3002  about the central peak wavelengths of the C-Band channel (e.g., 1547.57 nm and 1548.52 nm, in the illustrated example), but upstream optical spectrum  3004  exhibits a considerably higher noise floor outside of the peak wavelengths. The backscattering noise power is described further below with respect to  FIGS. 31-35 . 
       FIG. 31  is a graphical illustration depicting a comparative plot  3100  of reflected power against input power. Comparative plot  3100  depicts simulated results at a 26-km SMF transmission, and includes sub-plots  3102  representative of the reflection power (e.g., −35 dBm at 0 dBm input power) measured over different wavelengths of 1528.77 nm, 1548.52 nm, and 1567.54 nm, respectively. As can be seen from the example depicted in  FIG. 31 , the reflected power tracks fairly linearly with the input power, and is substantially agnostic of the various changes to the wavelength. 
       FIG. 32  is a graphical illustration depicting a comparative plot  3200  of reflected power against input power. Comparative plot  3200  is similar to comparative plot  3100 ,  FIG. 31 , however, comparative plot  3200  depicts simulated results at a 52-km SMF transmission, and includes sub-plots  3202  representative of the reflection power (e.g., again −35 dBm at 0 dBm input power) measured over the different wavelengths of 1528.77 nm, 1548.52 nm, and 1567.54 nm, respectively. As can be seen from the example depicted in  FIG. 32 , the reflected power tracks again fairly linearly with the input power at this larger transmission distance, and remains substantially agnostic of the various changes to the wavelength. 
       FIG. 33  is a graphical illustration depicting a comparative plot  3300  of reflected power against input power. Comparative plot  3300  is similar to comparative plot  3100 ,  FIG. 31 , however, comparative plot  3300  depicts simulated results at a 78-km SMF transmission, and includes sub-plots  3302  representative of the reflection power (e.g., again −35 dBm at 0 dBm input power) measured over the different wavelengths of 1528.77 nm, 1548.52 nm, and 1567.54 nm, respectively. As can be seen from the example depicted in  FIG. 33  as well, the reflected power still tracks fairly linearly with the input power at even larger transmission distances, and continues to remain substantially agnostic of the various changes to the wavelength. 
       FIG. 34  is a graphical illustration depicting a comparative plot  3400  of reflected power against input power. Comparative plot  3400  depicts simulated results at the 1528.77 nm wavelength, and includes sub-plots  3402  representative of the −35 dBm reflection power (i.e., at 0 dBm input power) measured over the different transmission distances of 26-km, 52-km, and 78-km, respectively. As can be seen from the example depicted in  FIG. 34 , the reflected power tracks fairly linearly with the input power, and is substantially agnostic of the various changes to the transmission distances at this wavelength. 
       FIG. 35  is a graphical illustration depicting a comparative plot  3500  of reflected power against input power. Comparative plot  3500  depicts simulated results at the 1548.52 nm wavelength, and includes sub-plots  3502  representative of the −35 dBm reflection power (i.e., at 0 dBm input power) measured over the different transmission distances of 26-km, 52-km, and 78-km, respectively. As can be seen from the example depicted in  FIG. 35 , the reflected power continues to remain fairly linear with respect to the input power, and is also fairly agnostic of the various changes to the transmission distances at this wavelength. However, a slight separation between the respective subplots  3502  can now be seen at the greater transmission distances. 
       FIG. 36  is a graphical illustration depicting a comparative plot  3600  of reflected power against input power. Comparative plot  3600  depicts simulated results at the 1567.54 nm wavelength, and includes sub-plots  3602  representative of the −35 dBm reflection power (i.e., at 0 dBm input power) measured over the different transmission distances of 26-km, 52-km, and 78-km, respectively. As can be seen from the example depicted in  FIG. 36 , the reflected power it is still substantially linear with respect to the input power, and is also somewhat agnostic of the various changes to the transmission distances at this higher wavelength. However, at this higher wavelength, the small separation between the respective subplots  3502  may nevertheless be seen more readily as the transmission distance increases. 
     Full-Duplex Coherent Passive Optical Networks 
     The embodiments described herein advantageously enable a number of unique architectures that provide for efficient implementation with a coherent passive optical network (PON). For example, coherent PON architectures for implementing the present techniques may include symmetrical and/or asymmetrical modulation schemes for downstream and upstream communications. In exemplary embodiments of the present systems and methods, up-conversion and down-conversion may be performed in the digital domain to mitigate the effects of Rayleigh Backscattering (RB) crosstalk noise (described above) for different reach and splitting ratio scenarios. 
     Conventional PON-based fiber-to-the-home (FTTH) networks are presently expected to deliver more capacity and bandwidth per user by increasing the bit rate at the OLT and ONU optical transceivers in order to satisfy the continuously growing traffic growth on these networks. However, although the relatively primitive signaling scheme used in these conventional access networks enables the use of low-cost equipment, the conventional signaling scheme ultimately significantly diminishes the bandwidth that is available to the end-users. 
     Coherent communication systems offer significantly improved power-efficiency and bandwidth-efficiency in comparison with the more primitive optical access networks. Coherent communication technology is theoretically able to feasibly transform the conventional access networks and enable ubiquitous new services and applications with uncontended, multi-gigabits-per-user broadband connections. Nevertheless, the more advanced technology of coherent communication systems is not readily capable of simply substituting for existing portions of conventional optical access networks, such as in a “plug and play” manner. Implementation of coherent technology into optical access networks requires significant modifications for the integration therewith to become economically viable. 
     Accordingly, in some exemplary embodiments described herein, in order to minimize system costs, a single laser source may be implemented at the transmitter side, or hub, to share for both the coherent transmitter and the coherent receiver at the ONU. In such embodiments, a unique wavelength may be provided for the downstream and upstream transmissions, respectively. In other embodiments, coherent technology may be uniquely integrated with some conventional technology schemes, such that some overlap between the downstream and upstream transmissions may occur. According to the present embodiments, coherent PONs are capable of realizing full duplex coherent optics in point-to-multipoint (P2MP) configurations, and achieving realistic and efficient bidirectional (BiDi) connections. 
       FIG. 37  is a schematic illustration of a coherent optics network architecture  3700 . Architecture  3700  is similar to architecture  100 ,  FIG. 1 , in general operation, and includes a transmitter end  3702 , a receiver end  3704 , and a transport medium/fiber  3706 . Transmitter end  3702  may represent a hub, and includes a downstream coherent transceiver  3708 . In an exemplary embodiment, downstream coherent transceiver  3708  includes one or more of a downstream laser  3710 , a downstream coherent transmitter  3712 , and a downstream coherent receiver  3714 . Downstream coherent receiver  3714  is depicted, in this example, as a burst mode coherent receiver. In an embodiment, transmitter end  3702  further includes a three-port downstream optical circulator  3716 . 
     Receiver end  3704  includes a plurality of upstream coherent transceivers  3718 . Each of upstream coherent transceivers  3718  may represent a node or an end user, and includes one or more of an upstream laser  3720 , an upstream coherent receiver  3722 , and an upstream coherent transmitter  3724 . In an embodiment, receiver end  3704  further includes a three-port upstream optical circulator  3726  for each coherent transceiver  3718 . Each of upstream coherent transceivers  3718  communicates over at least one short fiber  3728 , and are combined onto transport medium  3706  by a combiner  3730 . 
     In an exemplary embodiment, architecture  3700  is configured to implement both downstream and upstream coherent transmission and reception for a PON configuration. In this example, architecture  3700  is configured to transmit wavelength λ from a burst mode coherent receiver in the upstream direction, and broadcast and select in the downstream direction. 
       FIG. 38  is a schematic illustration of a coherent optics network architecture  3800 . Architecture  3800  is generally similar, in overall function and several structural elements, to architecture  3700 ,  FIG. 3700 . In the exemplary embodiment, architecture  3800  thus similarly includes a transmitter end  3802 , a receiver end  3804 , and a transport medium/fiber  3806 . Elements designated by the same label as elements in other drawings may be considered to have similar function and structure. Architecture  3800  differs though, from architecture  3700  in that, whereas architecture  3700  is configured to implement both downstream and upstream coherent transmission and reception 
     Accordingly, transmitter end  3802  may also represent a hub, and include a downstream coherent transceiver  3808 . In the exemplary embodiment depicted in  FIG. 38 , downstream coherent transceiver  3808  includes one or more of a downstream laser  3810 , a downstream coherent transmitter  3812 , and a downstream receiver  3814 . In this example, downstream receiver  3814  is a burst mode intensity receiver. In an embodiment, transmitter end  3802  further includes a three-port downstream optical circulator  3816 . 
     Receiver end  3804  includes a plurality of upstream coherent transceivers  3818 . Each of upstream coherent transceivers  3818  includes an upstream coherent receiver  3820  configured to receive the coherent transmission from downstream coherent transmitter  3812 , and an upstream intensity modulation transmitter  3822  configured to receive and modulate an upstream signal  3824  for transmission to downstream burst mode intensity receiver  3814 . In an embodiment, receiver end  3804  further includes a three-port upstream optical circulator  3826  for each coherent transceiver  3818 . Each of upstream coherent transceivers  3818  communicates over at least one short fiber  3828 , and are combined onto transport medium  3806  by a combiner  3830 . 
     In an exemplary embodiment, architecture  3800  is configured to implement an asymmetrical modulation scheme for wavelength λ, using coherent transmission and reception for downstream communications, and amplitude/intensity modulation and direct detection for upstream communications. In some embodiments, architecture  3800  is configured to implement, direct detection by external modulation. In other embodiments, direct detection is implemented by use of a reflective semiconductor optical amplifier (RSOA) configured to combine amplification and modulation functionality within a single device. Exemplary ONU structures for enabling such direct detection implementations are described further below with respect to  FIGS. 39-41 . 
       FIG. 39  is a schematic illustration of an exemplary ONU  3900 . In an exemplary embodiment, ONU  3900  is configured to implement external modulation and/or an external modulation scheme for upstream communications (e.g., at the receiver end of a communication network architecture). As depicted in  FIG. 39 , ONU  3900  includes one or more of an integrated coherent receiver (ICR)  3902 , an analog-to-digital converter (ADC)  3904 , a receiver digital signal processor (DSP)  3906 , an optical coupler  3908 , a local oscillator  3910 , and a modulator  3912 . 
     In exemplary operation, ONU  3900  is configured to receive a downstream optical signal  3914  (e.g., from a downstream transmitter at a hub) at ICR  3902 , which is then converted by ADC  3904 , processed by receiver DSP  3906 , and then output as reception data  3916 . In an exemplary embodiment, ICR  3902  is also configured to receive, through communication with optical coupler  3908 , a local oscillator signal from local oscillator  3910 . In further exemplary operation, modulator  3912  is configured to receive transmission data  3918 , modulate transmission data  3918  with the local oscillator signal from local oscillator  3910  (i.e., also through communication with optical coupler  3908 ), and output an upstream optical signal  3920 . 
       FIG. 40  is a schematic illustration of an exemplary ONU  4000 . In an exemplary embodiment, ONU  4000  is similar to ONU  3900 ,  FIG. 39 , in many structural and functional aspects. ONU  4000  though, differs from ONU  3900  in that ONU  4000  is configured to implement RSOA modulation and/or an RSOA modulation scheme for upstream communications. 
     As depicted in  FIG. 40 , ONU  4000  similarly includes one or more of an ICR  4002 , an ADC  4004 , a receiver DSP  4006 , an optical coupler  4008 , and a local oscillator  4010 . Different from ONU  3900 , instead of a modulator (e.g., modulator  3912 ,  FIG. 39 ), ONU  4000  implements an RSOA  4012  and an optical circulator  4014  (a three-port optical circulator, in this example). 
     In exemplary operation, ONU  4000  is similarly configured to such ICR  4002  is configured to receive both a downstream optical signal  4016  and the local oscillator signal from local oscillator  4010 . These signals are then converted by ADC  4004 , processed by receiver DSP  4006 , and output as reception data  4018 . In further exemplary operation, ONU  4000  may also be configured such that RSOA  4012  is configured to receive transmission data  4020 , and then amplify transmission data  4020  for combination, at optical circulator  4014 , with the local oscillator signal from local oscillator  4010  (i.e., through communication with optical coupler  4008 ), and output an upstream optical signal  4022 . 
     The exemplary configuration of ONU  4000  may, for example, be of particular advantageous use in implementations where a relatively larger power budget is desired/required (e.g., for longer distance transmissions). In comparison with an external modulator (e.g., ONU  3900 ), ONU  4000  may provide a lower cost option that reduces the relative LO power requirements. 
       FIG. 41  is a schematic illustration of an exemplary ONU  4100 . In an exemplary embodiment, ONU  4100  is similar to ONU  4000 ,  FIG. 40 , in many structural and functional aspects, but provides an alternative operational configuration for implementing RSOA modulation and/or an RSOA modulation scheme for upstream communications. That is, as depicted in  FIG. 41 , ONU  4100  similarly includes one or more of an ICR  4102 , an ADC  4104 , a receiver DSP  4106 , an optical coupler  4108 , a local oscillator  4110 , an RSOA  4112 , and an optical circulator  4114 . 
     In exemplary operation, ONU  4100  is configured to such ICR  4002  is configured to receive both a downstream optical signal  4116  (through optical coupler  4108 ) and the local oscillator signal directly from local oscillator  4110 . These signals are then converted by ADC  4104 , processed by receiver DSP  4106 , and output as reception data  4118 . In further exemplary operation, ONU  4100  is also configured such that RSOA  4112  receives transmission data  4120 , and then amplifies transmission data  4120  for combination, at optical circulator  4114 , with downstream optical signal  4116  (i.e., through communication with optical coupler  4008 , in this alternative configuration), and output an upstream optical signal  4022 . 
     The exemplary configuration of ONU  4100  may realize similar benefits to those achieved according to ONU  4000 , with respect to longer distance transmissions relative LO power requirements. ONU  4100  may realize still further advantages with respect to implementations where it is desirable to combine downstream and upstream optical signals, and particularly with respect to full duplex communications. 
     The foregoing embodiments illustrate and describe some particular schemes for implementing up/down-conversion in the digital domain to mitigate Rayleigh Backscattering in full duplex coherent optical systems. These embodiments are provided though, by way of example, and not in a limiting sense. That is, the person of ordinary skill in the art will appreciate that the architectures described herein are not limited to only the coherent signal generation and reception techniques described above. The present systems and methods may be advantageously implemented where different coherent signal generation and reception techniques and architectures are provided. One such alternative conversion architecture is described below with respect to  FIG. 42 . 
       FIG. 42  is a schematic illustration of an exemplary optical conversion architecture  4200 . Optical conversion architecture  4200  may be advantageously useful for either or both of digital up-conversion and digital down-conversion. In an exemplary embodiment, architecture  4200  is configured for conversion and complex path mixing (or splitting), and may be implemented with one or more of the embodiments described herein. 
     In the exemplary embodiment, architecture  4200  receives input signal  4202 . Input signal  4202  may represent, for example, a plurality of QAM symbols. Architecture  4200  further includes one or more of a digital filter  4204 , a mixer  4206 , a summing unit  4208 , a digital-to-analog converter (DAC)  4210 , a laser  4212 , and a modulator  4214 . In an embodiment, summing unit  4208  may be a summing amplifier, and DAC  4210  may be configured to convert the I and Q components into separate pathways before the respective components are modulated by modulator  4214 . 
     In exemplary operation, at mixer  4206 , the filtered QAM symbols are subject to e −2*rr*f     d     *t  in the case where conversion architecture  4200  is implemented for the upstream communication signals, or e 2*rr*f     u     *t , in the case where conversion architecture  4200  is implemented for the downstream communication signals. In the exemplary embodiment depicted in  FIG. 42 , architecture  4200  may thus operate considering the downstream frequency f d  (indicated, for example, as plot  4216 ) as being the same as the upstream frequency f u  (indicated, for example, as plot  4218 ), or f=f d =f u . 
       FIG. 43  is a graphical illustration  4300  depicting relative signal distributions  4302 ,  4304 ,  4306 ,  4308 ,  4310  for architecture  3700 ,  FIG. 37 . In an exemplary embodiment, signal distribution  4302  represents a spectral plot seen at a downstream coherent transmitter (e.g., downstream coherent transmitter  3712 ), signal distribution  4304  represents a spectral plot seen at an upstream coherent transmitter (e.g., upstream coherent transmitter  3724 ), signal distribution  4306  represents a spectral plot seen over a fiber link (e.g., transport medium  3706 ), signal distribution  4308  represents a spectral plot seen at an upstream coherent receiver (e.g., upstream coherent receiver  3722 ), and signal distribution  4310  represents a spectral plot seen at an downstream coherent receiver (e.g., downstream coherent receiver  3714 ). 
     In exemplary operation, signal distribution  4302  represents a substantially “pure” downstream transmission signal  4312  from the downstream coherent transmitter, and signal distribution  4304  represents a substantially pure upstream transmission signal  4314  from the upstream coherent transmitter. In the exemplary embodiment, both of transmission signals  4312 ,  4314  represent coherent optical signals centered around a frequency f, but where the respective upstream and downstream center frequencies are effectively frequency negatives of one another about a zero point on the frequency spectrum (e.g., f and “−f,” such as through operation of mixer  4206 ,  FIG. 42 ). Thus, at the fiber link, signal distribution  4306  depicts a relatively “clean” combination of both pure transmission signals  4312 ,  4314 . 
     Nevertheless, as indicated by signal distributions  4308 ,  4310 , the spectral distribution recovered at the respective upstream and downstream receivers is subject to a bleed over effect of the combined transmission signals  4312 ,  4314  on the fiber link. More particularly, although signal distribution  4308  indicates that the downstream coherent receiver receives downstream transmission signal  4312  substantially intact, the downstream coherent receiver also receives a downstream bleed over signal  4316  of upstream transmission signal  4314 . That is, downstream bleed over signal  4316  has a frequency distribution that substantially corresponds to a frequency distribution of upstream transmission signal  4314 , but at a significantly reduced amplitude. 
     Similarly, as indicated by a signal distribution  4310 , the upstream coherent receiver receives upstream transmission signal  4314  substantially intact, but also an upstream bleed over signal  4318  that substantially corresponds to the frequency distribution of downstream transmission signal  4312 , but a significantly lower amplitude. According to the exemplary systems and methods described herein, a full duplex communication architecture is advantageously able to transmit and receive the respective upstream and downstream coherent optical signals simultaneously over the same fiber link, but without substantial interference to one coherent transmission from the other. By effectively separating the downstream signal from the upstream signal (e.g., by operation of exemplary up-conversion and down-conversion techniques), the bleed over signal portions may be substantially ignored at the respective receiver. 
     Exemplary systems and methods of mitigating bleed over effects in full duplex communication networks are described in greater detail in co-pending U.S. patent application Ser. No. 16/177,428, filed Nov. 1, 2018, the disclosure of which is incorporated by reference herein. Additionally, the person of ordinary skill in the art will understand that the present embodiments are applicable to full duplex coherent communications with and without the bleed over effect, and that the embodiments herein are simplified for ease of explanation, and do not necessarily illustrate all components that may be implemented at the transmitter portion or the receiver portion, or within a hub or a fiber node. 
       FIG. 44  is a graphical illustration  4400  depicting relative signal distributions  4402 ,  4404 ,  4406 ,  4408 ,  4410  for architecture  3800 ,  FIG. 38 . Graphical illustration  4400  is therefore similar to graphical illustration  4300 ,  FIG. 43 , except that illustration  4400  depicts a case where coherent optical transmission is one-way, namely, in the downstream direction (e.g., by downstream coherent transmitter  3812 ). Transmission in the upstream direction is according to intensity modulation (e.g., by upstream intensity modulation transmitter  3822 ), in this example. 
     In an exemplary embodiment, signal distribution  4402  represents a spectral plot seen at the downstream coherent transmitter, signal distribution  4404  represents a spectral plot seen at the upstream intensity modulation transmitter, signal distribution  4406  represents a spectral plot seen over a fiber link (e.g., transport medium  3806 ), signal distribution  4308  represents a spectral plot seen at an upstream coherent receiver (e.g., upstream coherent receiver  3820 ), and signal distribution  4310  represents a spectral plot seen at an downstream receiver (e.g., downstream burst mode intensity receiver  3814 ). 
     In exemplary operation, signal distribution  4402  represents a substantially pure downstream transmission signal  4412  from the downstream coherent transmitter, and signal distribution  4404  represents a substantially pure upstream transmission signal  4414  from the upstream intensity modulation transmitter. In the exemplary embodiment, downstream transmission signal  4412  represents a coherent optical signal centered around a frequency f and upstream transmission signal  4414  represents an intensity modulated optical signal centered around the zero point on the frequency spectrum, with a bandwidth between the frequency f and its respective negative. At the fiber link, signal distribution  4406  depicts a combination of transmission signals  4412 ,  4414 , which are simultaneously transmitted in this example. 
     As indicated by signal distributions  4408 ,  4410 , the recovered spectral distribution at the respective upstream and downstream receivers in this embodiment is also subject to a bleed over effect of the combined transmission signals  4412 ,  4414  on the fiber link. That is, in signal distribution  4408 , the downstream coherent receiver receives downstream transmission signal  4412  substantially intact, but also receives a downstream bleed over signal  4416  of upstream transmission signal  4414  (i.e., substantially the same frequency distribution but lower amplitude). Similarly, in signal distribution  4410 , the upstream intensity modulated receiver is shown to receive upstream transmission signal  4414  substantially intact, but also an upstream bleed over signal  4418  substantially corresponding to the frequency distribution of downstream transmission signal  4412 , but at a lower amplitude. 
     The systems and methods described herein are therefore advantageously capable of resolving the deficiencies of conventional coherent transceiver systems that produce significant crosstalk. As described with respect to the embodiments herein, this crosstalk problem is substantially mitigated or essentially eliminated according to the present techniques. According to the innovative embodiments illustrated and described herein, an operator is able to realize significantly improved spectral efficiency (e.g., at least double) of existing fibers, whether for single channel or WDM channel operation, and without requiring significant regard to the transmission distance of the fiber(s), or to the particular wavelength(s) transmitted over the channel(s). 
     Full Duplex Coherent Optics—Single Fiber Connection 
     Recent studies indicate that approximately 20 percent of conventional cable access networks use single-fiber transmission topologies, similar to the embodiments described above, in which both downstream and upstream transmission to fiber nodes occurs on the same single strand of fiber. Because of the increasing trend to maximize capacity on each fiber strand, having dedicated fibers for each direction of transmission is becoming less desirable, and it is estimated that the percentage of single-fiber bidirectional access topologies will increase accordingly. 
     Therefore, to control the cost and fully utilize the existing infrastructure, the present systems and methods provide improved techniques for bidirectional transmission over a single fiber, utilizing coherent signals and technologies to better support single-fiber topologies and to facilitate redundancy of optical links. 
       FIG. 45  is a schematic illustration of a coherent optics network architecture  4500 . Architecture  4500  is similar in some respects to architecture  100 ,  FIG. 1 , and includes a hub  4502 , a fiber node  4504 , and an optical transport medium  4506  (e.g., an optical fiber) communicatively coupled therebetween. Architecture  4500  differs from architecture  100  though, in that optical transport medium  4506  includes a dedicated downlink fiber  4506   D  separate from a dedicated uplink fiber  4506   U . Thus, architecture  4500  may be representative of a standard conventional topology for implementing bidirectional transmission in the optical domain with a single laser. That is, conventional coherent optical bidirectional communication networks require two separate optical fibers. 
     In this example, architecture  4500  further includes at least one hub transceiver  4508  having a hub laser source  4510 , a hub transmitter  4512  configured to convert an electrical downlink signal into an optical downlink signal on downlink fiber  4506 , and a hub receiver  4514 . Architecture  4500  also includes at least one node transceiver  4516  having a node laser source  4518 , a node receiver  4520  configured to receive the optical downlink signal and recover the electrical downlink signal, and a node transmitter  4522  configured to convert an electrical uplink signal into an optical uplink signal for transmission to hub receiver  4514  over uplink fiber  4506   U . 
     Architecture  4500  may further include a first hub optical multiplexer  4524  for multiplexing the optical downlink signal from hub transmitter  4512  with optical downlink signals from other hub transmitters (not shown in  FIG. 45 ), and a second hub optical multiplexer  4526  for demultiplexing aggregated optical uplink signals sent from node transmitter  4522  and other node transmitters (not shown in  FIG. 45 ). Architecture  4500  may similarly include a first node optical multiplexer  4528  for demultiplexing the optical downlink signal to node receiver  4520 , and a second node optical multiplexer  4530  for multiplexing the optical uplink signal from node transmitter  4522  with optical uplink signals from other node transmitters. 
     In operation of this dual-fiber embodiment, both of transceivers  4508 ,  4516  perform at least two functional roles: (1) the optical signal source for the respective transmitter  4512 ,  4522 ; and (2) the reference local oscillator signal in the respective receiver  4514 ,  4520 . In this configuration, because the same wavelength from the same laser is used (i.e., laser  4510  and laser  4518  may operate at the same frequency, wavelength, modulation, etc., or be locked together), a second fiber is required to separate the uplink optical signals from the downlink optical signals. 
     Some conventional approaches avoid this dual-fiber approach through use of a single fiber for both the optical uplink and downlink, but with the optical uplink and downlink transmitted at different frequencies or wavelengths from one another, similar to HFC networks architectures described further above. This approach has conventionally required at least two lasers, disposed at respective opposing transceivers, and operating at different wavelengths to accomplish frequency/wavelength multiplexing. A single-fiber/two laser approach is described further below with respect to  FIG. 46 . 
       FIG. 46  is a schematic illustration of a coherent optics network architecture  4600 . Architecture  4600  is similar to architecture  4500 ,  FIG. 45 , and includes a hub  4602 , a fiber node  4604 , an optical transport medium  4606  (e.g., a single optical fiber, in this example) communicatively coupled therebetween, a hub transceiver  4608  having a first hub laser source  4610 ( 1 ), a second hub laser source  4610 ( 2 ), a hub transmitter  4612 , and a hub receiver  4614 , a node transceiver  4616  having a first node laser source  4618 ( 1 ), a second node laser source  4618 ( 2 ), a node receiver  4620 , and a node transmitter  4622 , a hub optical multiplexer  4624 , and a node optical multiplexer  4626 . 
     Architecture  4600  differs from architecture  4500  though, not only by the use of a single optical fiber instead of two, but also by the use of two lasers instead of one. In this configuration, the respective first laser source  4610 ( 1 ),  4618 ( 1 ) at transceivers  4608 ,  4616  may operate at a different frequency than the respective second laser source  4610 ( 2 ),  4618 ( 2 ) of that transceiver, but at the same frequency (or wavelength) as the first laser source of the other transceiver. In this configuration, optical multiplexers  4624 ,  4626  may be used to multiplex the different frequencies together at one end for optical transport, and demultiplex the multiplexed frequencies at the other end, e.g., following a wavelength management and allocation strategy for combining the different wavelengths over the same fiber. 
     Conventionally, this single-fiber/two laser approach has been helpful for increasing the signal capacity of a single fiber strand, but this approach has also been undesirably costly, not only in the monetary price of adding a second laser at each transceiver, but also from the significant increases to the power consumption, operational complexity, and transceiver footprint of a communication system according to architecture  4600 . 
     The embodiments herein overcome these conventional challenges through the implementation of innovative coherent optics architectures and techniques for the efficient use thereof. In an exemplary embodiment, a full duplex bidirectional coherent optics approach is achieved through leveraging two optical circulators on each end of a single fiber in an innovative configuration. In this unique configuration, the optical circulator is a passive, low-cost element (i.e., particularly in comparison with a second laser or a second fiber line installation), but nevertheless a directional device that functions analogously to a traffic roundabout for automobiles. That is, the optical circulator serves to re-route the optical path of incoming signals to different output directions. 
     The full duplex coherent optics embodiments described further herein realize the benefits of both conventional approaches described immediately above, but while also avoiding the costs and technical challenges associated with both approaches. More particularly, the full duplex coherent optics approaches herein advantageously achieve bidirectional transmission over a single fiber, but without requiring two lasers at the transceivers. The present full duplex coherent optics systems and methods realize bidirectional transmission using a single laser over a single-fiber connection in a coherent detection communication system. An exemplary full duplex coherent optics topology is described further below with respect to  FIG. 47 . 
       FIG. 47  is a schematic illustration of a coherent optics network architecture  4700 . Architecture  4700  is similar to architecture  4500 ,  FIG. 45 , and includes a hub  4702 , a fiber node  4704 , an optical transport medium  4706  (e.g., a single optical fiber) communicatively coupled therebetween, a hub transceiver  4708  having a hub laser source  4710 , a hub transmitter  4712 , and a hub receiver  4714 , a node transceiver  4716  having a node laser source  4718 , a node receiver  4720 , and a node transmitter  4722 , a first hub optical multiplexer  4724 , a second hub optical multiplexer  4726 , a first node optical multiplexer  4728 , and a second node optical multiplexer  4730 . 
     Architecture  4700  differs from architecture  4500  in that architecture  4700  further includes a hub optical circulator  4732  disposed between hub  4702  and optical transport medium  4706 , and which operably couples first hub optical multiplexer  4724  and second hub optical multiplexer  4726  with optical transport medium  4706 . Architecture  4700  further includes a node optical circulator  4734  disposed between node  4704  and optical transport medium  4706 , and which operably couples first node optical multiplexer  4728  and second node optical multiplexer  4730  with optical transport medium  4706 . 
     Embodiments following the unique configuration of architecture  4700  are uniquely suited to improving the reach of the cable network paradigm, which has conventionally focused on access environments having limited transmission distances. That is, unlike the backbone and metropolitan coherent optical network paradigms, the access network paradigm does not conventionally employ multiple directional optical amplifiers in cascade. As described above though, optical communication systems for coherent signal detection see higher OSNR sensitivity and higher tolerance to impairments from spontaneous Rayleigh backscattering (e.g., continuous reflection) and Fresnel reflection (e.g., discrete reflections) when compared with conventional intensity-modulated communication systems. 
     Furthermore, and as also described above, most existing conventional analog optics networks employ APCs to mitigate return loss from MPI, fusion or mechanical splices from jumper cables or optical distribution panels. Additionally, the threshold of the SBS nonlinear effect is significantly suppressed due to the phase-modulated signals reducing the optical carrier power and increasing the effective linewidth. Thus through effective implementation of this innovative dimension of direction-division multiplexing (DDM) in the optical domain, systems and methods according to the present embodiments are enabled to use essentially any coherent wavelength twice, i.e., once in each direction, thereby doubling the system capacity of each fiber. Furthermore, these full-duplex techniques are not wavelength-selective, i.e., the present embodiments are of particular value for both short and long wavelengths. The test results described herein still further demonstrate effective coverage for not only the entire C-Band, but effectively the entire fiber spectrum, in accordance with the different optical sources implemented within the system or network. 
       FIG. 48  is a schematic illustration of a coherent optics network architecture  4800 . In an exemplary embodiment, architecture  4800  represents a real-world testing implementation of the principles described above, and is similar to architecture  2100 ,  FIG. 21 . That is, architecture  4800  includes a first coherent transceiver  4802 , a second coherent transceiver  4804 , an optical fiber  4806 , a first transmitter  4808  at first coherent transceiver  4802 , a second transmitter  4810  at second coherent transceiver  4804 , a first receiver  4812  at first coherent transceiver  4802 , a second receiver  4814  at second coherent transceiver  4804 , a first optical circulator  4816 , and a second optical circulator  4818 . 
     In exemplary operation, architecture  4800  further included a variable attenuator  4820  on the receiver side with respect to each transmission direction of architecture  4800 . More specifically, in the examples depicted in  FIG. 48 , the disposition of variable attenuator  4820  at first receiver  4812  is labeled as architecture  4800 , and the disposition of variable attenuator  4820  at second receiver  4814  is labeled as architecture  4800 ′. In this manner, the real-world implementation of architecture  4800 / 4800 ′ measured, for test lengths of both 50 km and 80 km, the power penalty of the system with, and without, full duplex operation. According to this testing implementation, the power of both the received signal(s) and the returned impairment(s) were attenuated, and the penalty arose from the Rayleigh backscattering and Fresnel reflection along the entire link of architecture  4800 . The testing results are described further below with respect to  FIGS. 49A-B . 
       FIGS. 49A-B  are graphical illustrations of comparative plots  4900 ,  4902 , respectively, of bit error rate against received power for architecture  4800 ,  FIG. 48 . In a real-world testing implementation, plot  4900  represents the received power over an 50-km single fiber (SMF), and plot  4902  represents the received power over an 80-km single fiber (SMF). For both of plots  4900 ,  4902  the reflected power is measured as −34.7 dBm as the output power of the respective transmitter, TX 1  or TX 2 , is set to 0 dBm. As can be seen in the example depicted in  FIG. 49A , when compared with full duplex operation having single direction operation, an approximately 0.5 dB power penalty was observed for a fiber length of 50 km, and in the example depicted in  FIG. 49A , an approximately 1 dB power penalty was observed for a fiber length of 80 km. 
       FIG. 50  is a schematic illustration of a coherent optics network architecture  5000 . In an exemplary embodiment, architecture  5000  represents an alternative real-world testing implementation similar to architecture  4800 ,  FIG. 48 , and includes a first coherent transceiver  5002 , a second coherent transceiver  5004 , an optical fiber  5006 , a first transmitter  5008  at first coherent transceiver  5002 , a second transmitter  5010  at second coherent transceiver  5004 , a first receiver  5012  at first coherent transceiver  5002 , and a second receiver  5014  at second coherent transceiver  5004 . Different though, from architecture  4800 , architecture  5000  represents a three-circulator topology, in contrast with the two-circulator topology of architecture  4800 . 
     More specifically, architecture  5000  further includes a first optical circulator  5016 , a second optical circulator  5018 , and a third optical circulator  5020 , as well as an optical splitter  5022  disposed between first transmitter  5008  and first and second optical circulators  5016 ,  5018 , and an optical coupler  5024  disposed between first and second optical circulators  5016 ,  5018 , between first and third optical circulators  5016 ,  5020 , and between second and third optical circulators,  5018 ,  5020 . In this test example, architecture  5000  additionally included a variable attenuator (VA)  5026  at second transmitter  5010 , and a variable backreflector (VB)  5028  at first optical circulator  5016 . 
     In operation of testing architecture  5000 , variable backreflector  5028  served to measure the respective robustness of coherent signals at different return loss levels. That is, in contrast to the fixed reflection impairment testing setup of architecture  4800 , architecture  5000  implemented variable backreflector  5028  to purposely control the reflected power to the desired signal detection. As described further below with respect to  FIGS. 51A-B , a polarization controller (PC)  5030  was disposed between first optical circulator  5016  and optical coupler  5024 . According to this configuration of architecture  5000 , full duplex operation was effectively achieved by: (i) controlling the received power (i.e., the transmitted power-link loss) to be larger than the power sensitivity requirements of architecture  5000 ; and (ii) maintaining the OSNR (i.e., from reflection power) to be better than the OSNR sensitivity requirements of architecture  5000 . The testing results are described further below with respect to  FIGS. 51A-B . 
       FIG. 51A  is graphical illustration of a comparative plot  5100  of bit error rate against received power for architecture  5000 ,  FIG. 50 .  FIG. 51B  is graphical illustration of a comparative plot of required received power against reflected power for architecture  5000 . More specifically, plot  5100  depicts BER vs. power curves  5104  that were measured under different reflected power levels for 100G PM-QPSK signals, and plot  5102  depicts comparative power levels for the 80-km transmission case. In this implementation, the reflected power (e.g.,  FIG. 51B ) was measured before first receiver  5012  (RX 1 ), with the transmitter output power having been set to 0 dBm. 
     Further to this testing implementation, polarization controller  5030  was inserted in the testing setup (implementation of architecture  5000 ) to emulate a worst case scenario for polarization alignment. As can be seen from plot  5102 , the required receive power increases, in a nearly linear fashion, as the reflected power increases. More particularly, in the 80-km transmission case illustrated in  FIG. 51B , at region  5106  of plot  5102 , where the reflected power is at a −34.8 dBm value (i.e., approximately the value described above with respect to the test results depicted in  FIGS. 49A-B ), to maintain the required OSNR level of architecture  5000 , the required received power is approximately −26 dBm. It may be noted that, for this implementation, no error floor is observable, even with reflected power arriving at −24 dBm. 
       FIG. 52  is a schematic illustration of a coherent optics network architecture  5200 . Architecture  5200  represents a testing implementation similar to architecture  4800 ,  FIG. 48 , and includes a first coherent transceiver  5202 , a second coherent transceiver  5204 , an optical fiber  5206 , a first transmitter  5208  at first coherent transceiver  5202 , a second transmitter  5210  at second coherent transceiver  5204 , a first receiver  5212  at first coherent transceiver  5202 , a second receiver  5214  at second coherent transceiver  5204 , a first optical circulator  5216 , a second optical circulator  5218 , and a variable attenuator  5220 . Different though, from architecture  4800 , architecture  5200  locates variable attenuator  5220  on the transmitter side with respect to each transmission direction (i.e., labeled architecture  5200  at first receiver  5212 , and  5200 ′ at second receiver  5214 ). Similar to architecture  4800  though, architecture  5200  was also implemented for test lengths of both 50 km and 80 km, and the testing results are described further below with respect to  FIGS. 53A-C . 
       FIGS. 53A-C  are graphical illustrations of comparative plots  5300 ,  5302 ,  5304  of BER against received power for architectures  5200 ,  5202 ,  FIG. 52 . More particularly, similar to plots  4900 ,  FIG. 49 , plots  5300 ,  5302 ,  5304  demonstrate the BER performance of full duplex operation where the reflected power was measured as −34.8 dBm (i.e., at receivers  5012 ,  5014 ). Plots  5300 ,  5302 ,  5304  thus demonstrate the performance characteristics in the case where the reflected power is maintained at a relatively steady level, and the power of the desired signal (i.e., from transmitters  5008 ,  5010 ) is attenuated. 
       FIG. 54  is a schematic illustration of a coherent optics network architecture test subsystem  5400 . Subsystem  5400  is similar to portions of architecture  5000 ,  FIG. 50 , and refers to substantially equivalent components thereof by similar labels. More particularly, subsystem  5400  includes a hub coherent transmitter  5402  (e.g., TX 1 ) and a node coherent receiver  5404  (e.g., RX 2 ) in operable communication with one another over an optical fiber  5406 . In an embodiment, at least one EDFA  5408  is disposed along optical fiber  5406 . Subsystem  5400  further includes an optical splitter  5410  and an optical coupler  5412  in operable communication with optical splitter  5410  over a first transmission branch  5414  and a second transmission branch  5416 . In this example, subsystem  5400  further includes a first variable attenuator  5418  disposed between optical splitter  5410  and optical coupler  5412  along first transmission branch  5414 , and a second variable attenuator  5420  and a PC  5422  disposed between optical splitter  5410  and optical coupler  5412  along second transmission branch  5416 . 
     In operation of subsystem  5400 , reflected power was measured along optical fiber  5406  at a stage  5424  after optical coupler  5412 , that is, between optical coupler  5412  and node receiver  5404 , and second variable attenuator  5420  is disposed after optical splitter  5410 , but effectively disabled emulate coherent interference between the hub transmitter  5402  and node receiver  5404 . Testing results of subsystem  5400  are described further below with respect to  FIGS. 55 and 56 . 
       FIG. 55  is graphical illustration of a comparative plot  5500  of bit error rate against received power for architecture  5400 ,  FIG. 54 . More specifically, plot  5500  depicts BER vs. power curves  5502  that were measured under conditions where the power level of hub transmitter  5402  was zero dBm, and where the emulated reflection was attenuated, by first variable attenuator  5418 , by increasing values of 5, 10, 15, 20, and 25 dB. Similar to the example depicted in  FIG. 54 , inclusion of PC  5422  enables collection of valuable information regarding the worst polarization case. 
       FIG. 56  is graphical illustration of a comparative plot  5600  of required power against reflected power for architecture  5400 ,  FIG. 54 . More specifically, plot  5600  depicts comparative power levels within the required optical power at node receiver  5406  has a value of 4.5E-3. Similar to the embodiment depicted in  FIG. 51B , the required power level exhibits a relationship to the reflected power that is somewhat linear. 
       FIG. 57  is a schematic illustration of a coherent optics network architecture  5700 . Architecture  5700  represents a fiber-pair approach similar to architecture  4600 ,  FIG. 46 , and includes a hub  5702 , a fiber node  5704 , and an optical transport medium  5706  (i.e., a dedicated downlink optical fiber  5706   D , and a dedicated uplink optical fiber  5706   U , in this example) communicatively coupled therebetween. Different though, from architecture  4600 , hub  5702  further includes a hub transceiver  5708  having a hub laser source  5710 , a hub coherent modulator  5712  (i.e., in the place of a transmitter), and a hub coherent detector  5714  (i.e., in the place of a receiver). Similarly, node  5704  includes a node transceiver  5716  having a node laser source  5718 , a node coherent detector  5720 , and a node coherent modulator  5722 . Architecture  5700  further includes a hub optical multiplexer  5724 , and a node optical multiplexer  5726 . In operation, architecture  5700  functions in a manner substantially similar to the operation of architecture  4600 , except that architecture  5700  implements more coherent-detection-specific components than described above with respect to the more conventional transmitter/receiver pair implementations. 
       FIG. 58  is a schematic illustration of a coherent optics network architecture  5800 . Architecture  5800  is similar to architecture  5700 ,  FIG. 57 , and includes a hub  5802 , a fiber node  5804 , an optical transport medium  5806  (i.e., a single fiber, in this example) communicatively coupled therebetween, a hub transceiver  5808  having a hub laser source  5810 , a hub coherent modulator  5812  in communication with hub laser source  5810 , and a hub coherent detector  5814 . Similarly, node  5804  includes a node transceiver  5816  having a node laser source  5818 , a node coherent detector  5820 , and a node coherent modulator  5822 . Architecture  5800  further includes a hub optical multiplexer  5824 , and a node optical multiplexer  5826 . 
     However, different from architecture  5700 , within architecture  5800 , fiber node  5804  further includes a node local oscillator  5828  in operable communication with node coherent detector  5820 , and hub  5802  further includes a hub local oscillator  5830  in operable communication with hub coherent detector  5814 . In operation, architecture  5800  functions in a manner similar to the operation of the several architectures described above, except that architecture  5800  is configured to modulate two separate wavelengths, i.e., λ down  and λ up , on single fiber  5806 . That is, according to this innovative configuration of coherent detection techniques, implementation of local oscillators  5828 ,  5830  enable the modulation of two separate wavelengths in a single-fiber approach, but without requiring an additional laser, which is considerably more expensive than local oscillator, at each transceiver. 
       FIG. 59  is a schematic illustration of a coherent optics network architecture  5900 . Architecture  5900  is similar to architecture  5800 ,  FIG. 58 , and includes a hub  5902 , a fiber node  5904 , an optical transport medium  5906  (i.e., a single fiber, in this example) communicatively coupled therebetween, a hub transceiver  5908  having a hub laser source  5910 , a hub coherent modulator  5912  in communication with hub laser source  5910 , and a hub coherent detector  5914 . Similarly, node  5904  includes a node transceiver  5916  having a node laser source  5918 , a node coherent detector  5920 , and a node coherent modulator  5922 . Architecture  5800  further includes a hub optical multiplexer  5924 , a node optical multiplexer  5926 , and a node local oscillator  5928  in operable communication with node coherent detector  5920 . 
     However, different from architecture  5800 , within architecture  5900 , instead of a second local oscillator at hub  5902 , architecture  5900  instead includes a hub optical circulator  5930  between hub optical multiplexer  5924  and optical transport medium  5906 , and a node optical circulator  5932  between optical transport medium  5906  and node optical multiplexer  5926 . In operation, architecture  5900  functions in a manner similar to the architectures described above, except that architecture  5900  is configured to modulate a single wavelengths λ on single fiber  5906 . 
       FIG. 60  is a schematic illustration of a coherent optics network architecture  6000 . Architecture  6000  represents a dual-fiber, single-wavelength on a single-fiber transmission approach similar to the operation of architecture  5700 ,  FIG. 57 , and includes a hub  6002 , a fiber node  6004 , an optical transport medium  6006  (i.e., a downlink optical fiber  6006   D , and an uplink optical fiber  6006   u , in this example) communicatively coupled therebetween, a hub transceiver  6008  having a hub laser source  6010 , a hub coherent modulator  6012 , and a hub coherent detector  6014 , a node transceiver  6016  having a node laser source  6018 , a node coherent detector  6020 , and a node coherent modulator  6022 . Different though, from architecture  5700 , architecture  6000  does not require additional optical multiplexers/demultiplexers between hub  6002  and fiber node  6004 . 
       FIG. 61  is a schematic illustration of a coherent optics network architecture  6100 . Architecture  6000  represents a single-wavelength bidirectional system approach similar to the operation of architecture  4700 ,  FIG. 47 , and includes a hub  6102 , a fiber node  6104 , an optical transport medium  6106  (e.g., a single optical fiber) communicatively coupled therebetween, a hub transceiver  6108  having a hub laser source  6110 , a hub transmitter  6112 , and a hub receiver  6114 , a node transceiver  6116  having a node laser source  6118 , a node receiver  6120 , and a node transmitter  6122 , a hub optical multiplexer  6124 , a node optical multiplexer  6126 , a hub optical circulator  6128 , and a node optical circulator  6130 . Architecture  6100  differs from architecture  4700  in that, within the topology of architecture  6100 , optical circulators  6128 ,  6130  are disposed before the respective optical multiplexers  6124 ,  6126 . That is, hub optical circulator  6128  is located between hub  6102  and hub optical multiplexer  6124 , and node optical circulator  6130  is located between node  6104  and node optical multiplexer  6126 . 
       FIG. 62  is a schematic illustration of a coherent optics network architecture  6200 . Architecture  6200  is similar to architecture  300 ,  FIG. 3 , and represents an example of a real-world demonstration setup for an aggregation use case implementing DWDM transmission using multiplexers and demultiplexers. 
     For the actual setup, architecture  6200  included a hub  6202 , an aggregation fiber node  6204 , a transport medium/fiber  6206 , a hub coherent transceiver  6208 , a hub optical circulator  6210 , a node coherent transceiver  6212 , a node optical circulator  6214 , a plurality of hub transmitters  6216  (three hub transmitters, in this setup), a plurality of hub receivers  6218  (two hub receivers, in this setup), a plurality of node receivers  6220  (three node receivers, in this setup), a plurality of node transmitters  6222  (two node transmitters, in this setup), a first optical multiplexer  6224  at hub  6202 , a second optical multiplexer  6226  at node  6204 , a first optical demultiplexer  6228  at hub  6202 , and a second optical demultiplexer  6230  at node  6204 . 
     Different from architecture  300 , testing architecture  6200  further includes a hub optical modulation analyzer (OMA)  6232  disposed between hub optical circulator  6210  and first optical demultiplexer  6228 , and a node OMA  6234  disposed between node optical circulator  6214  and second optical demultiplexer  6230 . In the actual setup, transceivers  6208 ,  6212  utilized Acacia CFP/CFP2-DCO modules and EVBs, NeoPhotonics CFP-DCO module and EVBs, and OMAs  6232 ,  6234  utilized Keysight N4392A Optical Modulation Analyzers. Not shown, in  FIG. 62 , but also used in the demonstration setup, are 3 dB optical couplers, optical patch cables, an optical spectrum analyzer (OSA) for tracking and displaying two different directions on the same screen, optical power meters, display monitors for each of the optical spectra, the downstream BER, and the upstream BER, power supplies (e.g., 12V, 30V), and graphical user interfaces (GUIs) for the respective coherent CFP modules. 
     From this setup, testing architecture  6200  was able to demonstrate 100 G and 200 G data rates, for different modulation formats, implementing 50/100 GHz spacing, and for a number of wavelengths in the downstream direction (e.g., λ 1 , λ 2 , λ 3 ) that is different than the number of wavelengths (e.g., λ 1 , λ 2 ) in the upstream direction. 
       FIG. 63  is a schematic illustration of a coherent optics network architecture  6300 . Architecture  6300  is similar to architecture  6200 ,  FIG. 62 , and represents an example of a real-world demonstration setup for an aggregation DWDM channel use case using multiplexers and demultiplexers. Architecture  6300  differs though, from architecture  6200 , in that architecture  6300  was configured to transmit the same number of wavelengths (e.g., λ 1 , λ 2 ) in both the upstream and downstream directions. 
     Architecture  6300  includes a hub  6302 , an aggregation fiber node  6304 , a transport medium/fiber  6306 , a hub coherent transceiver  6308 , a hub optical circulator  6310 , a node coherent transceiver  6312 , a node optical circulator  6314 , a plurality of hub transmitters  6316  (two, in this setup), a plurality (e.g., two) of hub receivers  6318 , a plurality (e.g., two) of node receivers  6320 , a plurality (e.g., two) of node transmitters  6322 , a first optical multiplexer  6324  at hub  6302 , a second optical multiplexer  6326  at node  6304 , a first optical demultiplexer  6328  at hub  6302 , a second optical demultiplexer  6330  at node  6304 , a hub OMA  6332  disposed between hub optical circulator  6310  and first optical demultiplexer  6328 , and a node OMA  6334  disposed between node optical circulator  6314  and second optical demultiplexer  6330 . Operational results of architecture  6300  demonstrated that the present innovative principles are useful and advantageous for single-fiber approaches aggregating the upstream and downstream bidirectional wavelengths equally, or unequally. 
     Coherent Optics for Access Applications and Cable Deployment 
     The above principles represent significant improvements to conventional communication systems where the coexistence of different signals, and of different signal types, has been particularly challenging. The present full duplex coherent optics solutions overcome these challenges. 
     Cable access networks, for example, have been recently undergoing significant technological and architectural changes, which have been driven by an ever-increasing residential data service growth rate, as well as an increasing number of services types being supported (e.g., business services, cellular connectivity, etc.). Digital fiber technologies and distributed access architectures for fiber deep strategies have provided an improved infrastructural foundation for cable operators to deliver service quality to end users, but conventional technologies have not been capable of fully utilizing these structural improvements. User demand has been outpacing the structural capacity of the recent infrastructural improvements. 
     These challenges have been significant in the cable-specific fiber access environment, which includes only a few fibers available for a 500-household serving area. Because the cost of adding more fibers is considerably expensive, the present inventors have thus developed a number of innovative architectural and processing solutions based on coherent optics to significantly expand the capacity of the existing fibers. The present systems and methods thus demonstrate further long-term fiber access connectivity solutions of significant value in the next-generation cable access network paradigm, as well as point-to-point (P2P) coherent optics topologies in particular. The embodiments herein enable a smooth transition from digital coherent long-haul and metro solutions to coherent access network applications, while also enabling the network operators to best leverage existing fiber infrastructures to withstand the exponential growth in capacity and services for residential and business subscribers without significantly increasing the subscriber costs. 
     Conventional cable operators seeking to deploy coherent optics into their access networks (typically, 10 G networks) have heretofore had only two options: (1) deploy coherent optics on the existing 10G system; or (2) build a new coherent-only connection. Green field deployments of such coherent systems on fibers have been considered optimal, because such deployments would not require compensation devices, such as dispersion compensation modules (DCMs) and other wavelength channels. However, in practice, for such upgrades to be cost-effective, only one or a few channels may be upgraded in many brown field installations, depending on capacity demand. Therefore, for networks that already implement WDM analog DOCSIS and/or 10 G on-off keying (OOK), the different services would have to coexist with the new coherent system to support a hybrid scenario over the same fiber transmission. 
     Such hybrid configurations, however, have not been fully implemented due to lack of operator ability to support such hybrid systems for 100 G, and also due to particular concerns, such as cross-phase modulation (XPM) impairments in the fiber nonlinear regime. The embodiments described herein not only offer solutions to these concerns in the field, but also present extensive experimental verifications, under various coexistence scenarios, as proof of concept, while also providing useful operational and deployment guidance for such use cases. The present systems and methods thus provide effective techniques for simultaneous bi-directional transmission over single fiber and single wavelength using full duplex coherent optics, to effectively double the capacity of existing fibers in a coherent optics-based fiber distribution network. The present embodiments further successfully analyze and quantify impairments such as ORL and optical reflections, including all discrete reflections (Fresnel) and continuous reflections (backscatter), such that these impairments may be addressed or mitigated by network operators. 
     Coherent optics/coherent detection technology provides high receiver sensitivity through coherent amplification by the local oscillator. However, commercial use has been hindered by the additional complexity of active phase and polarization tracking. Additionally, the emergence of the cost-effective EDFA as an optical pre-amplifier reduced the urgency to commercialize coherent detection technology because the EDFAs, together with improved WDM techniques, served to extend the reach and capacity of many networks. Traffic demand, combined with per-bit/per-Hz cost reduction requirements, spectral efficiency increases, and advancements of CMOS processing nodes and DSP, have since served to exhaust the expanded capacity and reach of these extended networks. The coherent optics technology solutions herein avoid these limits. 
     Commercial coherent optical technology was first introduced in long haul applications to overcome fiber impairments that required complex compensation techniques when using direct detection receivers. The first-generation coherent optical systems were based on single-carrier polarization division multiplexed quadrature phase shift keying (PDM-QPSK) modulation formats, and the achieved spectral efficiency (SE) thereof was 2 bits/s/Hz over conventional 50-GHz optical grids. Commercial coherent technology has thus increased system capacity to approximately 10 Tb/s in a fiber C-band transmission window. Coherent solutions have since further moved from long haul to metro and access networks by leveraging the further developments in CMOS processing, design complexity reductions, and reduced costs for opto-electronic components. 
     Coherent optics technology and coherent detection communication systems therefore represent a clean progression from this initial long-haul technology development, into coherent optical access networks, and particularly in light of recent developments of application-specific integrated circuit (ASIC) DSP chips and corresponding optical modules. The present inventors contemplate implementation of programmable and comprehensive coherent DSPs into the several embodiments described herein, which DSPs may be capable of processing data rates in the range of 100 G-600 G per single wavelength (or greater), while supporting higher modulation formats such as 32/64 QAM (or greater) and high net coding gain (NCE) FEC. As described herein, additional solutions are provided for reducing the system/network power consumption, thereby enabling operators to meet the size and cost requirements of particular access applications. 
     The present coherent detection techniques further enable, for access networks, superior receiver sensitivity that allows for extended power budgets, as well as high spectral efficiency for dense WDM (DWDM). Moreover, the use of high-order modulation formats enables efficiently utilizing the spectral resource and benefiting futureproof network upgrades. In the cable access environment, coherent optics allows operators to best leverage the existing fiber infrastructure to deliver vastly increased capacity with even longer distances. 
     However, coherent technologies in long-haul optical systems have utilized best-in-class (i.e., the most expensive) discrete photonic and electronic components, including the latest DAC/ADC and DSP ASIC based on the most recent CMOS processing nodes. Coherent pluggable modules for metro solutions, on the other hand, have progressed from the CFP to the CFP2 form factor to realize a smaller footprint, lower cost, and lower power dissipation. Nevertheless, this progression in the metro paradigm is still considered over-engineered, too expensive, and too power hungry, to be successfully implemented for interoperability in the access network paradigm. The access network is a very different environment from the long haul and metro environments, and will often require hardened solutions for remote site locations where the temperature is not controlled. Standardization and interoperability are also important considerations in the access network paradigm, but standardization in the optical community has been driven mainly by short-reach metro/aggregation applications, where optical performance is not a differentiator. 
     The present systems and methods though, enable easy standardization and smooth interoperability in the access network paradigm, and particularly with respect to P2P coherent optics communication systems which enable support to the cable industry for the growing requirements of broadband access, such as, for example, toward Node+0 architectures and related substantial increase in the volume of optical connections to intelligent nodes resulting from this evolution. The present solutions are thus also fully compatible with recent short-reach coherent optical standardizations developed by: (i) the Optical Internetworking Forum (OIF), such as DWDM interfaces in DCI applications with reaches up to 120 km with multi-vendor interoperability; and (ii) IEEE, such as coherent optics for non-amplified applications beyond 10-km distance. 
     In an exemplary embodiment, the present coherent optics technology solutions may be leveraged in the cable environment as a means of multi-link aggregation, and/or through direct edge-to-edge connectivity to desired end-point. An exemplary edge-to-edge connectivity environment is described further below with respect to  FIG. 64 . 
       FIG. 64  is a schematic illustration of an optical communications network system  6400 . In an embodiment, system  6400  includes at least one core network  6402 , a hub  6404 , a plurality of aggregation nodes  6406 , and a plurality of end users  6408  (e.g., high speed wavelength services, remote PHY, remote MAC-PHY, backhauls, remote FTTH backhauls, 4G/5G wireless services, etc.). Aggregation nodes  6406  operably connect with hub  6404  over long (e.g., 20-80 km or greater) fibers  6410 , and with end users  6408  over short (e.g., typically less than 3-5 km) fibers  6412 . The present inventors therefore contemplate that, in accordance with capacity growth trends, it is likely that the aggregation use cases will initially out-number direct edge-to-edge connectivity use cases. The present aggregation use cases, for example, support any Distributed Access Architecture (DAA), including Remote PHY, Remote MAC-PHY, and Remote optical line terminal (OLT) architectures. 
     In the aggregation use case, a device host, such as an Optical Distribution Center (ODC) or aggregation node is configured to terminate a downstream P2P coherent optic link originating from the headend or hub, and then output multiple optical or electrical Ethernet interfaces, operating at lower comparative data rates, to connect devices that are either co-located with the ODC, and/or exist within or past a deeper secondary hub in the network. In some embodiments, such aggregation and disaggregation functionality is performed by a router, an Ethernet switch, or a muxponder depending on the relevant DOCSIS protocols, the PON system requirements, the business traffic demand, costs, scalability/flexibility/reliability requirements, or other operational considerations. In the example depicted in  FIG. 64 , each primary hub  6402  may be configured to support multiple (e.g., approximately 60 or greater) aggregation nodes  6406  for different services. 
     Commercial services have also been a rapidly growing and high revenue segment in the cable industry. The present coherent technology solutions therefore fully support the growing cable service portfolio for business connectivity, cellular backhaul, and wireless access point connectivity (including 5G connectivity). These services demand very high bandwidths, as well as robustness and flexibility, for supporting a diversity of service levels. The coherent optics technology embodiments described herein advantageously and fully address these growing service requirements for this market segment. 
     As described above, related patents to the present inventors demonstrate how direct wavelength services may overlay with aggregation connections, such as with 10 G/25 G intensity-modulated signals and/or 100 G coherent signals. This type of overlay is thus lambda/wavelength deep for edge-to-edge services. In the use cases described herein, the coherent optic links are effectively terminated at the edge customer, and then the WDM multiplexer/demultiplexer at the headend/hub may be used for aggregating multiple P2P optical links onto a single fiber. In an exemplary embodiment, such WDM systems may include a hybrid system having a mix of data rates and modulation formats. 
     The capability for coexistence with legacy optical channels is a significant advantage realized according to the present systems and methods. The following description illustrates several testing embodiments that verify these coexistence advantages. As described above, the expectation from cable operators has been that it is preferable to add more 100 G coherent services, by using free channels in the WDM grid, but without impacting existing services, that is, essentially creating a hybrid 10 G/100 G network in which multiple services coexist. However, 10 G signals based on analog amplitude modulation (AM) or OOK have a much higher power density than coherent 100 G, which causes the analog AM signals to have a much greater impact on the refractive index of the system for nonlinear effects, such as XPM and four-wave mixing (FWM). Additionally, crosstalk penalties in ITU-T grid networks having mixed rates have led to system degradation due to optical Mux/DeMux in-band residual power or non-uniform channel grid allocation in DWDM systems. 
     The following description therefore demonstrates the effectiveness of the present systems and methods to enable network operators to sufficiently support 100 G applications on existing networks infrastructures. The following experimental verification results quantitatively explore performance challenges in the proposed coexistence applications. Previous performance comparisons, due to the limited availability of the number of analog optical channels, tested three co-propagating analog DOCSIS channels along with single coherent channels. Nevertheless, the results of these previous experiments showed that coherent optics transmissions remain robust, even in close proximity to the much stronger analog optical carriers, and also that coherent optical carriers impose a negligible impact on the proximate analog optical carriers. The following experimental results therefore expand upon these previous discoveries, through the innovative testing techniques described herein to evaluate the transmission performance of fully-loaded coexisting systems over longer transmission distances. 
       FIGS. 65A-B  are graphical illustrations of comparative optical spectrum plots  6500 ,  6502 , respectively. More specifically, optical spectrum plot  6500  represents power over wavelength for a channel corresponding to CH 21 , and optical spectrum plot  6502  represents power over wavelength for a channel corresponding to CH 24 . Optical spectrum plots  6500 ,  6502  thus demonstrate the effectiveness of DWDM components for Mux and Demux  3 . 1 . 
     In the examples illustrated in  FIGS. 65A-B , results are depicted from testing three different kinds of optical multiplexors/de-multiplexors have been evaluated in the testing: (1) an 8-port thin-film filter (TFF); (2) a 40-port array waveguide grating (AWG)-based optical multiplexer/demultiplexer; and (3) a 48-port AWG-based optical multiplexer/demultiplexer. The TFF uses concatenated interference filters, each of which is fabricated with a different set of dielectric coatings designed to pass a single wavelength. Optical spectrum plot  6500  includes a first sub-plot  6504  illustrating the optical spectrum using the 8-port TFF, a second sub-plot  6506  illustrating the upstream optical spectrum using the 40-port AWG, and a third sub-plot  6508  illustrating the upstream optical spectrum using the 48-port AWG. Similarly, optical spectrum plot  6502  includes a first sub-plot  6510  for the 8-port TFF, a second sub-plot  6512  for the 40-port AWG, and a third sub-plot  6514  for the 48-port AWG. 
     As can be seen from plots  6500 ,  6502 , the TFFs demonstrate better optical performance, in terms of flatter passband ripple and higher isolation in neighboring channels, in comparison with the AWGs. The TFFs also work effectively in the case of low channel counts, and particularly for analog WDM systems, but present challenges in the case of higher channel counts and narrower spacing due to the TFFs requiring several hundred layers of coating, and therefore more strict error control. In contrast to TFFs, AWG devices use a parallel multiplexing approach based on planar waveguide technology, and have an advantage over TFFs in that the cost of an AWG devices is not dependent on wavelength count, which renders the AWG significantly more cost-effective for high channel count applications. Accordingly, existing long-haul coherent DWDM systems typically use AWG for Mux and DeMux implementations. In the experimental setup to achieve the results depicted in  FIGS. 65A-B , the insertion loss was approximately 1.5 dB for the TFFs, and approximately 3.5 dB for the 40-port AWG, and described further below with respect to  FIG. 66 . 
       FIG. 66  is a schematic illustration of a coherent optics network architecture test subsystem  6600 . Subsystem  6600  is similar to portions of the several network architectures described herein, and refers to substantially equivalent components thereof by similar labels. More particularly, subsystem  6600  includes a coherent transmitter side  6602  (e.g., TX) and a coherent receiver side  6604  in operable communication with one another over an optical fiber  6606  (e.g., an SMF-28). Transmitter side  6602  includes an 8-port multiplexer  6608  (e.g., TFF) for analog channels, and a 40-port multiplexer  6610  (e.g., AWG) for coherent channels, which operably communicate with an optical combiner  6612  (e.g., 3 dB, in this example) before amplification by an EDFA  6614  disposed between combiner  6612  and optical fiber  6606 . Receiver side  6604  similarly includes an 8-port demultiplexer  6616  for the analog channels, and a 40-port demultiplexer  6618  for the coherent channels, which operably communicate with an optical splitter  6620  (e.g., also 3 dB). 
     In operation, subsystem  6600  demonstrates an effective analog-plus-coherent coexistence implementation using an 8-port Mux/DeMux (e.g., 8-port multiplexer  6608 /demultiplexer  6616 ). In this experimental setup of a first case with 40-port multiplexer  6610 /demultiplexer  6618  (the topology to substitute or add 48-port AWG Mux/DeMux is not significantly different), all of eight analog DOCSIS channels (up to 1.2 GHz, with channel labels depicted in  FIG. 66  corresponding to the standard ITU-T wavelength grid) are multiplexed through TFF-based 8-port multiplexer  6608 , while four coherent channels are multiplexed through AWG-based 40-port multiplexer  6610  with a 100-GHz optical grid spacing. Both of these different types of multiplexed signals are then combined at optical combiner  6612 . 
     The nonuniform selection of the analog wavelength plan thus serves to mitigate fiber nonlinear impairments, and FWM in particular. The selection of the coherent wavelength plans, on the other hand, considers which criteria may create the worst nonlinear crosstalk impairments. Accordingly, once the combined analog-plus-coherent signals are amplified by EDFA  6614  (e.g., configured for long-distance analog signal amplification, and a maximum output power of approximately 18 dBm), and the amplified signals are transmitted over optical fiber  6606  (e.g., 80-km SMF), and then split at optical splitter  6620  prior to reaching the respective demultiplexer  6616 ,  6618  for analog and coherent channels. The launched and received power of both types of channels, as well as the gain/attenuation of optical devices along the optical link of subsystem  6600 , are shown below in Table 2. For this experimental setup, a 10-dB power difference was set between the analog and coherent channels. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Tx 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 Output 
                 Mux 
                 Coupler 
                 EDFA 
                 Fiber 
                 Splitter 
                 DeMux 
                 Received 
               
               
                 Signal 
                 Power 
                 Loss 
                 Loss 
                 Gain 
                 Attenuation 
                 Loss 
                 Loss 
                 Power 
               
               
                 Type 
                 (dBm) 
                 (dB) 
                 (dB) 
                 (dB) 
                 (dB) 
                 (dB) 
                 (dB) 
                 (dBm) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Analog 
                 9.5 
                 −1.5 
                 −3 
                 +2.5 
                 −5.5  
                 for 
                 −3 
                 −1.5 
                 −2.5  
                 for 
               
               
                   
                   
                   
                   
                   
                 26.1  
                 km; 
                   
                   
                 26.1  
                 km; 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 −6.5  
                 for 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 51.8  
                 km 
               
               
                 Coherent 
                 −1 
                 −3.5 
                 −3 
                 +5 
                 −9.5  
                 for 
                 −3 
                 −3.5 
                 −14.4  
                 for 
               
               
                   
                   
                   
                   
                   
                 51.8  
                 km 
                   
                   
                 26.1 
                 km; 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 −18.4  
                 for 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 51.8  
                 km 
               
               
                   
               
            
           
         
       
     
       FIGS. 67A-B  are graphical illustrations of comparative optical spectrum plots  6700 ,  6702 , respectively. More specifically, optical spectrum plot  6700  represents power over wavelength for all analog and coherent signals transmitted over test subsystem  6600 ,  FIG. 66 , before and after optical amplification. That is, optical spectrum plot  6700  includes a first sub-plot  6704  illustrating the optical spectrum of the hybrid channels prior to amplification by EDFA  6614 , and a second sub-plot  6706  illustrating the optical spectrum of the combined channels after amplification by EDFA  6614 . Similarly, optical spectrum plot  6702  represents power over wavelength for the hybrid channels before and after optical fiber transmission, and includes a first sub-plot  6708  illustrating the optical spectrum of the hybrid channels prior to optical transmission over optical fiber  6606 , and a second sub-plot  6710  illustrating the optical spectrum of the transmitted channels after transmission over optical fiber  6606 . In both of plots  6700 ,  6702 , it may be seen that CH 23 , CH  25 , CH  26 , and CH 27  are coherent channels having wider spectra and lower power in comparison with the to eight analog channels. 
       FIG. 68A  is a graphical illustration of a comparative plot  7000  of modulation error ratio (MER) against carrier frequency for the analog channels transmitted by subsystem  6600 ,  FIG. 66 . More specifically, for the system setup of subsystem  6600 , the MER transmission performance of analog channel CH  52  illustrates that a negligible penalty is observed after 26.3-km or 51.8-km fiber transmission, evaluating the respective impact with and without coherent signals on the analog channel.  FIG. 68A  illustrates the performance effect on only CH  52  for ease of explanation, and not in a limiting sense. Similar performance test results were obtained by evaluating the other analog channels under the substantially similar transmission conditions, i.e., with and without coherent channel signals over the same fiber. 
       FIG. 68B  is a graphical illustration of a comparative plot  7002  of BER against modulation format for the coherent channels transmitted by subsystem  6600 ,  FIG. 66 . More specifically, and also for the system setup of subsystem  6600 , the BER transmission performance of coherent channel CH  26  illustrates that only a minor BER difference is observed for 8 QAM-based and 16 QAM-based 200 Gbps channels, at 0-dBm transmitter output power. That is, in contrast to plot  7000 , which illustrates the impact of coherent channels on analog channels, plot  7002  illustrates the reverse case of the impact of analog channels on coherent channels. Nevertheless, when compared with back-to-back coherent signal sensitivity, a power penalty less than 0.5-dB is found for the coherent transmission having an analog channel overlay through amplification by EDFA  6614  over the same fiber (e.g., optical fiber  6606 ). 
       FIG. 69  is a graphical illustration of a comparative optical spectrum plot  6900 . More specifically, optical spectrum plot  6900  represents power over wavelength for an analog-plus-OOK-plus-coherent coexistence using a 16-port multiplexer. As depicted in  FIG. 69 , plot  6900  illustrates the optical spectrum distribution of analog signals  6902  (e.g., eight channels), coherent CFP2 signals  6904  (e.g., two coherent CFP2-DCO channels), coherent 400 G signals  6906  (e.g., two coherent channels), and NRZ signals  6908  (e.g., two 10 G NRZ channels), all coexisting in a transmission over the same optical fiber. 
     As shown in  FIG. 69 , an effective coexistence hybrid scheme is provided by the present systems and methods, which is shown to include essentially all the major modulation formats in use today, as well as services under different data rates/baud rates. To generate the test results depicted in  FIG. 69 , a pair of 16-channel TFF-based WDMs were used for channel multiplexing and demultiplexing. 
     Further to this testing example, the corresponding input power level, i.e., the power at the output port of each transmitter before entering the WDM Mux, was measured for several typical operating conditions, and for different detection schemes. The results of these measurements, namely, the optical transmitted power of the analog, OOK, and coherent signals, are shown below Table 3. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Application  
                 Channel  
                 Input Power  
               
               
                 Scenarios 
                 Index 
                 (dBm) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 DOCSIS 
                 21 
                 9.64 
               
               
                 Analog 
                 24 
                 9.48 
               
               
                   
                 28 
                 9.43 
               
               
                   
                 33 
                 9.64 
               
               
                   
                 39 
                 9.48 
               
               
                   
                 52 
                 9.11 
               
               
                   
                 60 
                 9.01 
               
               
                   
                 62 
                 8.92 
               
               
                 400G 
                 22 
                 2.68 
               
               
                 Coherent 
                 61 
                 3.15 
               
               
                 CFP2 
                 36 
                 0.08 
               
               
                 Coherent 
                 44 
                 −0.14 
               
               
                  10G  
                 26 
                 −0.89 
               
               
                 NRZ 
                 57 
                 −0.75 
               
               
                   
               
            
           
         
       
     
     For the measurements shown in Table 3, the power level for the analog channels was set to approximately 9.5 dBm, and the power level for the 56-GBaud 400 G coherent channels was set to approximately 3 dBm. To improve the spectral efficiency and confine the optical power within each WDM channel, in some embodiments, the coherent signals may be further shaped using square root raised cosine filters. 
       FIG. 70  is a graphical illustration of a comparative plot  7000  of normalized power penalty against data rate. More specifically, plot  7000  represents an alternative demonstration for the analog-plus-OOK-plus-coherent coexistence scheme depicted in  FIG. 69 , and also using a 16-port multiplexer. In the example depicted in  FIG. 70 , plot  7000  illustrates the normalized power requirements (i.e., penalties) for net data rates of 200 G, 300 G, and 400 G. Plot  7000  further illustrates these experimental results, with respect to coherent channels, for back-to-back (B2B) operation, and for 50-km transmission with and without coexistence of analog-plus-NRZ channels. As can be seen from  FIG. 70 , the performance difference is insignificant for the analog channels, in comparison with coexistence schemes using an 8-port Mux. That is, a power penalty of less than 0.6 dB was observed in the coexistence scenarios relative to the non-coexistence scenarios. 
     Coherent Optics Access Interfaces 
     The typical optical access network includes some components that are generally considered to be fundamental. In this regard, “fundamental” refers to those components that have been heretofore most widely used in the access network, and which are expected to play a significant role in the access networks of the future. Such components are generally grouped into three categories: (1) the optical transmitter; (2) the optical channel; and (3) the optical receiver. Optical transceivers may typically include both an optical transmitter and an optical receiver. These components are described further below with respect to the following embodiments. 
       FIG. 71  is a schematic illustration of a transceiver  7100  having a dual optical interface structure. In the embodiment depicted in  FIG. 71 , transceiver  7100  is disposed with respect to an electrical client side  7102  (or host side  7102 ) and an optical line side  7104 . In this example, transceiver  7100  includes an electrical receive interface  7106  and an electrical transmit interface  7108  at client side  7102 , and optical transmit interface  7110  and an optical receive interface  7112  at line side  7104 . That is, client side/host side  7102  corresponds to electrical interfaces  7106 ,  7108 , and line side  7104  corresponds to optical interfaces  7110 ,  7112 . In an exemplary embodiment, transceiver  7100  further includes a management interface  7114  in communication with a control layer  7116  of transceiver  7100 . 
     Dual optical interface transceiver  7100  thus utilizes separate optical interfaces  7110 ,  7112  capable of operable communication with the transmit and receive functions of transmitter and receiver portions  7118  of transceiver  7100 , respectively. In an embodiment, transceiver  7100  further includes a framing unit  7120  in communication with electrical interfaces  7106  and  7108  at host side  7102  (e.g., a 100 Gb Ethernet physical coding sublayer (PCS)  7122  and/or an optional optical transport unit (OTN)  7124 ). 
       FIG. 72  is a schematic illustration of a transceiver  7200  having a single optical interface structure. In the embodiment depicted in  FIG. 72 , transceiver  7200  is similar to transceiver  7100 ,  FIG. 71 , and is similarly disposed with respect to an electrical client/host side  7202  and an optical line side  7204 , and includes both an electrical receive interface  7206  and an electrical transmit interface  7208  at client side  7202 . Transceiver  7200  differs from transceiver  7100  though, in that transceiver  7200  includes a single optical interface  7210  at line side  7204 . Optical interface  7210  is further able to communicate with a directional element  7212  of transceiver  7200 , and is also capable of functionally directing transmitted and received optical signals between the respective transmitter and receiver portions  7214  of transceiver  7200 . 
     In an exemplary embodiment, transceiver  7200  still further includes a management interface  7216  in communication with a control layer  7218  of transceiver  7200 . In some embodiments, transceiver  7200  also similarly includes a framing unit  7220  in communication with electrical interfaces  7206  and  7208  at host side  7202  (e.g., a 100 Gb Ethernet PCS  1122  and/or an optional optical transport unit  7224 ). 
     Implementation of the single optical interface structure of transceiver  7200  is particularly useful in the case where only a single fiber is available from the hub to the node. In such cases, signal direction functionality may be incorporated into transceiver  7200  (e.g., by directional element  7212 ) to enable the transmitter optical signal (i.e., Opt. Tx) to be directed to the single optical interface (i.e., optical interface  7210 ), while enabling the receiver optical signal (i.e., Opt. Rx), incoming through the single optical interface, to be directed to the respective receiver of transmitter/receiver portions  7214 , with a performance impact that is expected to be negligible, and while also utilizing the same communication frequency in both the transmit and receive directions. 
     Further architectural considerations using a single interface transceiver are described functionally below with respect to  FIGS. 73 and 74 , which are provided for illustration purposes, but not in a limiting sense. Other transmitter and receiver implementations, used for similar purposes, may follow different sequences and different feedback dependencies. 
       FIG. 73  is a functional schematic illustration of a transmitter  7300 . In an exemplary embodiment, transmitter  7300  is configured to perform relevant transmitter functions occurring, for example, in a coherent optical transceiver (e.g., transceiver  7100 ,  FIG. 71 , transceiver  7200 ,  FIG. 72 ) from the electrical input on the host side (e.g., host side  7102 ,  FIG. 71 , host side  7202 ,  FIG. 72 ) to the optical output on the line side (e.g., line side  7104 ,  FIG. 71 , line side  7204 ,  FIG. 72 ). 
     In exemplary operation, transmitter  7300  includes one or more functional units, which may operate in the order listed, or in some cases in a different order, and which may be individually implemented by hardware elements, software modules, or by combinations of hardware and software. In some embodiments, transmitter  7300  may include fewer functional units, or additional functional units to those described herein, without departing from the scope of this description. 
     Transmitter  7300  may, for example, include without limitation one or more of an Ethernet mapping and optional optical transport network (OTN) framing unit  7302 , an FEC coding unit  7304 , a symbol mapping unit  7306 , a linear and nonlinear pre-emphasis unit  7308 , a DAC unit  7310 , an IQ modulation and polarization combining unit  7310 , and an optional directional element unit  7314 . That is, directional element unit  7314  may be implemented in the case of a single optical interface transceiver (e.g., transceiver  7200 ,  FIG. 72 ), but may not be needed in the case of a dual optical interface transceiver (e.g., transceiver  7100 ,  FIG. 71 ). 
     In an exemplary embodiment, the optical signal transmitted through transmitter  7300  may be described using parameters including, without limitation, one or more of the encoding scheme, line rate, polarization imbalance, quadrature and polarization skew, transmitter clock jitter, frequency tolerance, optical output power, laser wavelength, laser linewidth, and transmitter OSNR. The optical distribution medium of the access network (e.g., cable environment, telecommunication environment, etc.) may include various elements over the respective link, including one or more of an optical fiber, optical splitters, optical circulators, wavelength multiplexers, wavelength demultiplexers, and other optical passive components. Various impairments that may impact the optical signals traversing the link may include optical loss or gain, chromatic dispersion, polarization mode dispersion (PMD), polarization dependent loss, polarization rotation, optical crosstalk, and optical SNR degradation. 
     An optical signal that is generated by such an “imperfect” implementation of transmitter  7300  may be further degraded by one or more impairments from the optical distribution medium, such as those entering the line side of the transceiver (e.g., transceiver  7100 ,  FIG. 71 , transceiver  7200 ,  FIG. 72 ) for detection, compensation and processing by a receiver portion thereof. An exemplary receiver configuration is described further below with respect to  FIG. 74 . 
       FIG. 74  is a functional schematic illustration of a receiver  7400 . In an exemplary embodiment, receiver  7400  is configured to perform relevant receiver functions occurring, for example, in the coherent optical transceiver (e.g., transceiver  7100 ,  FIG. 71 , transceiver  7200 ,  FIG. 72 ) from the optical input on the line side (e.g., line side  7104 ,  FIG. 71 , line side  7204 ,  FIG. 72 ) to the electrical output on the host side (e.g., host side  7102 ,  FIG. 71 , host side  7202 ,  FIG. 72 ). In exemplary operation, receiver  7400  includes one or more functional units which generally correspond to relevant functional units of transmitter  7300 ,  FIG. 73 , and which may similarly operate in the order listed, or in a different order, and may be individually implemented by hardware elements, software modules, or by combinations of hardware and software. 
     The receiver functional units may include, without limitation, one or more of an optional directional element unit  7402  (e.g., in the case of a single optical interface transceiver such as transceiver  7200 ,  FIG. 72 ), a detection unit  7404  for detecting I and Q orthogonal channels for each X and Y polarizations, an ADC unit  7406 , a deskew and orthogonality compensation unit  7408 , a chromatic dispersion estimation and compensation unit  7410 , a PMD compensation and polarization multiplexing unit  7412 , a clock recovery unit  7414 , a carrier frequency offset estimation and compensation unit  7416 , a carrier phase estimation and compensation unit  7418 , a symbol demapping unit  7420 , an FEC decoding unit  7422 , and an Ethernet demapping and optional OTN framing unit  7424 . In an embodiment, receiver  7400  may include one or more feedback channels  7426  to ADC unit  7406 , from one or more of units  7408 ,  7410 ,  7412 ,  7414 , respectively. 
     In an exemplary embodiment, the optical signal processed through receiver  7400  may be described using parameters including, without limitation, one or more of the modulation, symbol rate, symbol mapping, FEC, line rate, encoding scheme, frequency tolerance, frame format and mapping, optical input power, laser wavelength, laser linewidth, receiver OSNR, polarization imbalance, quadrature and polarization skew, transmitter clock jitter, chromatic dispersion, polarization dispersion, and polarization rotation (SoP track). 
     With respect to both transmitter  7300  and receiver  7400 , some general transceiver characteristics may also be considered, such as (i) the end-to-end link latency, which includes both the transmitter and receiver latencies, as well as the transmission delay of the optical channel, and (ii) the reflectances of the transmitter and receiver optical interfaces, which are characterized by its optical return loss. Additionally, the operation of the transceiver may also be impacted by the ambient temperature, for which some additional compensation may be desirable. With respect to receiver  7400  in particular, the present systems and methods may further utilize the data reacquisition time as a useful metric to indicate the time the receiver takes to turn back on after loss of signal. 
     With respect to the optical ports and frequencies of the transceivers described above (e.g., transceiver  7100 ,  FIG. 71 , transceiver  7200 ,  FIG. 72 ), these transceivers may be configured to have either one or two optical ports to handle the respective transmit and receive functions. A transceiver having two separate transmit and receive ports, for example, may use the same frequency. In an exemplary embodiment, a transceiver may use a single port for both transmit and receive, either by using two separate frequencies, or through bi-directional transmission on a single fiber at the same frequency. In at least one embodiment, a two-frequency, two-port solution is achieved through a hybrid implementation of these approaches. In some cases, a single optical interface transceiver may relax some of its minor requirements relative to those provided for a dual optical interface transceiver. 
     Accordingly, a transceiver according to the above embodiments is expected to support either one or two optical ports for transmit and receive functions, and also to support use of the same frequency for transmit and receive functions. Nevertheless, the transceiver may be further configured to support transmitting and receiving on two different frequencies. 
     That is, referring back to  FIGS. 71 and 72 , two different line side interface options are described herein, namely, a dual optical interface option (e.g., transceiver  7100 ), and a single optical interface option (e.g. transceiver  7200 ). According to the present systems and methods, either interface option is capable of supporting a single frequency for transmitting and receiving, on the one hand, or separate frequencies for transmitting and receiving come on the other hand. Thus, the present embodiments are configured to support both line side interface options. In an exemplary embodiment, the transceiver is configured to support use of the same frequency for transmitting and receiving, and may further be optionally configured to be capable of supporting transmitting and receiving using different frequencies. 
     The transmitter optical output power is defined as the total optical launch power, measured in dBm, from the output port of a transceiver in operation, and this parameter may be measured with a calibrated optical power meter (OPM) capable of power measurement, for example, in the 1550 nm wavelength range. During a transmitter startup (e.g., power up, power down, or a change of wavelength sequence), the transmitter may generate “fast transients,” such as sudden spikes in power across a range of frequencies, which may briefly impact any operating channels that are on the same optical plant as the starting up transceiver. Accordingly, in at least one embodiment, the present transceiver may be further configured with blanking capability, which enables the transceiver to suppress optical output until, for example, the transceiver output has stabilized. Accordingly, different transmit power requirements may be provided for the dual optical interface and single optical interface approaches, namely, to account for the loss incurred by the directional element within the single optical interface transceiver. 
     For example, in the exemplary case of a transceiver supporting the present dual optical interface approach, the transceiver may be configured to support a transmitter optical output power of at least −6 dBm (or higher). In contrast, in the case of a transceiver supporting the present single optical interface approach, the transceiver may be configured to support a transmitter optical output power of −6.75 dBm or higher. Consistent with the operation of the several embodiments described herein, the transceiver may be still further configured to prevent a transmitter optical output power above a particular desired value (e.g., +7 dBm or higher). 
     In some embodiments, the transceiver may be further configured to include capability to: (i) report its minimum and maximum supported optical output powers; (ii) report its transmitter optical output power with an accuracy of ±1.5 dB; (iii) optionally support adjustment of its transmitter optical output power (e.g., adjustments in steps of 0.1 dB); and (iv) optionally support blanking to protect the optical plant during startup. 
     Under other conditions, such as in the case of an optical amplifier being close to the receiving transceiver, the optical received power may be considered relatively high, but the OSNR may be considered to be relatively low in comparison. This discrepancy may occur due to the noise added by optical amplification, and because optical amplifiers may boost both of the signal and the noise power levels. In this case, the transceiver will be limited by its sensitivity to OSNR, rather than power, which is referred to as an “OSNR-limited case,” and represents a baseline requirement for received OSNR. 
     In addition to the baseline received OSNR requirement that applies to the dual optical interface transceiver option, an adjustment (described above) in the receive power requirement of the single optical interface option may also be introduced to account for the loss incurred by directional elements within the transceiver to enable the single optical interface option. 
     Accordingly, where the transceiver supports dual optical interfaces, the transceiver may be configured, for example, to achieve a post-FEC BER of ≤10 −15  when the link OSNR is ≥14.5 dB and the received optical power is ≥−10 dBm (i.e., the “baseline received optical power requirement”). In contrast, where the transceiver supports a single optical interface, the transceiver may be alternatively configured to achieve a post-FEC BER of ≤10 −15  when the link OSNR is ≥14.5 dB and the received optical power is ≥−9.25 dBm (i.e., the “baseline received OSNR requirement”). In these examples, the transceiver may thus be further configured to report the received OSNR. 
     In some implementations of the single optical interface transceiver approach described herein, a performance degradation in OSNR (i.e., received OSNR penalty) may occur due to discrete optical reflections and from back scattering caused by fiber imperfections. High-quality fiber splicing, cleanliness of fiber-optic connector mating surfaces, and the use of angle-polished connectors, for example, may contribute to a lower back reflection power. Thus, in an optical link using a single optical interface transceiver, a 0.5 dB penalty in link OSNR may be observed with an aggregate back reflection power of −33 dBm over a 50-km SMF transmission and a receive optical power of ≥−20 dBm. In this scenario, a link OSNR≥15 dB may be desirable to address this reflection impairment. 
     Based on the systems and methods described herein, and as verified by the innovative testing schemes demonstrating proofs of concept thereof, several coexistence measurement experiments confirmed that: (i) both coherent and analog/NRZ signals work may be effectively configured to coexist, with approximately a 0.6-dB maximum power penalty in the case of 100-GHz channel spacing and 50/80-km fiber transmission distance; (ii) legacy analog system components and devices (e.g., analog EDFA, optical Mux/DeMux) may be effectively configured for multiplexing and amplification of coherent signals, which exhibit strong robustness when deployed with traditional analog DWDM transmission; and (iii) conventional AWG-based optical Mux and DeMux, which are conventionally used for coherent channels, are undesirable for use with conventional analog channels. 
     Therefore, as the industry evolves toward Node+0 architectures, the volume of optical connections to intelligent nodes will increase substantially in comparison with conventional, traditional architectures. The present coherent optics technology-based systems and methods though, offer future-proofing solutions that will be of particular value for cable operators to meet bandwidth demand without having to retrench new fibers. 
     As described herein, extensive experimental verification under different coexistence scenarios provides significant operational and deployment guidance for the use cases described, and for similar use cases that would be understood by persons of ordinary skill in the art. These experimental results demonstrate that coherent optics transmissions are robust, even in close proximity to much stronger analog and intensity-modulated optical carriers. According to these advantageous embodiments, cable operators are effectively enabled to support 100 G (or higher) coherent channels on existing cable access networks without significant performance degradation. 
     Exemplary embodiments of full duplex coherent optics systems and methods for communication networks are described above in detail. The systems and methods of this disclosure though, are not limited to only the specific embodiments described herein, but rather, the components and/or steps of their implementation may be utilized independently and separately from other components and/or steps described herein. 
     Although specific features of various embodiments of the disclosure may be shown in some drawings and not in others, this convention is for convenience purposes and ease of description only. In accordance with the principles of the disclosure, a particular feature shown in a drawing may be referenced and/or claimed in combination with features of the other drawings. 
     Some embodiments involve the use of one or more electronic or computing devices. Such devices typically include a processor or controller, such as a general purpose central processing unit (CPU), a graphics processing unit (GPU), a microcontroller, a reduced instruction set computer (RISC) processor, an application specific integrated circuit (ASIC), a programmable logic circuit (PLC), a field programmable gate array (FPGA), a digital signal processing (DSP) device, and/or any other circuit or processor capable of executing the functions described herein. The processes described herein may be encoded as executable instructions embodied in a computer readable medium, including, without limitation, a storage device and/or a memory device. Such instructions, when executed by a processor, cause the processor to perform at least a portion of the methods described herein. The above examples are exemplary only, and thus are not intended to limit in any way the definition and/or meaning of the term “processor.” 
     This written description uses examples to disclose the embodiments, including the best mode, and also to enable any person skilled in the art to practice the embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.