Patent Publication Number: US-2022239377-A1

Title: Optical communication systems and methods

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 17/169,600, filed on Feb. 8, 2021. U.S. application Ser. No. 17/169,600 is a continuation of U.S. application Ser. No. 16/600,324, filed on Oct. 11, 2019, now U.S. Pat. No. 10,917,177, issued Feb. 9, 2021. U.S. application Ser. No. 16/600,324 is a continuation of U.S. application Ser. No. 15/861,303, filed on Jan. 3, 2018, now U.S. Pat. No. 10,447,404, issued Oct. 15, 2019. U.S. application Ser. No. 15/861,303 is a continuation of U.S. application Ser. No. 15/283,632, filed on Oct. 3, 2016, now U.S. Pat. No. 9,912,409, issued Mar. 3, 2018. U.S. application Ser. No. 15/283,632 claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/321,211, filed Apr. 12, 2016. All of these prior applications are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     The field of the disclosure relates generally to optical communication networks, and more particularly, to optical networks utilizing wavelength division multiplexing. 
     Telecommunications networks include an access network through which end user subscribers connect to a service provider. Bandwidth requirements for delivering high-speed data and video services through the access network are rapidly increasing to meet growing consumer demands. At present, data delivery over the access network is growing by gigabits (Gb)/second for residential subscribers, and by multi-Gb/s for business subscribers. Present access networks are based on passive optical network (PON) access technologies, which have become the dominant system architecture to meet the growing high capacity demand from end users. 
     Gigabit PON and Ethernet PON architectures are conventionally known, and presently provide about 2.5 Gb/s data rates for downstream transmission and 1.25 Gb/s for upstream transmission (half of the downstream rate). 10 Gb/s PON (XG-PON or IEEE 10G-EPON) has begun to be implemented for high-bandwidth applications, and a 40 Gb/s PON scheme, which is based on time and wavelength division multiplexing (TWDM and WDM) has recently been standardized. A growing need therefore exists to develop higher/faster data rates per-subscriber to meet future bandwidth demand, and also increase the coverage for services and applications, but while also minimizing the capital and operational expenditures necessary to deliver higher capacity and performance access networks. 
     One known solution to increase the capacity of a PON is the use of WDM technology to send a dedicated wavelength signal to end users. Current detection scheme WDM technology, however, is limited by its low receiver sensitivity, and also by the few options available to upgrade and scale the technology, particularly with regard to use in conjunction with the lower-quality legacy fiber environment. The legacy fiber environment requires operators to squeeze more capacity out of the existing fiber infrastructure to avoid costs associated with having to retrench new fiber installment. Conventional access networks typically include six fibers per node, servicing as many as 500 end users, such as home subscribers. Conventional nodes cannot be split further and do not typically contain spare (unused) fibers, and thus there is a need to utilize the limited fiber availability in a more efficient and cost-effective manner. 
     Coherent technology has been proposed as one solution to increase both receiver sensitivity and overall capacity for WDM-PON optical access networks, in both brown and green field deployments. Coherent technology offers superior receiver sensitivity and extended power budget, and high frequency selectivity that provides closely-spaced dense or ultra-dense WDM without the need for narrow band optical filters. Moreover, a multi-dimensional recovered signal experienced by coherent technology provides additional benefits to compensate for linear transmission impairments such as chromatic dispersion (CD) and polarization-mode dispersion (PMD), and to efficiently utilize spectral resources to benefit future network upgrades through the use of multi-level advanced modulation formats. Long distance transmission using coherent technology, however, requires elaborate post-processing, including signal equalizations and carrier recovery, to adjust for impairments experienced along the transmission pathway, thereby presenting significant challenges by significantly increasing system complexity. 
     Coherent technology in longhaul optical systems typically requires significant use of high quality discrete photonic and electronic components, such as digital-to-analog converters (DAC), analog-to-digital converters (ADC), and digital signal processing (DSP) circuitry such as an application-specific integrated circuit (ASIC) utilizing CMOS technology, to compensate for noise, frequency drift, and other factors affecting the transmitted channel signals over the long distance optical transmission. Coherent pluggable modules for metro solution have gone through C Form-factor pluggable (CFP) to CFP2 and future CFP4 via multi-source agreement (MSA) standardization to reduce their footprint, to lower costs, and also to lower power dissipation. However, these modules still require significant engineering complexity, expense, size, and power to operate, and therefore have not been efficient or practical to implement in access applications. 
     BRIEF SUMMARY 
     In one aspect, an injection locked transmitter for an optical communication network includes a master seed laser source input substantially confined to a single longitudinal mode, an input data stream, and a laser injected modulator including at least one slave laser having a resonator frequency that is injection locked to a frequency of the single longitudinal mode of the master seed laser source. The laser injected modulator is configured to receive the master seed laser source input and the input data stream, and output a laser modulated data stream. 
     In another aspect, an optical network communication system includes, an input signal source, an optical frequency comb generator configured to receive the input signal source and output a plurality of phase synchronized coherent tone pairs. Each of the plurality of phase synchronized coherent tone pairs includes a first unmodulated signal and a second unmodulated signal. The system further include a first transmitter configured to receive the first unmodulated signal of a selected one of the plurality of phase synchronized coherent tone pairs as a seed source and to output a first modulated data stream, and a first receiver configured to receive the first modulated data stream from the first transmitter and receive the second unmodulated signal of the selected one of the plurality of phase synchronized coherent tone pairs as a local oscillator source. 
     In yet another aspect, an optical network communication system includes an optical hub including an optical frequency comb generator configured to output at least one phase synchronized coherent tone pair having a first unmodulated signal and a second unmodulated signal, and a downstream transmitter configured to receive the first unmodulated signal as a seed source and to output a downstream modulated data stream. The system further includes a fiber node and an end user including a downstream receiver configured to receive the downstream modulated data stream from the downstream transmitter and receive the second unmodulated signal as a local oscillator source. 
     In a still further aspect, a method of optical network processing includes steps of generating at least one pair of first and second unmodulated phase synchronized coherent tones, transmitting the first unmodulated phase synchronized coherent tone to a first transmitter as a seed signal, adhering downstream data, in the first transmitter, to the first unmodulated phase synchronized coherent tone to generate a first modulated data stream signal, optically multiplexing the first modulated data stream signal and the second unmodulated phase synchronized coherent tone together within a hub optical multiplexer, and communicating the multiplexed first modulated data stream signal and the second unmodulated phase synchronized coherent tone to a first receiver, by way of fiber optics, for downstream heterodyne detection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 an exemplary fiber communication system in accordance with an exemplary embodiment of the present disclosure. 
         FIG. 2  is a schematic illustration depicting an exemplary transmitter that can be utilized with the fiber communication system depicted in  FIG. 1 . 
         FIG. 3  is a schematic illustration depicting an alternative transmitter that can be utilized with the fiber communication system depicted in  FIG. 1 . 
         FIG. 4  is a schematic illustration depicting an alternative transmitter that can be utilized with the fiber communication system depicted in  FIG. 1 . 
         FIG. 5  is a schematic illustration depicting an alternative transmitter that can be utilized with the fiber communication system depicted in  FIG. 1 . 
         FIG. 6  is a schematic illustration depicting an exemplary upstream connection that can be utilized with the fiber communication system depicted in  FIG. 1 . 
         FIG. 7  is a schematic illustration depicting an exemplary processing architecture implemented with the fiber communication system depicted in  FIG. 1 . 
         FIG. 8  is a flow chart diagram of an exemplary downstream optical network process. 
         FIG. 9  is a flow chart diagram of an exemplary upstream optical network process that can be implemented with the downstream process depicted in  FIG. 8 . 
     
    
    
     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 the 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. 
     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. 
       FIG. 1  is a schematic illustration of an exemplary fiber communication system  100  in accordance with an exemplary embodiment of the present disclosure. System  100  includes an optical hub  102 , a fiber node  104 , and an end user  106 . Optical hub  102  is, for example, a central office, a communications hub, or an optical line terminal (OLT). In the embodiment shown, fiber node  104  is illustrated for use with a passive optical network (PON). End user  106  is a downstream termination unit, which can represent, for example, a customer device, customer premises (e.g., an apartment building), a business user, or an optical network unit (ONU). In an exemplary embodiment, system  100  utilizes a coherent Dense Wavelength Division Multiplexing (DWDM) PON architecture. 
     Optical hub  102  communicates with fiber node  104  by way of downstream fiber  108 . Optionally, where upstream communication is desired along system  100 , optical hub  102  further connects with fiber node  104  by way of upstream fiber  110 . In operation, downstream fiber  108  and upstream fiber  110  are typically 30 km or shorter. However, according to the embodiments presented herein, greater lengths are contemplated, such as between 100 km and 1000 km. In an exemplary embodiment, fiber node  104  connects with end user  106  by way of fiber optics  112 . Alternatively, fiber node  104  and end user  106  may be integrated as a single device, such as a virtualized cable modem termination system (vCMTS), which may be located at a customer premises. Where fiber node  104  and end user  106  are separate devices, fiber optics  112  typically spans a distance of approximately 5000 feet or less. 
     Optical hub  102  includes an optical frequency comb generator  114 , which is configured to receive a high quality source signal  116  from an external laser  118  and thereby generate multiple coherent tones  120 ( 1 ),  120 ( 1 ′), . . .  120 (N),  120 (N′). Optical frequency comb generator  114  utilizes, for example, a mode-locked laser, a gain-switched laser, or electro-optic modulation, and is constructed such that multiple coherent tones  120  are generated as simultaneous low-linewidth wavelength channels of known and controllable spacing. This advantageous aspect of the upstream input signal into system  100  allows a simplified architecture throughout the entire downstream portion of system  100 , as described further below. 
     Generated coherent tones  120  are fed into an amplifier  122 , and the amplified signal therefrom is input into a first hub optical demultiplexer  124 . In an exemplary embodiment, amplifier  122  is an erbium-doped fiber amplifier (EDFA). Optical hub  102  further includes a downstream transmitter  126  and a hub optical multiplexer  128 . In an embodiment, optical hub  102  optionally includes a hub optical splitter  130 , an upstream receiver  132 , and a second hub optical demultiplexer  134 . 
     Downstream transmitter  126  includes a downstream optical circulator  136  and a downstream modulator  138 . In an exemplary embodiment, downstream modulator  138  is an injection locked laser modulator. Upstream receiver  132  includes an upstream integrated coherent receiver (ICR)  140 , an upstream analog to digital converter (ADC)  142 , and an upstream digital signal processor (DSP)  144 . In the exemplary embodiment, fiber node  104  includes a node optical demultiplexer  146 . In an alternative embodiment, where upstream transmission is desired, fiber node  104  further includes a node optical multiplexer  148 . In the exemplary embodiment, node optical demultiplexer  146  and node optical multiplexer  148  are passive devices. 
     End user  106  further includes a downstream receiver  150 . In an exemplary embodiment, downstream receiver  150  has a similar architecture to upstream receiver  132 , and includes a downstream ICR  152 , a downstream ADC  154 , and a downstream DSP  156 . For upstream transmission, end user  106  optionally includes end user optical splitter  158 , which may be located within downstream receiver  150  or separately, and an upstream transmitter  160 . In an exemplary embodiment, upstream transmitter  160  has a similar architecture to downstream transmitter  126 , and includes an upstream optical circulator  162 , and an upstream modulator  164 . 
     In operation, system  100  utilizes optical frequency comb generator  114  and amplifier  122  convert the input high quality source signal  116  into multiple coherent tones  120  (e.g., 32 tones, 64 tones, etc.), which are then input to first hub optical demultiplexer  124 . In an exemplary embodiment, high quality source signal  116  is of sufficient amplitude and a narrow bandwidth such that a selected longitudinal mode of signal  116  is transmitted into optical frequency comb generator  114  without adjacent longitudinal modes, which are suppressed prior to processing by comb generator  114 . First hub optical demultiplexer  124  then outputs a plurality of phase synchronized coherent tone pairs  166 ( 1 ),  166 ( 2 ), . . .  166 (N). That is, the generated coherent frequency tones  120  are amplified by amplifier  122  to enhance optical power, and then demultiplexed into multiple separate individual phased synchronized coherent tone source pairs  166 . For simplicity of discussion, the following description pertains only to coherent tone pair  166 ( 1 ) corresponding to the synchronized pair signal for the first channel output, which includes a first unmodulated signal  168  for Ch 1  and a second unmodulated signal  170  for Ch 1 ′, and their routing through system  100 . 
     With source signal  116  of a high quality, narrow band, and substantially within a single longitudinal mode, coherent tone pair  166 ( 1 ), including first unmodulated signal  168  (Ch 1 ) and second unmodulated signal  170  (Ch 1 ′), is output as a high quality, narrowband signal, which then serves as both a source of seed and local oscillator (LO) signals for both downstream and upstream transmission and reception directions of system  100 . That is, by an exemplary configuration, the architecture of optical frequency comb generator  114  advantageously produces high quality continuous wave (CW) signals. Specifically, first unmodulated signal  168  (Ch 1 ) may function as a downstream seed and upstream LO throughout system  100 , while second unmodulated signal  170  (Ch 1 ′) concurrently may function as an upstream seed and downstream LO for system  100 . 
     According to the exemplary embodiment, within optical hub  102 , first unmodulated signal  168  (Ch 1 ) is divided by hub optical splitter  130  and is separately input to both downstream transmitter  126  and upstream receiver  132  as a “pure” signal, and i.e., substantially low amplitude, narrow bandwidth continuous wave does not include adhered data. First unmodulated signal  168  (Ch 1 ) thus becomes a seed signal for downstream transmitter  126  and an LO signal for upstream receiver  132 . In an exemplary embodiment, within downstream transmitter  126 , first unmodulated signal  168  (Ch 1 ) passes through downstream optical circulator  136  into downstream modulator  138 , in which one or more laser diodes (not shown in  FIG. 1 , described below with respect to  FIGS. 2-5 ) are excited, and adhere data (also not shown in  FIG. 1 , described below with respect to  FIGS. 2-5 ) to the signal that then exits downstream optical circulator  136  as downstream modulated data stream  172  (Ch 1 ). 
     In an exemplary embodiment, downstream optical circulator  136  is within downstream transmitter  126 . Alternatively, downstream optical circulator  136  may be physically located separately from downstream transmitter  126 , or else within the confines of downstream modulator  138 . Downstream modulated data stream  172  (Ch 1 ) is then combined in hub optical multiplexer  128  with the plurality of modulated/unmodulated data stream pairs from other channels (not shown) and transmitted over downstream fiber  108 , to a node optical demultiplexer  174  in fiber node  104 , which then separates the different channel stream pairs for transmission to different respective end users  106 . At end user  106 , because the data stream pair  170 ,  172  entering downstream receiver  150  is a phase synchronized, digital signal processing at downstream DSP  156  is greatly simplified, as described below with respect to  FIG. 7 . 
     Where upstream reception is optionally sought at optical hub  102 , second unmodulated signal  170  (Ch 1 ′) is divided, within end user  106 , by end user optical splitter  158  and is separately input to both downstream receiver  150  and upstream transmitter  160  as a “pure” unmodulated signal for Ch 1 ′. In this alternative embodiment, second unmodulated signal  170  (Ch 1 ′) thus functions a seed signal for upstream transmitter  160  and a “pseudo LO signal” for downstream receiver  150  for the coherent detection of Ch 1 . For purposes of this discussion, second unmodulated signal  170  (Ch 1 ′) is referred to as a “pseudo LO signal” because it uses an LO signal from a remote source (output from first hub optical demultiplexer  124 ), and is not required to produce an LO signal locally at end user  106 . This particular configuration further significantly reduces cost and complexity of the architecture of the system  100  by the reduction of necessary electronic components. 
     For upstream transmission, in an exemplary embodiment, a similar coherent detection scheme is implemented for upstream transmitter  160  as is utilized for downstream transmitter  126 . That is, second unmodulated signal  170  (Ch 1 ′) is input to upstream optical circulator  162  and modulated by upstream modulator  164  to adhere symmetric or asymmetric data (not shown, described below with respect to  FIG. 6 ) utilizing one or more slave lasers (also not shown, described below with respect to  FIG. 6 ), and then output as an upstream modulated data stream  176  (Ch 1 ′), which is then combined with similar modulated data streams from other channels (not shown) by a node multiplexer  178  in fiber node  104 . Second unmodulated signal  170  (Ch 1 ′) is then transmitted upstream over upstream fiber  110 , separated from other channel signals by second hub optical demultiplexer  134 , an input to upstream receiver  132 , for simplified digital signal processing similar to the process described above with respect to downstream receiver  150 . 
     By this exemplary configuration, multiple upstream channels from different end users  106  can be multiplexed at fiber node  104  (or a remote node) and sent back to optical hub  102 . Thus, within optical hub  102 , the same coherent detection scheme may be used at upstream receiver  132  as is used with downstream receiver  150 , except that upstream receiver  132  utilizes first unmodulated signal  168  (Ch 1 ) as the LO and upstream modulated data stream  176  (Ch 1 ′) to carry data, whereas downstream receiver  150  utilizes the data stream pair (Ch 1 , Ch 1 ′) in reverse. That is, downstream receiver  150  utilizes second unmodulated signal  170  (Ch 1 ′) as the LO and downstream modulated data stream  172  (Ch 1 ) to carry data. 
     Implementation of the embodiments described herein are useful for migrating hybrid fiber-coaxial (HFC) architectures towards other types of fiber architectures, as well as deeper fiber architectures. Typical HFC architectures tend to have very few fiber strands available from fiber node to hub (e.g. fibers  108 ,  110 ), but many fiber strands could be deployed to cover the shorter distances that are typical from legacy HFC nodes to end users (e.g., fiber optics  112 ). In the exemplary embodiments described herein, two fibers (i.e., fibers  108 ,  110 ) are illustrated between optical hub  102  and fiber node  104 , which can be a legacy HFC fiber node. That is, one fiber (i.e., downstream fiber  108 ) is utilized for downstream signal and upstream seed/downstream LO, and another fiber (i.e., upstream fiber  110 ) is utilized for upstream signal. Additionally, three fibers (i.e., fiber optics  112 A-C) are illustrated for each end user from fiber node  104  (e.g., legacy HFC fiber node) to end user  106 . By utilization of the advantageous configurations herein, fiber deeper or all-fiber migration schemes can utilize an HFC fiber node as an optical fiber distribution node, thereby greatly minimizing the need for fiber retrenching from an HFC node to an optical hub. 
     The architecture described herein, by avoiding the need for conventional compensation hardware, can therefore be structured as a significantly less expensive and more compact physical device than conventional devices. This novel and advantageous system and subsystem arrangement allows for multi-wavelength emission with simplicity, reliability, and low cost. Implementation of optical frequency comb generator  114 , with high quality input source signal  116 , further allows simultaneous control of multiple sources that are not realized by conventional discrete lasers. According to the embodiments herein, channel spacing, for example, may be 25 GHz, 12.5 GHz, or 6.25 GHz, based on available signal bandwidth occupancy. 
     The embodiments described herein realize still further advantages by utilizing a comb generator (i.e., optical frequency comb generator  114 ) that maintains a constant wavelength spacing, thereby avoiding optical beat interference (OBI) that may be prevalent in cases with simultaneous transmissions over a single fiber. In the exemplary embodiment illustrated in  FIG. 1 , fiber node  104  is shown as a passive system, and is thus expected to maintain a higher reliability than other migration approaches. Nevertheless, one of ordinary skill in the art, after reading and comprehending present application, will understand how the embodiments disclosed herein may also be adapted to a remote PHY solution, or to a remote cable modem termination system (CMTS) that is included in the fiber node. 
     As illustrated and described herein, system  100  may utilize an architecture of coherent DWDM-PON incorporate novel solutions to meet the unique requirements of access environment, but with cost-efficient structures not seen in conventional hardware systems. Optical frequency comb generator  114  produces a plurality of simultaneous narrow width wavelength channels with controlled spacing, thereby allowing simplified tuning of the entire wavelength comb. This centralized comb light source in optical hub  102  therefore provides master seeding sources and LO signals for both downstream and upstream directions in heterodyne detection configurations in order to reuse the optical sources throughout the entirety of system  100 . This advantageous configuration realizes significant cost savings and reduction in hardware complexity over intradyne detection schemes in long-haul systems, for example. 
       FIG. 2  is a schematic illustration depicting an exemplary downstream transmitter  200  that can be utilized with fiber communication system  100 , depicted in  FIG. 1 . Downstream transmitter  200  includes downstream optical circulator  136  (see  FIG. 1 , above) in two-way communication with a laser injected modulator  202 , which includes a laser diode  204 , which receives data  206  from an external data source  208 . In an alternative embodiment, downstream transmitter  200  may include two separate fiber receivers (not shown), which would substitute, and eliminate the need, for downstream optical circulator  136  in the structural configuration shown. 
     In operation, downstream transmitter  200  performs the same general functions as downstream transmitter  126  ( FIG. 1 , described above). Laser injected modulator  202  utilizes laser diode  204  as a “slave laser.” That is, laser diode  204  is injection locked by external laser  118 , which functions as a single frequency or longitudinal mode master, or seed, laser to keep the frequency of a resonator mode of laser diode  204  close enough to the frequency of the master laser (i.e., laser  118 ) to allow for frequency locking. The principle of downstream transmitter  200  is also referred to as “laser cloning,” where a single high quality master laser (i.e., laser  118 ) transmits a narrow bandwidth, low noise signal (i.e., source signal  116 ), and a relatively inexpensive slave laser (e.g., laser diode  204 ) can be used throughout system  100  to transmit data modulated signals, such as downstream modulated data stream  172  (Ch 1 ). In an exemplary embodiment, laser diode  204  is a Fabry Perot laser diode (FP LD), or a vertical-cavity surface-emitting laser (VCSEL), in comparison with the considerably more expensive distributed feedback laser diodes (DFB LD) that are conventionally used. In an alternative embodiment, laser diode  204  is an LED, which can perform as a sufficient slave laser source according to the embodiments herein due to the utilization of the high quality source signal  116  that is consistently utilized throughout system  100 . 
     More specifically, first unmodulated signal  168  (Ch 1 ) exiting hub optical splitter  130  is input to downstream optical circulator  136 , which then excites laser diode  204 , that is, laser diode  204  emits light at a specified modulation rate. Laser injected modulator  202  adheres data  206  to the excited Ch 1  signal, and the resultant modulated Ch 1  signal with adhered data is output from downstream optical circulator  136  as downstream modulated data stream  172  (Ch 1 ). According to this exemplary embodiment, first unmodulated signal  168  (Ch 1 ) is input to downstream transmitter  126  as an unmodulated, low amplitude, narrow bandwidth, low noise “pure” source, and is modulated by laser diode  204 , which is a high amplitude, wide bandwidth device, and resultant downstream modulated data stream  172  (Ch 1 ) is a high amplitude, narrow bandwidth, low noise “pure” signal that can be transmitted throughout system  100  without the need for further conventional compensation means (hardware and programming). Suppression of adjacent longitudinal modes from laser diode  204 , for example, is not necessary because of the exciting source signal (i.e., signal  168 ) is of such high quality and narrow bandwidth that output downstream modulated data stream  172  (Ch 1 ) is substantially amplified only within the narrow bandwidth of external laser  118 . In the exemplary embodiment illustrated in  FIG. 2 , laser injected modulator  202  implements direct modulation. 
     Optical injection locking as described herein thus improves upon the performance of the relatively less expensive, multi-longitudinal slave laser source (i.e., laser diode  204 ) in terms of spectral bandwidth and noise properties. With respect to heterodyne coherent detection, incoming signals (upstream or downstream) can be combined with the LO or pseudo-LO and brought to an intermediate frequency (IF) for electronic processing. According to this exemplary configuration, part of the LO/pseudo-LO optical power can also be employed as the master/seed laser for the reverse transmission direction, at both optical hub  102 , and at end user  106  (described below with respect to  FIG. 6 ), and thus a fully coherent system having a master seed and LO delivery from an optical hub can be achieved in a relatively cost-effective manner comparison with conventional systems. 
       FIG. 3  is a schematic illustration depicting an alternative downstream transmitter  300  that can be utilized with fiber communication system  100 , depicted in  FIG. 1 . Downstream transmitter  300  is similar to downstream transmitter  200  ( FIG. 2 ), including the implementation of direct modulation, except that downstream transmitter  300  alternatively utilizes polarization division multiplexing to modulate the Ch 1  signal into downstream modulated data stream  172  (Ch 1 ). 
     Downstream transmitter  300  includes downstream optical circulator  136  (see  FIG. 1 , above) in two-way communication with a laser injected modulator  302 , which includes a polarization beam splitter (PBS)/polarization beam combiner (PBC)  304 , which can be a single device. Laser injected modulator  302  further includes a first laser diode  306  configured to receive first data  308  from an external data source (not shown in  FIG. 3 ), and a second laser diode  310  configured to receive second data  312  from the same, or different, external data source. 
     In operation, downstream transmitter  300  is similar to downstream transmitter  200  with respect to the implementation of direct modulation, and master/slave laser injection locking. Downstream transmitter  300  though, alternatively implements dual-polarization from the splitter portion of PBS/PBC  304 , which splits first unmodulated signal  168  (Ch 1 ) into its x-polarization component P 1  and y-polarization component P 2 , which separately excite first laser diode  306  and second laser diode  310 , respectively. Similar to downstream transmitter  200  ( FIG. 2 ), in downstream transmitter  300 , first unmodulated signal  168  (Ch 1 ) exiting hub optical splitter  130  is input to downstream optical circulator  136 , the separate polarization components of which then excite laser diodes  306 ,  310 , respectively, at the specified modulation rate. Laser injected modulator  302  adheres data first and second data  308 ,  312  to the respective excited polarization components of the Ch 1  signal, which are combined by the combiner portion of PBS/PBC  304 . The resultant modulated Ch 1  signal with adhered data is output from downstream optical circulator  136  as downstream modulated data stream  172  (Ch 1 ). 
     In an exemplary embodiment, the polarized light components received by first and second laser diodes  306 ,  310  are orthogonal (90 degrees and/or noninteractive). That is, first laser diode  306  and second laser diode  310  are optimized as slave lasers to lock onto the same wavelength as external laser  118  (master), but with perpendicular polarization directions. By this configuration, large data packets (e.g., first data  308  and second data  312 ) can be split and simultaneously sent along separate pathways before recombination as downstream modulated data stream  172  (Ch 1 ). Alternatively, first data  308  and second data  312  may come from two (or more) separate unrelated sources. The orthogonal split prevents data interference between the polarized signal components. However, one of ordinary skill in the art will appreciate that, according to the embodiment of  FIG. 3 , first unmodulated signal  168  (Ch 1 ) can also be polarized at 60 degrees, utilizing similar principles of amplitude and phase, as well as wavelength division. First unmodulated signal  168  (Ch 1 ) can alternatively be multiplexed according to a spiral or vortex polarization, or orbital angular momentum. Additionally, whereas the illustrated embodiment features polarization multiplexing, space division multiplexing and mode division multiplexing may be also alternatively implemented. 
     According to this exemplary embodiment, master continuous wave signal for Ch 1 , namely, first unmodulated signal  168 , is received from optical frequency comb generator  114  and is split to be used, in the first part, as the LO for upstream receiver  132 , and in the second part, to synchronize two slave lasers (i.e., first laser diode  306  and second laser diode  310 ) by the respective x-polarization and y-polarization light portions such that both slave lasers oscillate according to the wavelength of the master laser (i.e., external laser  118 ). Data (i.e., first data  308  and second data  312 ) is directly modulated onto the two slave lasers, respectively. This injection locking technique thus further allows for frequency modulation (FM) noise spectrum control from the master laser to the slave laser, and is further able to realize significant improvements in FM noise/phase jitter suppression and emission linewidth reduction. 
     As described herein, utilization of optical injection with a dual-polarization optical transmitter (i.e., downstream transmitter  300 ) by direct modulation may advantageously implement relatively lower-cost lasers to perform the functions of conventional lasers that are considerably more costly. According to this configuration of a dual-polarization optical transmitter by direct modulation of semiconductor laser together with coherent detection, the present embodiments are particular useful for short-reach applications in terms of its lower cost and architectural compactness. Similar advantages may be realized for long reach applications. 
       FIG. 4  is a schematic illustration depicting an alternative downstream transmitter  400  that can be utilized with fiber communication system  100 , depicted in  FIG. 1 . Downstream transmitter  400  is similar to downstream transmitter  200  ( FIG. 2 ), except that downstream transmitter  400  alternatively implements external modulation, as opposed to direct modulation, to modulate the Ch 1  signal into downstream modulated data stream  172  (Ch 1 ). Downstream transmitter  400  includes downstream optical circulator  136  (see  FIG. 1 , above) and a laser injected modulator  402 . Downstream optical circulator  136  is in one-way direct communication with a separate external optical circulator  404  that may be contained within laser injected modulator  402  or separate. Laser injected modulator  402  further includes a laser diode  406 , which receives the low amplitude, narrow bandwidth, first unmodulated signal  168  (Ch 1 ) and emits an excited, high amplitude, narrow bandwidth, optical signal  408  back to external optical circulator  404 . Laser injected modulator  402  still further includes an external modulating element  410 , which receives data  412  from an external data source  414 , and adheres data  412  with optical signal  408  to be unidirectionally received back by downstream optical circulator  136  and output as downstream modulated data stream  172  (Ch 1 ). 
     In this exemplary embodiment, downstream transmitter  400  performs the same general functions as downstream transmitter  126  ( FIG. 1 , described above), but uses external modulation as the injection locking mechanism to lock laser diode  406  to the wavelength of the master laser source (e.g., external laser  118 ). To implement external modulation, this embodiment regulates optical signal flow through mostly unidirectional optical circulators (i.e., downstream optical circulator  136 , external optical circulator  404 ). External modulating element  410  may optionally include a demultiplexing filter (not shown) as an integral component, or separately along the signal path of downstream modulated data stream  172  (Ch 1 ) prior to input by downstream receiver  150 . In an exemplary embodiment, external modulating element  410  is a monitor photodiode, and injection locking is performed through a rear laser facet. 
       FIG. 5  is a schematic illustration depicting an alternative downstream  500  transmitter that can be utilized with fiber communication system  100 , depicted in  FIG. 1 . Downstream transmitter  500  is similar to downstream transmitter  300  ( FIG. 3 ), including the implementation of direct modulation and polarization division multiplexing, except that downstream transmitter  500  further implements quadrature amplitude modulation (QAM) to modulate the Ch 1  signal into downstream modulated data stream  172  (Ch 1 ). That is, further external modulating elements may be utilized per polarization branch ( FIG. 2 , above) to generate QAM signals. 
     Downstream transmitter  500  includes downstream optical circulator  136  (see  FIG. 1 , above) in two-way communication with a laser injected modulator  502 , which includes a PBS/PBC  504 , which can be a single device or two separate devices. Additionally, all of the components of laser injected modulator  502  may themselves be separate devices, or alternatively all contained within a single photonic chip. Laser injected modulator  502  further includes a first laser diode  506  configured to receive first data  508  from an external data source (not shown in  FIG. 5 ), a second laser diode  510  configured to receive second data  512  from the same, or different, external data source, a third laser diode  514  configured to receive third data  516  from the same/different, external data source, and a fourth laser diode  518  configured to receive fourth data  520  from the same/different external data source. 
     In operation, downstream transmitter  500  implements dual-polarization from the splitter portion of PBS/PBC  504 , which splits first unmodulated signal  168  (Ch 1 ) into its x-polarization component (P 1 ) and y-polarization component (P 2 ). Each polarization component P 1 , P 2  is then input to first non-polarized optical splitter/combiner  522  and second non-polarized optical splitter/combiner  524 , respectively. First and second optical splitters/combiners  522 ,  524  each then further split their respective polarization components P 1 , P 2  into their I-signals  526 ,  528 , respectively, and also into their Q-signals  530 ,  532 , respectively. Generated I-signals  526 ,  528  then directly excite laser diodes  506 ,  514 , respectively. Before directly communicating with laser diodes  510 ,  518 , respectively, generated Q-signals  530 ,  532  first pass through first and second quadrature phase shift elements  534 ,  536 , respectively, each of which shifts the Q-signal by 45 degrees in each direction, such that the respective Q-signal is offset by 90 degrees from its respective I-signal when recombined at splitters/combiners  522 ,  524 . 
     The resultant modulated Ch 1  signal, with adhered data, is output from downstream optical circulator  136  of downstream transmitter  500  as downstream modulated data stream  172  (Ch 1 ), and as a polarized, multiplexed QAM signal. According to this exemplary embodiment, utilization of a photonic integrated circuit allows for directly modulated polarization of a multiplexed coherent system, but utilizing significantly lower cost hardware configurations than are realized by conventional architectures. In an exemplary embodiment, laser diodes  506 ,  510 ,  514 ,  516  are PAM-4 modulated laser diodes capable of generating 16-QAM polarization multiplexed signals. 
       FIG. 6  is a schematic illustration depicting an exemplary upstream transmitter  600  that can be utilized with the fiber communication system  100 , depicted in  FIG. 1 . In the embodiment illustrated in  FIG. 6 , upstream transmitter  600  is similar to downstream transmitter  300  ( FIG. 3 ) in structure and function. Specifically, upstream transmitter  600  includes upstream optical circulator  162  (see  FIG. 1 , above) in two-way communication with a laser injected modulator  602  (not separately illustrated in  FIG. 6 ), which includes a PBS/PBC  604 , which can be a single device or separate devices. Laser injected modulator  602  further includes a first laser diode  606  configured to receive first data  608  from an external data source (not shown in  FIG. 6 ), and a second laser diode  610  configured to receive second data  612  from the same, or different, external data source. Similar to the embodiments of  FIGS. 2-5 , above, downstream transmitter  600  may also eliminate for upstream optical circulator  162  by the utilization of at least two separate fiber receivers (not shown). 
     Upstream transmitter  600  is thus nearly identical to downstream transmitter  300  ( FIG. 3 ), except that upstream transmitter  600  utilizes second unmodulated signal  170  (Ch 1 ′) as the end user seed source, in laser injected modulator  602 , to combine or adhere with data (e.g., first data  608 , second data  612 ) to generate upstream modulated data stream  176  (Ch 1 ′) to carry upstream data signals to an upstream receiver (e.g., upstream receiver  132 ). In operation, first laser diode  606  and second laser diode  610  also function as slave lasers by injection locking to the master signal from external laser  118 . That is, symmetric or asymmetric data for Ch 1 ′ (e.g., first data  608 , second data  612 ) is modulated onto the two slave lasers (i.e., first laser diode  606  and second laser diode  610 ) with polarization multiplexing, much the same as the process implemented with respect to downstream transmitter  300  ( FIG. 3 ) in optical hub  102 . 
     In this example, upstream transmitter  600  is illustrated to substantially mimic the architecture of downstream transmitter  300  ( FIG. 3 ). Alternatively, upstream transmitter  600  could equivalently mimic the architecture of one or more of downstream transmitters  200  ( FIG. 2 ),  400  ( FIG. 4 ), or  500  ( FIG. 5 ) without departing from the scope of the present disclosure. Furthermore, upstream transmitter  600  can conform to any of the embodiments disclosed by  FIGS. 2-5 , irrespective of the specific architecture of the particular downstream transmitter utilized within optical hub  102 . By utilization of high-quality, narrow bandwidth, low noise external laser source  118 , the master/slave laser relationship carries through the entirety of system  100 , and the plurality of end users  106  that receive modulated/unmodulated signal pairs (which may be 32, 64, 128, or as many as 256 from a single fiber line pair, e.g., downstream fiber  108  and upstream fiber  110 ). 
     The significant cost savings according to the present embodiments are thus best realized when considering that as many as 512 downstream transmitters (e.g., downstream transmitter  126 ,  FIG. 1 ) and upstream transmitters (e.g., upstream transmitter  160 ,  FIG. 1 ) may be necessary to fully implement all available chattel pairs from a single optical hub  102 . The present embodiments implement a significantly lower cost and less complex hardware architecture to utilize the benefits accruing from implementation of high-quality external laser  118 , without having to add expensive single longitudinal mode laser diodes, or other compensation hardware necessary to suppress adjacent longitudinal modes from inexpensive lasers or the noise components produced thereby. 
       FIG. 7  is a schematic illustration depicting an exemplary processing architecture which can be implemented for upstream receiver  132 , downstream receiver  150 , and fiber communication system  100 , depicted in  FIG. 1 . The respective architectures of upstream receiver  132  and downstream receiver  150  are similar with respect to form and function (described above with respect to  FIG. 1 ), except that upstream receiver  132  receives a first data stream pair  700  for Ch 1 , Ch 1 ′, in reverse of a second data stream pair  702 , which is received by downstream receiver  150 . In other words, as described above, first data stream pair  700  includes first unmodulated signal  168  (Ch 1 ) as the LO and upstream modulated data stream  176  (Ch 1 ′) to carry data, whereas second data stream pair  702  includes unmodulated signal  170  (Ch 1 ′) as the LO and downstream modulated data stream  172  (Ch 1 ) to carry data. 
     First and second data stream pairs  700 ,  702  the multiplexed phase synchronized pairs modulated/unmodulated of optical signals that are converted into analog electrical signals by ICR  140  and ICR  152 , respectively. The respective analog signals are then converted into digital domain by ADC  142  and ADC  154 , for digital signal processing by DSP  144  and DSP  156 . In an exemplary embodiment, digital signal processing may be performed by a CMOS ASIC employing very large quantities of gate arrays. A conventional CMOS ASIC, for example, can utilize as many as 70 million gates to process incoming digitized data streams. In the conventional systems, modulated data streams for Ch 1  and Ch 1 ′ are processed independently, which requires significant resources to estimate frequency offset, drift, and digital down conversion compensation factors (e.g., e{circumflex over ( )}-jωt, where ω represents the frequency difference between first unmodulated signal  168  and upstream modulated data stream  176 , and ω is held constant for coherent tone pair  166 , as extended throughout system  100 ). 
     According to the exemplary embodiments disclosed herein, on the other hand, the modulated and unmodulated signals from Ch 1  and Ch 1 ′ are phase synchronized together such that the difference between ω of the signal pair is always known, and phase synchronized to maintain a constant relationship. In contrast, conventional systems are required to constantly estimate the carrier phase to compensate for factors such as draft which requires considerable processing resources, as discussed above. According to the present embodiments though, since Ch 1  and Ch 1 ′ are synchronized together as first and second data stream pairs  700 ,  702 , the offset ω between the pairs  700 ,  702  need not be estimated, since it may be instead easily derived by a simplified subtraction process in DSP  144  and DSP  156  because the signal pairs will drift together by the same amount in a constant relationship. By this advantageous configuration and process, digital signal processing by a CMOS ASIC can be performed utilizing as few as one million gates, thereby greatly improving the processing speed of the respective DSP, and/or reducing the number of physical chips required to perform the processing (or similarly increasing the amount of separate processing that may be performed by the same chip). At present, implementation of the embodiments described herein may improve downstream and upstream data transmission speeds by as much as 5000 times faster than conventional systems. 
       FIG. 8  is a flow chart diagram of an exemplary downstream optical network process  800  that can be implemented with fiber communication system  100 , depicted in  FIG. 1 . Process  800  begins at step  802 . In step  802 , coherent tone pairs  166  are generated and output by optical frequency comb generator  114 , amplifier  122 , and first hub optical demultiplexer  124 . Similar to the discussion above, for simplification purposes, the following discussion addresses specific coherent tone pair  166 ( 1 ) for Ch 1 , Ch 1 ′. Coherent tone pair  166  includes first unmodulated signal  168  (Ch 1 ) and second unmodulated signal  170  (Ch 1 ′). Once coherent tone pair  166  is generated, process  800  proceeds from step  802  to steps  804  and  806 , which may be performed together or simultaneously. 
     In step  804 , first unmodulated signal  168  (Ch 1 ) is input to an optical splitter, e.g., optical splitter  130 ,  FIG. 1 . In step  806 , second unmodulated signal  170  (Ch 1 ′) is transmitted to a multiplexer, e.g., hub optical multiplexer  128 ,  FIG. 1 . Referring back to step  804 , first unmodulated signal  168  (Ch 1 ) is split to function both as an LO for upstream detection, and as a seed for downstream data transmission. For upstream detection, step  804  proceeds to step  808 , where first unmodulated signal  168  (Ch 1 ) is received by an upstream receiver, i.e., upstream receiver  132 ,  FIG. 1 . For downstream data transmission, step  804  separately and simultaneously proceeds to step  810 . 
     Step  810  is an optional step, where polarization division multiplexing is desired. In step  810 , first unmodulated signal  168  (Ch 1 ) is split into its x-component and y-component parts P 1 , P 2 , respectively (e.g., by PBS/PBC  304 ,  FIG. 3  or PBS/PBC  504 ,  FIG. 5 ) for separate direct or external modulation. Where polarization division multiplexing is not utilized, process  800  skips step  810 , and instead proceeds directly from step  804  to step  812 . In step  812 , first unmodulated signal  168  (Ch 1 ), or its polarized components if optional step  810  is implemented, is modulated by direct (e.g.,  FIGS. 2, 3, 5 ) or external (e.g.,  FIG. 4 ) modulation. Process  800  then proceeds from step  812  to step  814 . Step  814  is an optional step, which is implemented if optional step  810  is also implemented for polarization division multiplexing. In step  814 , the x-component and y-component parts P 1 , P 2  are recombined (e.g., by PBS/PBC  304 ,  FIG. 3  or PBS/PBC  504 ,  FIG. 5 ) for output as downstream modulated data stream  172  (Ch 1 ). Where polarization division multiplexing was not utilized, process  800  skips step  814 , and instead proceeds directly from step  812  to step  816 . 
     In step  816 , second unmodulated signal  170  (Ch 1 ′) and downstream modulated data stream  172  (Ch 1 ) are optically multiplexed, i.e., by hub optical multiplexer  128 ,  FIG. 1 , as a phase synchronized data stream pair (e.g., second data stream pair  702 ,  FIG. 7 ). Process  800  then proceeds from step  816  to step  818 , where the phase synchronized data stream pair is transmitted over an optical fiber, i.e., downstream fiber  108 ,  FIG. 1 . Process  800  then proceeds from step  818  to step  820 , where the synchronized data stream pair is optically demultiplexed, e.g., by node optical demultiplexer  174  in fiber node  104 . Process  800  then proceeds from step  820  to step  822 , where both components of the demultiplexed data stream pair (e.g., second unmodulated signal  170  (Ch 1 ′) and downstream modulated data stream  172  (Ch 1 )) are received by a downstream receiver (e.g., downstream receiver  150 ,  FIG. 1 ) for heterodyne coherent detection. 
     Where an end user (e.g., end user  106 ) further includes upstream transmission capability, process  800  further includes optional steps  824  and  826 . In step  824 , and prior to downstream reception in step  822 , second unmodulated signal  170  (Ch 1 ′) is optically split (e.g., by end user optical splitter  158 ,  FIG. 1 ), and additionally transmitted, in step  826 , to an upstream transmitter of the end user (e.g., upstream transmitter  160 ,  FIG. 1 ) as a seed signal for a modulator (e.g., modulator  164 ,  FIG. 1 ) for upstream data transmission, as explained further below with respect to  FIG. 9 . 
       FIG. 9  is a flow chart diagram of an exemplary upstream optical network process  900  that can be optionally implemented with fiber communication system  100 , depicted in  FIG. 1 . Process  900  begins at optional step  902 . In step  902 , where polarization division multiplexing is utilized in the upstream transmitter (e.g., upstream transmitter  160 ,  FIG. 1 ), second unmodulated signal  170  (Ch 1 ′) (from step  826 ,  FIG. 8 ) is split into its x-component and y-component parts (e.g., by PBS/PBC  604 ,  FIG. 6 ) for separate direct or external modulation. Where polarization division multiplexing is not utilized, step  902  is skipped, and process  900  instead begins at step  904 . 
     In step  904 , second unmodulated signal  170  (Ch 1 ′), or its polarized components if optional step  902  is implemented, is injection locked to the master source laser (e.g., external laser  118 ,  FIG. 1 ), as described above with respect to  FIGS. 1 and 6 . Step  904  then proceeds to step  906 , where injection locked signal is modulated by direct or external modulation. Process  900  then proceeds from step  906  to step  908 . Step  908  is an optional step, which is implemented if optional step  902  is also implemented for polarization division multiplexing. In step  908 , the x-component and y-component parts of the excited Ch 1 ′ signal are recombined (e.g., by PBS/PBC  604 ,  FIG. 6 ) for output as upstream modulated data stream  176  (Ch 1 ′). Where polarization division multiplexing was not utilized, process  900  skips step  908 , and instead proceeds directly from step  906  to step  910 . 
     In step  910 , upstream modulated data stream  176  (Ch 1 ′) is optically multiplexed, i.e., by node optical multiplexer  178 ,  FIG. 1 , with other upstream data stream signals (not shown). Process  900  then proceeds from step  910  to step  912 , where upstream modulated data stream  176  (Ch 1 ′) is transmitted over an optical fiber, i.e., upstream fiber  110 ,  FIG. 1 . Process  900  then proceeds from step  912  to step  914 , where upstream modulated data stream  176  (Ch 1 ′) is optically demultiplexed, e.g., by second hub optical demultiplexer  134 , which separates the selected data stream from the other upstream data stream signals, for transmission to a particular upstream receiver tuned to receive the modulated data stream. Process  900  then proceeds from step  914  to step  916 , where both components (e.g., first unmodulated signal  168  (Ch 1 ),  FIG. 8 , and upstream modulated data stream  176  (Ch 1 ′)) of the upstream data stream pair, e.g., first data stream pair  700 ,  FIG. 7 , are received by an upstream receiver (e.g., upstream receiver and  32 ,  FIG. 1 ) for heterodyne coherent detection. 
     As illustrated in the exemplary embodiment, a difference between upstream and downstream signal transmission is that an entire synchronized modulated/unmodulated channel pair (e.g., second data stream pair  702 ,  FIG. 7 ) can be transmitted in the downstream direction, whereas, in the upstream direction, only a data modulated signal (e.g., upstream modulated data stream  176  (Ch 1 ′)) to be transmitted over the upstream fiber connection, i.e., upstream fiber  110 . An advantage of the present configuration is that the LO for upstream coherent detection (e.g., at upstream receiver  132 ,  FIG. 1 ) comes directly from the split signal, i.e., first unmodulated signal  168  (Ch 1 ) generated from optical frequency comb generator  114  within optical hub  102 , after separation by first hub optical demultiplexer  124 , as depicted in  FIG. 1 . Conventional systems typically require LO generation at each stage of the respective system. According to the present disclosure, on the other hand, relatively inexpensive slave lasers can be implemented throughout the system architecture for modulation and polarization multiplexing in both optical hub  102  and end user  106  components, without requiring an additional LO source at the end user. 
     According to the present disclosure, utilization of dual-polarization optical transmitters, and by direct modulation of semiconductor lasers with coherent detection, is particularly beneficial for not only longhaul applications, but also for shortreach applications to reduce the cost of electronic hardware, while also rendering the overall network system architecture more compact. The present systems and methods further solve the conventional problem of synchronizing two laser sources over a long period of time. Utilization of the phase synchronized data stream pairs and slave lasers herein allows continual synchronization of the various laser sources throughout the system during its entire operation. These solutions can be implemented within coherent DWDM-PON system architectures for access networks in a cost-efficient manner. 
     Utilization of the high quality optical comb source at the front end of the system thus further allows a plurality of simultaneous narrow bandwidth wavelength channels to be generated with easily controlled spacing, and therefore also simplified tuning of the entire wavelength comb. This centralized comb light source in the optical hub provides master seeding sources and LO signals that can be reused throughout the system, and for both downstream and upstream transmission. The implementation of optical injection, as described herein, further improves the performance of low-cost multi-longitudinal slave laser sources in terms of spectral bandwidth and noise properties. Access networks according to the present systems and methods thus achieve more efficient transmission of wavelengths through optical fibers, thereby increasing the capacity of transmitted data, but at lower power, increased sensitivity, lower hardware cost, and a reduction in dispersion, DSP compensation, and error correction. 
     Exemplary embodiments of fiber communication systems and methods 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. Additionally, the exemplary embodiments can be implemented and utilized in connection with other access networks utilizing fiber and coaxial transmission at the end user stage. 
     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. 
     Although specific features of various embodiments of the disclosure may be shown in some drawings and not in others, this is for convenience 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 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.